the radio continuum morphology of spiral galaxies

Copyright © 1976 by Annual Reviews Inc. All rights reserved

THE RADIO CONTINUUM

MORPHOLOGY OF SPIRAL

GALAXIES

P. C. van der Kruit and R. J. Allen

:-:2103

Kapteyn Astronomical I nstitute, University of Groningen, Groningen, The Netherlands

INTRODUCTION

Once Jansky realized that the extraterrestial microwave radiation which he had discovered was not coming from the Sun but from a direction near to that of the Galactic center (Jansky 1933) and after he established that the emission was associated with the Milky Way (Jansky 1935), he naturally concluded that either stars in the disk or interstellar matter in the Galaxy was responsible for the emission ofthis radiation. Reber ( 1940) confirmed this and was the first to try to detect similar

emission from the nearest external spiral galaxy M31 in Andromeda. He gives a positive result at 160 MHz of about 3800 Jy, 1 corresponding to a main-beam brightness temperature of � lOooK assuming that the true beam width of his telescope was about 12°. In a later paper (Reber 1944) he gives an inconclusive detection of about 960 Jy corresponding to � 25°K. We know now that the latter value of flux density is an order of magnitude too high to be a detection; the brightness temperature indicates that what Reber observed was probably part of the complicated structure in the galactic background [see for example the all-sky map of Landecker & Wielebinski ( 1970)]. However, with the Galaxy dominating the radio sky it is understandable that the nearest spiral galaxies were expected to be detectable. Ryle, Smith & Elsmore (1950) were therefore not surprised that four ofthe weakest sources in their first Cambridge Survey appeared to be associated with the brightest spiral galaxies. Their quoted flux densities are now known also to be in error; in their case it is most likely that their observations were adversely affected by confusion with neighboring sources.

Detection of spiral galaxies turned out to be possible only with telescopes that were not confused at their detection limit and that had a resolving power comparable with the size of the galaxies. It was with such an instrument that Hanbury Brown & Hazard ( 1951) made the first incontrovertible observation of the

lIly = 10-26 W m-2 Hz-l. We of ten use the milli-lansky (mly), which is 1O-29 W m- 2 Hz-l.

417

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.

418 VAN DER KRUIT & ALLEN

radio emission from M31 with a beam of 2° halfpower width (at 158.5 MHz). The flux density was 100 Jy. Although the source was apparently extended, the beamwidth was too large to permit any statement about the distribution of the emission over the nebula. The first hint of this distribution came from interferometer measurements made by Baldwin (1954) at Cambridge, who carried out observations at various position angles of the interferometer baseline at sp�cings up to 18 wavelengths at 8 1 MHz. His strip distribution could be interpreted as consisting of a component "distributed like the stars" plus a spherical component, the latter containing two-thirds of the total flux density. The existence of the spherical component, which in later years became known as the halo, is still a matter of debate, as discussed in Section 7. These measurements, however, confirmed that in general terms the distribution of radio continuum emission in M31 was roughly similar to that of the Galaxy (Westerhout & Oort 195 1).

The radio-continuum surveys of M31 are summarized in Table 1. With angular resolutions of the order of 1°, very little detail is visible over the disk of the galaxy. Large et al. ( 1959) and MacLeod (1964) produced the first maps that showed some

Table t Radio-continuum surveys of the Andromeda Nebula

Frequency Authors Year (GHz) Beamwidth

Hanbury Brown & Hazard 1951 0.159 2° x 2° Baldwin 1954 0.081 18Jc spacing inter-

ferometer Seeger, Westerhout & Conway 1957 0.400 2�2 x 1 �8 Hanbury Brown & Hazard 1959 0.158 2�2 x 1 �6

0.237 1�5xl�1 Large, Mathewson & Haslam 1959 0.408 40' x 56' Baldwin & Costain 1960 0.038 0�8x4�4

0.178 13:5 x 4�6 Leibacker 1964 1.417 53' x 53' Kraus 1964 1.415 11' x 35' MacLeod 1964 0.610 16' x 16' Kenderdine & Baldwin 1965 0.038 45' x 45' Cooley, Roberts & Swenson 1967 2.695 11' xii' Pooley 1969a 0.408 80" x 120"; 4' x 4'

1.407 23" x 35" Durdin & Terzian 1972 0.074 100' x 80'

0.111 67' x 52' 0.197 33' x 37'

van der Kruit 1972 1.415 23" x 35" ; 3:9 x 5:4 Spencer 1973 2.695 �9"

8.085 �3" Berkhuijsen & Wielebinski 1973, 1974 2.695 4:8 x 4:8 Dennison, Balonek, Terzian & Balick 1975 1.410 10:4 x 10:4

2.695 5:2 x 5:2

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.

RADIO CONTINUUM MORPHOLOGY OF SPIRAL GALAXIES 419

structure in the disk, and they speculated that part of the emission was associated with the spiral structure. Cooley et aL ( 1967), however, attributed most of the structure in their map to a clustering of radio sources within the optical image of the galaxy. A great step forward was made when Pooley ( 1969a) observed M31 with the Cambridge One-Mile telescope. His map at 408 MHz, smoothed to a 4' beam and with most small-diameter sources removed, showed a structure remarkably similar to the distribution of H II regions, even though the emission had a non thermal spectrum. Pooley concluded that the emission originated in the spiral arms, caused either by energetic electrons from supernovae or by enhanced magnetic field strength. His observations were extended by van der Kruit ( 1972), who resolved the nuclear region into a few components, and further by Berkhuijsen & Wielebinski ( 1973, 1974). The effects of increased resolution are shown in Figure 1.

Here we review and discuss those radio-continuum observations of galaxies that show structural details in the brightness distribution, such as, for example, in Figure lb. We do not review in detail the information obtained from the extensive surveys of the total radio flux densities of spiral galaxies, such as the radio luminosity function or correlations involving the total radio power. The same applies to continuum surveys of the galactic background, which are only touched upon when a comparison with external systems is made. As this is a review of spiral galaxies, we also exclude most of the information obtained for the Magellanic Clouds, but again this is mentioned particularly in connection with the observations of H II regions and possible supernova remnants in extra galactic systems. We limit ourselves to normal galaxies ; the radio morphology of peculiar galaxies such as M82 and NGC 1275, as well as the structure of radio galaxies, is excluded. For recent reviews on H I 21 -cm line observations the reader is referred to Oort ( 1974) and Allen ( 1975a). Finally, we do not discuss the possible association of radio sources and spiral galaxies that are in close proximity on the sky.

We believe our literature survey to be complete up to August 1975. Recent reviews on the subject have been given by Oort (1974), Ekers ( 1974, 1975), and Allen (1975a).

2 THE DISK RADIO EMISSION IN SPIRAL GALAXIES

We review here those radio observations that have been made with sufficient angular resolution to provide some structural details of the radio continuum emission from the disks of spiral galaxies. In order to unambiguously separate any nuclear component from the possibly present disk emission, the halfpower beamwidth of the radio telescope should be less than about one-fifth of the galaxy Holmberg diameter. Examination of published results confirms this choice of minimum relative resolution, since in cases where the beamwidths are larger, the authors are generally unable tq present a discussion of the nucleus and disk emission separately. Table 2 summarizes several pertinent references to relatively low-resolution studies of normal galaxies. We do not discuss these results any further here; correlations with other properties of the galaxies have been investigated by several of the authors referenced in Table 2 and have been recently reviewed by Ekers ( 1975). A compendium of radio-continuum observations of bright galaxies has recently been provided by Haynes et al. (1975),

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.


-10

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.


and many useful references to earlier radio-continuum-structure observations of

bright galaxies can be found in the bibliography by Brosche et al. ( 1974). In Table 3 we reference those observations of spiral galaxies that satisfy our

selection criterion. In general, the authors have published contour maps and/or

41° 00' -

(b) Figure 1 Radio-continuum observations of M 3 1 at various resolutions. (a) Map at 196.5 MHz with a beam of 33' x 37' by Durdin & Terzian (1972). Contour interval is 6 K. (b) Map of the disk at 408 MHz by Pooley (1969a) with a 4' x 4' beam superposed on the distribution of H II regions. This map contains more information than the 4' resolution, because the initially higher resolution (�1:5) enabled the subtraction of small-diameter sources, which might have confused the picture. Contour interval is 2.5 K. (e) The nucleus at 1415 MHz observed with a 23" x 35" beam by van def Kruit [1972; the figure appeared in this form in Ekers (1975)]. All of the complex is contained within the central source in Pooley's map. The cross indicates the optical nucleus. The contour unit is 1.6 K (2 mly/beam).

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.


Declination (1950.0)

41· 01' 30"

41· 00' 00"

- , --

, ,

-'

(c)

Figure Ie See pages 420-21.

I I I I 1\ I \ I

, ( / \ / I 0/

I I ' .... --�

,.-\ c, ........ / \ '" -- , I \

"\ LJ I , ,"'.1 I I " , ... J

Oh39m4S'

/ J '"

r I \ "

, I ,

Right Ascension{1950.0)

Table 2 References to some recent analyses of relatively low-resolution radio observations of spiral and irregular galaxies (telescope HPBW is greater than about one-fifth of the Holmberg diameter of the galaxy)

Observing frequency Total number of

Reference�a (GH;.:) galaxies detected

Slee ( 1972a,b) 0.08 30 Cameron (1971a,b,c) 0.408 92 Lequeux (1971a) 1.421 54 Kazes et al. (1970) 2.695 47 Le Squeren & Crovisier (1974) 4.851 15 McCutcheon (1973) 6.631, 10.7 17

, Further references may be found in the papers listed in column one.

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.


Table 3 Radio-continuum observations of spiral galaxies for which the HPBW of the telescope is less than about one-fifth of the Holmberg diameter of the galaxy

References

Landecker & Wi elebinski (1970)'

Durdin & Terzian (1972) Pooley (1969a)b de Jong (1965) van del' Kruit (1972)' B erkhuijsen & Wielebinski (1974)

Slee ( l972b) Cameron (1971c)

Terzian & Pankonin (1972) de Jong (1965) van der Kruit (I 973a)b Huchtmeier (1972), Lequeux (1971b) de long (1967)

Baldwin & Pooley (1973)

van def Kruit (1973a)

van der Kruit (1973b)

van def Kruit, OOTt & Mathewson (1972)

van der Kruit (l973a)b, von Kap-herr, Jones & Wielebinski (1975)

Pooley (1969b) van der Kruit (1973a) Lcqueux (1971a)

Baldwin & Pooley (1973)

van der Kruit ( l97I)b

Cameron (1971b)

Mathewson, van der Kruil & Brouw (1972)

[srael, Goss & Allen (1975)

Lequeux (1971b) Ekers (1975)

Allen & Raimond (1972)

a This reference is included pro memori.

Observing frequency

(GIlz)

0,085, 0,150

0,196 DADS, 1.407 0,750,1.415 1.415 2,695

O,OSO OAOS

0,606 0,750 1.415 1.41S 2,695

O,40S

1.415

1.415

1.415

1.415 4,SOO

OA08, IA07 1.415 1.421

OAOS

1.415

OA08

1.415

0,610, 1.415

1.421 1.415

1.415

Galaxies

our Galaxy

M31 � N224

N253

M33 � N59S

NS91

N2403, N2903, M64 � N4826, N5055

M74 � N62S, lC342,b N3432, N3556, N4321,' N7331'

N4258

M81 � N3031

N4631

N4656/7

M94 � N4736

N4945, N5236, N55

M51 � N5194/5

M101 � N5457

N6946

Maffei 2

b The intensity of disk emission is unreliable owing to the absence of short interferometer spacings

in the observations.

cHI known to be present in the published continuum map.

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.


cross-cuts, which permit an identification of radio emISSIOn extended over the optical image of the galaxy. In several favorable cases, the angular resolution and sensitivity are great enough to distinguish bright spiral structures and giant H IT com plexes in the radio continuum emission, but most of the observations discussed here provide only a smoothed-out picture of the emission; it is this component that we refer to as the disk. This ineludes the spiral-arm emission and, therefore, IS different from the base disk, which is referred to in Section 3.

2.1 Observed Variations of Disk Brightness and Spectral Index with Radius

Since the earliest rudimentary mappings of the radio emission from spiral galaxies carried out at Nancay, CaJtech, Green Bank, and 10drell Bank, it has been known that if a galaxy shows a detectable radio disk component at all, the emission is roughly coincident with the optical disk. This property is confirmed by the results of the observations listed in Table 3. These newer results show further that the brightness of the radio disk generally decreases with increasing distance from the center of the galaxy, although in several notable cases (e.g. NGC 253, 4656, 891, 55) the position of the centroid of the disk brightness does not seem to correspond to the position of the galaxy nucleus.

In M51 the observed disk brightness is reasonably well described by an exponential

function exp ( -: Rj Ro), and the value for the scale length Ro is in the range of what is observed for the exponential disks of Sbc galaxies in optical surface photometry (Allen 1975a). This characteristic seems to obtain approximately for several other galaxies referred to in Table 3. The radial dependence of the radio volume emissivity at 408 MHz for NGC 891 (observed by Baldwin & Pooley 1973) can also be approximated by an exponential, and Price ( 1974) was able to obtain a reasonable fit with an exponential emissivity function applied to the observations of the Galactic radio continuum. These apparent correlations deserve a more thorough investigation with a comparison of both the radio and optical surface photometry of the same galaxy.

The mean radio disk brightness does not seem to correlate with any of the usual integral properties of spiral galaxies such as the total mass or luminosity (van der Kruit 1973c), and, although a tendency has been noted for the brightest disks to occur in the intermediate Hubble types (van der Kruit 1973c, Ekers 1 975), there are several notable exceptions. The range in brightness among galaxies is also very large. For instance, the mean brightness of the disk of M51 (Sbc) exceeds that of MIOl (Sc) by about a factor of 10 at 1415 MHz. An interesting diagram results when the

luminosity2 of the nucleus is plotted against the average brightness temperature of the disk (van der Kruit 1973c, Ekers 1975). Figure 2 is reproduced from the latter paper. One should keep in mind that the selection effects due to sensitivity (below

P 1= 8 X 1018 W Hz-1 sr-I the sample is incomplete) are severe mainly in the lower right corner. If the relation were to hold for a complete sample, it would constitute

2 We express the radiated power at a particular freq uency in W Hz - I sr - 1 and refer to it as the (monochromatic) luminosity.

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.


a strong argument for a significant contribution by the nucleus to the relativistic particles in the disk.

An important selection effect in all the observations mentioned here is a preference for the brightest spirals. Although it is known that the intrinsically faint spirals (M pg > - 20) are less likely to be radio emitters than the bright spirals (e.g. Fanti et al. 1973), the separate properties of the'di�k and nuclear emission of faint spirals are not known at present.

� 1 L.. Q) ...... \I)

,. I � Ui ::J Q)

U ::J c -� �

cL

1021r-------�1-----------.-1----------.-,-,

1019 r-

.M101

1018 -

1017 - I

o 0.1 K

.4051

.M81

.253

.2 903 .69�6

.M51

• IC31.2 .891

_ 4826

1.211. .1.631

_ .5035 + 1. 258 � 7331 .1.736

.4656 .galaxy

21.03 •

I o

1.0 K Ts (1415MHz)

-

-.1.1.90

-

I

Fiqure 2 Plot of the monochromatic luminosity of the central sources of spiral galaxies against the disk average brightness temperature at 1415 MHz (Ekers 1975).

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.


Only a limited amount of information is available on the variation of spectral index with radius. Israel et al. ( 1975) have determined spectral indices between 610 and 1415 MHz for several positions in the disk of M101 (Holmberg radius about 1 2:5). No significant variation in the mean (flux density) value of -0.4 was found for distances ranging from 4 to 15 kpc from the nucleus (2' to 7:5). In NGC 891 the spectral index is - 0.6 out to 3' from the center as determined from observations at 610, 1415, and 4995 MHz with the Westerbork telescope (Allen, Baldwin, and Sancisi ; see Sancisi et al. 1975).

A further notable result on the spectra of the disk emission in galaxies has been obtained by Slee ( 1972b). The total disk emission from NGC 253 has a strongly convex spectrum, essentially flat between 80 and 408 MHz and curving sharply down until the flux density at 2650 MHz is about a factor of 10 below the 80 MHz value. Similar "anomalies" were found in other bright galaxies (Slee 1972a), but the observations could not be separated into nuclear and disk components, as was possible for NGC 253. The radio spectra of regions near the plane of the Galaxy show similar, but much less pronounced, flattening at low frequencies. Further low-frequency measurements of galaxies with angular resolutions of order l' are clearly desirable in order to investigate these phenomena further.

2.2 Comments On the Origin of Disk Radio Emission in Galaxies

Based largely on the available one-dimensional radio brightness distributions, Lequeux (1971b) proposed that the radial distribution of radio continuum emission closely followed that of the "young Population I" typified by the H II regions. Lequeux further pointed out that the occurrence of supernovae as a function of radius in galaxies resembled the radio continuum distribution, and suggested that these supernovae are the sources of the relativistic electrons, which in turn generate the radio emission. Ilovaisky & Lequeux ( 1972) further applied this model to the Galactic radio emission. The new morphological information now available from the observations referred to in Table 3 allows us to make several qualifying remarks with regard to these suggestions.

First, the radial distributions of radio continuum emission do not resemble the distributions of extreme Population I, namely, the H I gas. As has been known for some time, the gas generally extends to larger radii than does the presently detectable continuum emission.

Second, the central regions of many spiral galaxies are often relatively devoid of H II regions, whereas the nonthermal continuum disks appear to increase to maxima located on or near the nuclei of the galaxies.

Finally, if further studies verify that the optical surface photometry and the radio surface brightness are as well correlated as has been indicated above, then one could just as well conclude that the radio continuum emission is associated with Population II and the general mass distribution in a galaxy rather than with Population I.

Although Figure 2 suggests a relation between radio emission from the nucleus and from the disk, the role of the nuclei remains unclear owing to selection effects (see above). As would be expected from the energy losses, Le Squeren & Crovisier ( 1974) have found a steeper spectrum for the disks than for the central complexes, whereas

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.


Ekers (1975) has found these spectra to be generally similar. It is therefore hazardous to draw definite conclusions. That some relation between the radio emission from the central regions and the disks might exist is evident from the correlation of the former with the total optical magnitude, but it is too early to speculate on the meaning of this relation.

3 SPIRAL STRUCTURE

In this section we review the observations of spiral structure in maps of the radio continuum distributions in galaxies. The discussion in the foregoing section on the general disk emission required observations with telescope beam widths of about one-fifth or less of the galaxy diameter. In order to observe spiral structure the requirements are even more severe (Allen 1975a) ; as the example of M3 1 shows (Figure 1), the beam has to be less than about one-tenth of the optical diameter.

As remarked in Section 1, the first hint of structure in the radio continuum maps, which seemed to be associated with the spiral arms, came from the synthesis observations ofM31 by Pooley (1969a). The distribution (Figure Ib) compared well with that of the H II regions. However, the spectral index is - 0.7 to - 1.0 between 408 and 1407 MHz [later verified up to 2695 MHz by Berkhuijsen & Wielebinski ( 1973)J , which rules out the possibility that the observed radiation is the integrated, thermal emission from the H II regions. Pooley concluded that an impossibly high supernova rate or implausibly long lifetimes for their remnants would be required to explain the emission as the collective effect of many remnants of Type II supernovae. Also there probably is no reasonable mechanism to confine the electrons produced in supernova events effectively enough to the spiral arms, although this is a very complex problem. Pooley mentioned a third possibility that the emission near the spiral arms is enhanced by an increase in the magnetic field strength when the gas is compressed by a density wave. This possibility remained a conjecture, since various predictions (such as the exact position of the compression regions with respect to the bright optical arms) could not be verified owing to the high inclination of M31 to the plane of the sky.

3.1 Density Wave Compression in M51

A significant step was made when Mathewson et al. (1972) observed the radio emission from M51 with the Westerbork telescope. Their map (see Figure 3) shows two ridges of radio emission that followed the two spiral arms quite accurately. The resolution of the telescope and the intrinsic sharpness of the ridges (they were essentially unresolved by the beam) enabled them to show that these ridges coincide accurately with the dust lanes that are visible on optical photographs at the inner edges of the arms. This is exactly what Roberts (1969) and Roberts & Yuan ( 1970) predicted from (magneto-) hydrodynamical investigations of gas motions in a disk galaxy with a density wave potential field.

In a galaxy with trailing spiral structure and a rotation of the pattern slower than the material, shocks will develop at the inner edge of the spiral potential well. At these positions, where star formation is supposed to be triggered and which are

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.


Figure 3 Radio-continuum map of M51 and NGC 5195 at 1415 MHz obtained with a 24" x 32" beam (Mathewson et al. 1972). Note the central sources and the strong ridges of spiral structure, which are displaced toward the inner edges of the optical arms and are coincident with the dust lanes. The source in the eastern arm is a supernova-remnant candidate (see Section 5). Two background sources are visible in the map. The contour unit is 0.8 K (\ mJy/beam).

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.


outlined in optical pictures by thin dust lanes, the gas compression is accompanied by a strong enhancement in magnetic field strength, because the frozen-in magnetic field is compressed along with the gas. For the synchrotron mechanism the volume emissivity is proportional to KB1-" where K is the number density of elections, B the magnetic field strength, and IY. the spectral index of the radio emission. Therefore, in a simplified approach the enhancement in brightness temperature of the radio continuum emission will be by a factor (pJ/pO)2-" where pJ/po is the relative-gasdensity contrast in the compression regions. Mathewson et al. have shown that with rx = - 0.7 this can reproduce the observed distribution, if it is assumed that the unperturbed disk of continuum emission (the so-called base disk) is four times as thick as the compression regions. The latter is reasonable because this same factor applies to the thickness of the radio disk of our Galaxy compared with the H I disk.

More recently Mouschovias et al. (1974) have shown that the enhancement in radio emission will be much less than expected from the simple one-dimensional compression discussed above. This is due to the fact that the magnetic field will buckle in the perpendicular direction owing to the onset of the Parker instability, so that the increase in magnetic field strength will be less than linearly proportional to the density contrast. Furthermore, the electrons will be redistributed by their own pressure when a Parker instability is formed and will tend to clump in the regions of lowest magnetic field strength. This mechanism may aid in explaining the formation of a radio disk, which is thicker than the gas disk, and it accounts in a more natural way for the fact that the emission contrast in M51 is lower than expected from simple one-dimensional compression.

The observations of M51 have provided what is now probably the most direct and straightforward observational verification of the density-wave theory for spiral structure. The most important feature is the exact coincidence of the ridges of radio emission with the dust lanes. An important piece of information derives from this, since the displacement of the radio ridge and the optical arms indicates the time between the onset and completion of the collapse in star formation. With reasonable values for the pattern speed, a value of five to ten million years can be found (Mathewson et al. 1972).

3.2 Observed Spiral Morphology in Other Galaxies

M33 has no observable spiral structure in the nonthermal emission (van der Kruit 1973a), and its emission is dominated in interferometer maps by the presumably thermal radiation from H II regions. M81 and M101 (see Figure 4) have a faint spiralarm component in the radio emission (van der Kruit 1973a, Israel et at. 1975), which is nowhere near as dramatic as in M51. Further evidence for spiral structure is clearly seen in the maps of NGC 4258 (van der Kruit et al. 1972 ; see Figure 7 and Section 6) and IC 342. Although less clear, spiral structure may also be present in the radio maps of NGC 2903, 4321 , and possibly 5055 (van der Kruit 1973a,b). Only in IC 342 is there definite evidence that the radio ridge is displaced toward the inner edges of the optical arms; in all other systems either the emission is too weak or the resolution too poor to determine this with much accuracy. Spiral structure is

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.


:

Figure 4 Radio-continuum map of M 101 at 610 MHz obtained with a I' x r2 beam (Israel et al. 1975). Identified H II regions have been indicated by their NGC number. Source # 20 is either a background source or a supernova remnant. Contour levels are at 3, 6, 9, 15, 20.5, and 27 K (I K � 1 mJyjbeam). The scale is 11.4" mm - ' .

apparently absent in the maps of Maffei II (Allen & Raimond 1 972) and NGC 2403 (van der Kruit 1973a).

It now seems that the intense radio spiral structure in M51 is exceptional. The same can be said of the optical spiral structure. Indeed, this has led to the obvious hypothesis that the hright radio arms in M51 have something to do with the presence of the nearby companion NGC 5195, and possibly with the origin of spiral structure through expulsion of companions from nuclei (Arp 1969) or through interaction between galaxies (Toomre & Toomre 1972).

In the cases of observed radio spiral structure the density-wave interpretation can be tested further, because this predicts that the radio spectrum of the ridges should be nonthermal. This is the case, as indicated above, for M3 1 (Pooley 1969a, Berkhuijsen & Wielebinski 1 974). For M51 there is yet no published study; observations at 5 GHz by Segalowitz and Shane at Westerbork, however, confirm the non thermal nature. Von Kap-herr et al. ( 1975) have remarked that the radio emission from the regions of the spiral arms in M81 has flat radio spectra. They compared their 4.8 GHz observations at Bonn with Westerbork data at 1.4 GHz; however, if the necessary (but uncertain) correction for the suppression of the zerolevel in the synthesis observations (due to the absence of short interferometer spacings) is made, the spectral index is more like -0.6.

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.


3.3 Compression Strength For the majority of observed galaxies of Table 3 no detailed information on the spiral morphology can be obtained. However, for about a dozen of them, it is possible to get some very crude estimates of the.amount of compression (van der Kruit 1973c). These estimates are based on the notion that at the position of the compression regions there is an enhancement of the general disk emission and that this enhancement is directly related to the density contrast and hence the shock strength. In determining this, one makes use of the fact that one can always choose an interferometer baseline longward of which the base disk has a very low fringe visibility, while the small-scale compression regions do not. In maps where the base disk is almost absent, the total flux density in the spiral arms can be found from planimetry ofthe contours (which have to be corrected for the emission from the nucleus and the shift in zero-level due to the absence of short spacings) and compared with the total flux density obtained from single-dish measurements. In this way a rough estimate for the flux density of the base disk can be made.

This procedure assumes that the thermal contribution of H II regions to the flux density is small, which is reasonable, since around 1415 MHz the overall spectra of the total flux density are strongly nonthermal (except possibly for M33, which has a rather flat spectrum; cf Israel & van der Kruit 1974). In this way a base disk was always found. Since the results are very crude, the twelve galaxies were simply divided into three groups with weak (e.g. M33, NGC 2403), intermediate (e.g. M31, M81, M 101), and strong (e.g. M51, N GC 2903) com pression, depending on the relative flux densities of spiral structure and base disk.

The density wave compression depends on the velocity with which the gas approaches the shock. If the pattern speed of the density wave equals the rotation speed of the outermost H II region, it is easily seen that such velocities on the average will be larger in galaxies in which the rotation curve reaches its maximum at a radius (Rmax) much smaller than that of the outermost H II region (Ropt) and that consequently the compression will be stronger.

The compression strength correlates well with the property Rmaxl Ropt and also with the luminosity classification of van den Bergh, which describes the narrowness of the optical arms. It is interesting that for this small sample the compression strength correlates well with the velocity perpendicular to the shock as determined from dynamics and theoretical spiral patterns fitted to these systems (Roberts et al. 1975), while in these calculations the shock strength is also found to correlate with the luminosity class. Also the "star formation strength" determined by Schweizer (1974) in four systems seems to correlate well with the compression strength. We may conclude that observations of the radio continuum morphology provide support for at least the fundamental ideas behind the density-wave theory for spiral structure (see also van der Kruit 1973d,e).

A further property of the radio ridges to be compared with the compression strength is the thickness of the compression regions. In M3 1 the apparent width is 3' (2.6 kpc) on the minor axis (van der Kruit 1972), but this is also partly due to the z extent. Berkhuijsen & Wielebinski ( 1974) have measured 1-2 kpc at the line of nodes

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.


where this effect should have little influence. This shows that the minor axis value is almost entirely due to the z extent, which then is about 0.6 kpc. In IC 342, which is more face-on, a value of 1 .5 2 kpc is found for thc width (van der Kruit 1 973b) and the weak emission in M81 suggests a similar value (van der Kruit 1973a). In M5 1 the radio ridge still appears to be unresolved (less than 0.4 kpc); the dust lanes have a width of 0.5 kpc. These values suggest that, as expected, the thickness is smallest in galaxies with strong compression.

3.4 Remarks on Our Galaxy

Price ( 1974) has found from surveys of nonthermal radio emission along the Galactic plane that also in our Galaxy there is a base disk and a spiral structure component, each containing about half of the total power. This is not inconsistent with the value of Rmax/Ropt (van der Kruit 1973b) or with the velocity of the gas perpendicular to the arms (Roberts et al. 1975). The compression strength is probably intermediate.

4 H II REGIONS

In this section, we summarize some of the recent results obtained from radio continuum observations of H II regions in galaxies. A review of this subject has been published recently by Israel ( 1975); we therefore limit the present discussion to a few historical notes, highlights from the recent results, and some additional remarks. A review of optical observations of extragalactic H II regions including many references has been presented by Hodge (1974).

Although radio emission from H II regions is prominent on centimeter-wavelength surveys of the Galactic plane, these objects are small and faint when compared to the general extended non thermal emission found in most galaxies. The long operating wavelengths and large beam widths of the early surveys of galaxies in the 1950s prevented the detection of individual H II regions in emission. Mills (1955) suggested that a bright feature on his 86-MHz map of the Large Magellanic Cloud (LMC) was 30 Doradus. Radio spectral information was lacking at that time, and we now know that part of the radio emission of 30 Dor is nonthermal (Le Marne 1968). Nevertheless, the presence of a significant amount of thermal radio emission was confirmed by Shain (1959), who found a slight depression in the 19.7-MHz isophotes of the LMC at the position of 30 Dor. Shain proposed that the extended nonthermal radio background from the LMC and from distant unresolved radio sources was being absorbed by the free-free mechanism in ionized gas within 30 Dor, thereby recording the first (and still the only) occasion of radio absorption in an extragalactic H II region. Further studies of H II regions in the MageJlanic Clouds were made later with the CSIRO 21O-foot reflector, and identifications of many H II regions based on surveys at 2695 and 1410 MHz were proposed by Mathewson & Healey ( 1964). Table 4 [adapted from Israel ( 1975)] lists references to the most recent published results obtained from observations of the Magellanic Clouds with beam-forming radio telescopes.

The high angular resolution (of the order 10") and sensitivity (of the order 1 mly) necessary to measure radio emission from H II regions in more distant galaxies is

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.

-Table 4 References to some recent, published, radio-continuum observations of extragalactic H II regions (abridged from Israel 1 975)

Approximate Number of Faintest Observing angular H II region Some notable H II detection frequency resolutions

Galaxy identifications regions studied (mJy) (GHz) (arcmin) R eferences

24 180 5, 2.7, 0.4 4, 7.7, 3.2 McGee & Newton (1972)

LMC 2.7, 1.4, 0.4 7.5, 14, 48 Mathewson & Healey (1964) 30Dor ::c >

0.4 3.2 Le Marne (1968) 0 5 (')

11 270 0.4 3.2 Mills & Aller ( 1 971) 0 Z ....,

SMC 6 1 70 0.4 3.2 Mills & Aller (1971) Z C C io::

67 NGC 595, 604 1.5 1 .4 0.5 Israel & van def Kruit (1 974) io:: 0 ::c M33 NGC 595 8, 2.7 0.05,0. 1 5 Spencer ( 1973)

"" ::r: 0 r 1 .4 2 . 1 Wright (1971)

0 Cl NGC 604 ><:

0 8, 2.7 0.05, 0.1 5 Spencer (1973) "r1

en ""

M51 VS53/55 8, 2.7 0.05,0.1 5 Spencer (1973) :;:; > r Cl

II Zw40 1 .4 0.4 x 6 Jaffe (1972) > r > ;>< NGC 5447, 5455, 5461 , t;;

en

M 101 8 5462,5471 2.7 5, 1.4, 0.6 0. 13, 0.45, 1 . 1 Israel, Goss & Allen ( 1975)

-I'>-NGC 5455 10.7,5, 1 .3, 0.13, Allen, Goss, Ekers & de Bruyn (1 976) v.> v.>

1 .4, 0.6 0.45, 1 . 1

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.


becoming available only recently with the advent of powerful aperture synthesis instruments. Table 4 also lists references to published results on M33 and M 101; observations of several other nearby spiral galaxies are presently being analysed. Rudimentary details of the radio continuum structure of extragalactic H II regions are now becoming available; the state of the art is well illustrated by Figure 5 (Allen et al. 1976), which shows a recent 5-GHz map of NGC 5455. We note that this map

Figure 5 Contour map of the 5-GHz Wrs terbork observations of NGC 5455 in MlOl (Allen et al. 1976). drawn over a photograph of the region taken by A. Sandage with the 200-inch (S-meter) telescope. The cross marks the position of SN 1970g (see Section 5). Contour interval is 0.75 mJy/beam; rms noise is 0.5 mJy/beam. Negative contours are dashed; the zero contour is omitted.

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.


resembles the radio maps of Galactic H II regions obtained with the radio telescopes that were in operation 20 years ago.

A summary of some notable results from the studies of M33 and M 101 (Israel & van der Kruit 1974, Israel et al. 1975) follows.

1. The giant complexes in M 101 (cf Table 4) contain several distinct optical condensations. At a distance of 7 Mpc, the complexes as a whole have linear sizes of the order of I kpc and emit about 50 x 1017 W Hz-I sr-1 at centimeter wavelengths; these values considerably exceed those corresponding to the largest H II regions in the Galaxy (e.g. about 0. 1 kpc and 3 x 1017 W Hz-1 8r-1 for W51), in M33 (e.g. about 0.3 kpc and 3 x 1017 W Hz-1 sr-1 for NGC 604), and in the LMC (about 0.3 kpc and 7 x 1017 W Hz-1 sr-1 for 30 Dor).

2. Several of the giant complexes show a "core-envelope" structure. For the objects in M 101, the cores have rms electron densities of about 10-30 cm -3 and total masses of ionized gas of about 106 Mo. The envelopes have rms electron densities of about 1 cm-3 and total ionized gas masses of the order of 107_108 Mo. About 500 stars of spectral type 05 are needed to account for the total observed ionization.

3. The presence of large amounts of dust internal to the H II regions in M33 and the giant complexes in M101 is indicated from a comparison of the optical and radio results. Such a result was found initially for the H II regions in the Magellanic Clouds by Mills & Aller (1971). In M 101, for instance, the nature and distribution of the dust within the giant complexes must be such that about 2 mag of optical extinction are produced with very little reddening. Allen ( 1975b) has pointed out that, for a homogeneous distribution of gas within the complex, the volume of the combined Stromgren spheres of the exciting stars is approximately equal to that of the complex as a whole. Considerable density inhomogeneities both in the gas and the dust, therefore, must exist if the dust is to be shielded from the ultraviolet radiation.

4. The radio-luminosity distribution function for the H II regions in MI01 (the number-flux density relation) differs substantially in shape from that of M33. The largest H II regions in galaxies are commonly used as distance indicators (e.g. Sandage & Tammann 1974), and the dissimilarity of the H II region populations in M33 and M 101 raises several related questions, which urgently await answers. For instance, does the dissimilarity disappear when one confines the comparison to galaxies of the same luminosity class? Phrased another way, do all Sc I galaxies have a physically homogeneous population of several giant H II complexes? Preliminary negative answers are provided by Israel (1975): Within the Sc I galaxies, the brightest H II complex in M51 is intrinsically weaker by a factor of four at radio wavelengths when compared with the brightest complex in M 101. For the Sc III class, the brightest complex in NGC 2403 is about eight times stronger than NGC 604 in M33. Results for a larger sample of galaxies are clearly needed.

5 SUPERNOVA REMNANTS

As is the case in the study of extragalactic H II regions, the proximity of the Magellanic Clouds to the Galaxy accounts for the successes in detecting radio emission from supernova remnants (SNR) there. The first identifications of suspected

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.


radio SNR in an extragalactic system were made by Mathewson & Healey ( 1964; see also Mathewson et al. 1963), who found three candidate sources in the LMC. These objects were subsequently confirmed as SNR by Westerlund & Mathewson ( 1966) from the characteristics of their optical spectra; as is the case for Galactic SNR, the [S II] lines are about equal in strength to the HI)( line. This optical method of confirming the SNR identification for the emission of radio sources found in surveys of the Magellanic Clouds was pursued by Mathewson & Clarke (1972; 1973a,b,c) until a total of 12 identifications in the LMC and 2 in the SMC were made.

De Bruyn (1973) mentions several relatively bright radio sources observed in the disks of nearby galaxies in the course of survey work carried out with the Westerbork telescope. M51 (Figure 3), NGC 4258 (Figure 7), and NGC 6946 each contain at least one bright, nonthermal source situated close to the Population I trace�s of spiral structure. If they are actually located in these galaxies, the objects produce more than ten times as much radio emission as presently emitted by Cas A. Although thcre is a chance that some of these sources are background radio galaxies, such as source # 20 in M101 (see Figure 4; cf Israel et al. 1975), the association with Population I material makes it plausible that a few are actually SNR. Their ages, unfortunately, are unknown. Spencer & Burke ( 1972) have confirmed the non thermal spectrum and small size of one such object originally observed in the survey of M51 by Mathewson et al. (1972); the object lies in a spiral arm (see Figure 3) near an H II region and has a spectral index of about -1.5. No definite evidence for or against the supernova-remnant hypothesis could be given.

An extrapolation to earlier epochs of the known temporal dependence of the radio emission from Cas A predicts large intensities, which should be easily detectable in external galaxies. Large intensities for young objects can also be inferred from the apparent systematics of the surface brightness-linear diameter diagram. De Bruyn ( 1973) has made a search for radio emission from 35 optically recorded supernovae in 22 extragalactic systems using the Westerbork telescope at 14 1 5 MHz. The supernovae ranged in age from 1 month to 86 years ; none were detected. The upper limits imply levels of radio emission less than from two to seven times the present emission of Cas A at 1415 MHz, and are below the values expected from simple extrapolations of Cas A. An even more stringent upper limit is available for the supernova S And in M31 (see also Spencer & Burke 1973), which presently emits less than 6% of the emission of Cas A at 1415 MHz.

Several mechanisms have been proposed to explain the absence of radio emission from young SNR: (a) suppression of the radio emission owing to absorption, (b) a deficiency of low-energy electrons, (e) insufficient magnetic field strength, and (d) combinations of these effects (de Bruyn 1973, Gull 1973). Sincc the efficiency of these mechanisms was generally accepted, it came as somewhat of a surprise when Gottesman et al. ( 1972) reported variable radio emission at 2695 MHz from the Type II SN 1970g in M lO1. The phenomenon was confirmed by Goss et al. (1973), and information is presently available at wavelengths from 49 to 2.8 cm over a time interval of more than four years (Allen et al. 1976). Figure 6 shows the observations of Allen et al. along with those of Gottesman et al. at 2695 MHz (as corrected by Allen et al.). The radio emission was first observed about half a year to one year after

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.


1 8 O A 49 c m • 2 1

1 6 o 1 1 S N 19709 a n d NGC 5455 >- 1 4

0 6

rtH � 6 2.8 .§. 1 2

I +

f f � :,,, ?: I Vi 1 0 c: I '" 0 8 I +1I y x 1:£ t �

6 " ::J 6 r-u: 28

4 f- I S N

2 r-

0 1970 1971 1972 1973 1914 1975

Figure 6 Collection of all available measurements of the combined radio emission of SN 1970g and NGC 5455 from Allen et al. (1976). The small insert on the right side gives the estimates at the different wavelengths for the nanvariable flux density of the giant H II complex NGC 5455.

maximum light, reaching 6 mly at 1415 MHz at an age of 1.4 years. It was observed to be still at about that level at an age of 3.4 years ; however, by 4.2 years the emission had decreased below 2 mly. The maximum observed radio luminosity at 1415 MH z is 2.8 x 1 0 1 8 W HZ- I Sr - I (using a distance of 7 Mpc) ; this value is about ten times the present radio luminosity of Cas A and is not substantially higher than the upper limits found by de Bruyn (1973) for other extragalactic SNRs. Since the object is expanding slower than the speed of light, the brightness temperature is at least 107 OK at an age of 1 .4 years. The emitted radio spectrum almost certainly is nonthermal.

The incompatibility of these results with curr�nt models for the early development of radio emission from SNR is discussed further in Allen et al. (see also Marscher 1(74). The origin of the relativistic particles and magnetic fields in quantities and strengths sufficient to produce the observed radio emission by the synchrotron mechanism is still obscure. Getting the radio emission out of the SNR is also difficult, given the efficiency with which various absorption mechanisms are thought to operate. In Allen et aI., the presence of extensive optical data on SN 1970g provided the valuable suggestion that the absorbing gas may have condensed into filaments just before the radio emission was observed.

6 CENTRAL RADIO SOURCES

With the currently available detection limit of 5 mly at 1400 MHz, one is able to observe central complexes3 similar to the one in our Galaxy out to distances of only

3 We purposely use the term central complexes, because we can observe only over linear size scales of a few hundred parsecs. In the Galactic center this means inclusion of the extended component of Sgr A and condensations such as Sgr B2. It might be considered for the future to reserve the term nuclei for radio sources of a size of 1 pc or less.

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.


about 10 Mpc. The presently available samples, therefore, are biased strongly toward the brighter sources. This is extensively discussed in the review by Ekers (1974).

The study of M31 by Pooley & Kenderdine (1967) indicated that the nuclear complex is an order of magnitude weaker than in our Galaxy, which increases the sensitivity requirements even further for an extensive study of a large sample. The nuclei of normal galaxies contribute generally less than 20% to the total flux density of the galaxy (van der Kruit 1973c), and possible relations between the radio luminosity of spirals and the Byurakan type, such as discussed by Tovmassian & Terzian (1973), therefore, do not constitute very strong evidence for the presence of bright central radio sources. In fact, this correlation is not present for the sample studied by van der Kruit ( 1973c), except for the case of the Seyfert galaxies.

Among the nuclei of the bright, nearby galaxies there is a large diversity in radio properties. M31 has been resolved into a few components (van der Kruit 1972), as illustrated in Figure Ie. The linear scale is similar to the center of our Galaxy, but the radio output is an order of magnitude lower. Furthermore, the distribution is more spherical than the inclination of M31 would indicate, so that the sources might nut all be in the plane. The spectrum is non thermal. No point source is present in the center of M33 above 3 mly at 1415 MHz, although a fcw H II regions have been detected nearby (van der Kruit 1973a, Israel & van der Kruit 1974). M81 does have a small-diameter radio source at the nucleus, about twice as bright as in our Galaxy (Wade 1968, Lequeux 1971a, van der Kruit I 973a), hut whereas the spectral index for most spiral nuclei is - 0.7, that of M81 has a value of +0.3 (Ekers 1974, von Kapherr et al. 1975). M51 has a nuclear source that is an order of magnitude brighter than the Galactic center and that at high resolution breaks up into two or more components (Spencer & Burke 1972). M101 has nuclear emission slightly weaker than our Galactic center, but it is at least partly thermal (Israel et al. 1975). The center of our Galaxy has an extended non thermal source, in which thermal complexes are embedded (Gordon 1974), but for the present discussion it is the extended source and its nonthermal spectrum that are to be compared with the central radio sources in other galaxies.

The properties of nuclei have been studied with increasingly better resolution and/ or larger samples by Wade (1968), Lequeux (1971a,b), van der Kruit (1972 ; 1973a,b,c), and Ekers (1974, 1 975). Several conclusions follow from these samples : The nuclei of Seyfert galaxies emit two to three orders of magnitudes more energy at radio wavelengths than does the average normal galaxy nucleus. There is no good correlation between the luminosity ofthe nucleus and the Hubble type or the presence or absence of a bar. On the other hand, the radio emission appears correlated with the 10-/1 infrared emission (van der Kruit 1971, 1973c ; Rieke & Low 1972), although, as is remarked in the first two papers, for the normal galaxies this may be due to the incompleteness of the sample. For the Seyfert galaxies the sample is complete but small. An extension of the sample hopefully will be available in the near future. The tightness (for the Seyfert sample) of the correlation is surprising. The physics is not understood and may be rather complicated if the 10-/1 emission is (thermal) reradiation of ultraviolet or X-ray emission by dust (see also de Bruyn & Willis 1974). In NGC 253 the extent of the radio and 10-/1 source is about the same (Becklin et al.

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.


1973), but Ricke & Low (1975) argue that at least the infrared source is short-lived, and is possibly related to a phase of strongly enhanced star formation during a period of violent activity in the nucleus.

The radio spectra have a broad distribution in spectral index, centered at about - 0.7 (Lequeux 1971a, Ekers 1 974), but it is narrower for the Seyfert nuclei, for which the mean is - 0.8 (de Bruyn & Willis 1974). For the normal systems this breadth might arise from the diversity in size and structure and the possibility of contributions of thermal emission. In the Seyfert nuclei the nonthermal emission dominates.

In the case of interacting galaxies, it appears that the total radio luminosity as well as that fraction of it that is associated with the nucleus are normal compared to spiral galaxies in general (Allen & Hartsuiker 1972, Purton & Wright 1972, Allen et al. 1973, Allen & Sullivan 1973, Burke & Miley 1973). This result has been interpreted as supporting a gravitational rather than an explosive origin of the bridges and tails.

There are, nevertheless, a few cases in which the radio morphology in the disk suggests that it is affected by activity in the nucleus. An outstanding example is NGC 4258 (van der Kruit et al. 1972 ; see Figure 7), in which ridges of non thermal radio emission accompanied by smooth HIX arms are interpreted as having resulted from gas expulsion by the nucleus. Some support for this model is given by measurements ofthe velocity field, both optically (van der Kruit 1974b) and with the use of the 21-cm line (van Albada & Shane 1975). The model and the related suggestionthat we might witness here the birth of spiral structure-have been discussed and reviewed at various occasions by Oort ( 1974, 1975). Other possible cases are NGC 4736 (van der Kruit 1 971 , 1 974a) and NGC 463 1 (van der Kruit 1973b), in which there appears to be a triplet of radio sources centered on the nucleus. For NGC 4736, extensive optical, radio continuum, and 21-cm line observations are being analysed. In the case of NGC 4631 there still is the possibility that in this edge-on system we are seeing the integrated emission from long path lengths tangential to the spiral arms (Pooley 1 969b, van der Kruit 1973a).

7 Z EXTENT OF THE RADIO EMISSION AND HALOS

In this section we discuss the distribution in radio emission perpendicular to the planes of galaxies. It should be noted at the outset that when the z distribution is broad compared with, for example, that of the neutral hydrogen or the optical light, we refer to the existence of a "halo," although in some cases a more descriptive term would be "thick disk." A clear morphological separation is not yet observable in any galaxy. For the sake of a separation in the discussion, we will distinguish "halos" [rom "thick disks" if the axial ratio of the faintest reliable radio contours exceeds 0.5 ; no physical distinction is intended by this use of nomenclature since the sensitivity of the observations may affect the axial ratios.

7.1 Halos

Baldwin (1954) showed that his interferometer measurements were consistent with the presence of two components in the radio emission (at 8 1 MHz) ofM3! : (a) a disk component, which was "distributed like the stars," and (b) a spherical component of

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.


Figure 7 Map of the radio emission from NGC 4258 at 1415 MHz with a 24" x 32" beam (van dt:r Kruil el al. 1972). The two ridges o[ radio emission that run oUl radially [rom the nucleus are identified with "arms" of smooth He< emission. The normal spiral arms are also visible. The contour unit is 1 mJy/beam or 0.9 K. The scale is 5.0" mm - ' .

radius at least 10 kpc, which contained two-thirds of the total flux density. The actual presence ofa halo in M 3 1 has been discussed by most observers listed in Table 1 with the usual conclusion that such a component probably exists. The main difficulty concerns how much of the emission that is observed in the surroundings of M31 is due to a combination of the Galactic background plus confusion with

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.


extragalactic sources. Pooley (1969a) found that the total flux density in the spiral arms, nucleus, and background sources is significantly less at 408 MHz than the total flux density of Largc et al. (1959) (35 Jy versus 140 Jy), and he interpreted this result as supporting the halo hypothesis. A similar conclusion was reached by Durdin & Terzian (1972) at 73.8 MHz under the assumption that the 5C3 sources all have a spectral index of - 0.7. The halo then measures 2° x 3° (25 x 40 kpc) total extent and probably has a steeper spectrum than the disk emission. Consistent with the steep spectrum of the halo, Dennison et al. (1975) have found no halo emission at 1.4 and 2.7 GHz. Wielebinski ( 1976) has argued that most of the emission at 408 MHz is due to extragalactic background sources, so that a radio halo, if present at all, is much weaker than previously thought. Upon evaluating all of the results, we feel that a faint radio halo probably does exist around M31, but the question of its brightness is not yet satisfactorily resolved.

The situation in our Galaxy is hardly clearer. Baldwin ( 1955) pointed out the possibility of a radio halo, but emphasized again later that the evidence is not conclusive (Baldwin 1967), owing to uncertainties in the low-frequency brightness of the extragalactic component. The presence of a halo was seriously questioned by Burke (1967) from a 234 MHz survey. Price ( 1974) suggested that the Sun might be near a local nonthermal spiral feature, so that a higher local volume emissivity could also explain the excess brightness temperatures observed at high galactic latitudes. Webster (1975) showed that there is evidence for a halo in the background surveys if the assumption is made that its spectrum is steeper than that of the disk. The total radio luminosity of the halo is then about the same as that of the disk, and the volume emissivity is about 30 times less than that of the disk.

The most favorable extragalactic systems in which to search for extended disk and halo emission are, or course, the edge-on spirals. This was first carried out by Pooley ( 1969b) in NGC 4631 , who established an upper limit to the brightness temperature of a halo of lOOK at 408 MHz. More recently a radio halo has indeed been detected around this galaxy with the Westerbork telescope by Ekers and Sancisi (see Ekers 1 975). The emission extends to 12 kpc (distance = 4 M pc) from the plane and the halo has an axial ratio of about 0.6. The brightness temperatures are 2°K at 610 MHz and �0.2°K at 1415 MHz at the height mentioned, consistent with Pooley's upper limit. The halo constitutes only a few percent of the total flux density from NGC 4631 (at 610 MHz), but NGC 4631 has a strong disk and nuclear source.

7.2 Thick Disks

With a beam width of about 1:6, Baldwin & Pooley ( 1973) found that the disk of NGC 891 has a full thickness between half intensity in the z direction of about l' at 408 MHz. Here we can be sure of dealing with a highly flattened stellar system seen nearly (within 1 �5) edge-on ; thus a reasonably straightforward interpretation of the result is possible. First, at 5 GHz the disk seems to have two components (Allen, Baldwin and Sancisi ; see Ekers 1975, Sancisi et a1. 1975 ; Figure 8). One has a thickness of less than 0.5 kpc (distance = 13 Mpc), similar to the H I and which might be thermal, while a second non thermal disk has a thickness of about 4 kpc ( = 1 ', in agreement with the 408 MHz result of Baldwin and Pooley). The radio spectra

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.


0..5

0.4 c E

0.3 t: 0

"->-

0..2

0..1

0. . . .

NGC 891 f... 6.o. cm

H PBW 8" =5o.Dpc

. 0

. . .

l' = 3.8 kpc d istance a l ong mi nor axi s

str ip integrat ions paral lel to major axis

o 0

Figure 8 The z distribution of the 5 GHz radio continuum emission in NGC 891 from Allen, Baldwin, and Sancisi (unpublished). The original map has been smoothed in 4'-long strips parallel to the major axis in order to improve the signal-to-noise ratio. The angular resolution parallel to the minor axis is 8". The assumed distance is 13 Mpc (H = 50. km sec- 1 Mpc- 1).

generally seem to be steeper than those closer to the plane. Emission can be traced out to about 7 kpc from the plane, which is indicative of a halo. The non thermal disk in our Galaxy has a thickness of 750. pc (Baldwin 1967), while it is less than 70.0. pc in NGC 463 1 (Pooley 1969b). However, due to distance uncertainties and differences in resolution a comparison is hazardous. Cameron ( 1971c) observed the somewhat edge-on systems NGC 55, 253, and 4945 at 40.8 MHz. No halo or thick disk was detected among the three ; in the last two he quotes an upper limit of 10% of the total flux density for that of a halo. No straightforward interpretation of the measurements on NGC 4656 could be.made by Baldwin & Pooley ( 1973).

Faint extensions appear on. the map of NGC 3556 at 1415 MHz (van der Kruit 1973b). The galaxy is not exactly edge-on, but �he thickness of the plane is less than about 1 kpc. The same value is obtained for NGC 3432 (van der Kruit 1973b). These observations contain no evidence for an extended radio halo in these galaxies, although "thick disks" are possible.

8 CONCLUDING REMARKS

The study of the radio morphology of spiral galaxies at resolutions significantly less than the diameters has started only recently, and, although some very important

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.


results have been obtained so far (e.g. the existence of density-wave-compression regions in M51 and a few other systems), much remains to be done.

The most obvious need is for extensive surveys of spiral galaxies at various frequencies to provide global properties such as disk and nucleus luminosity and their spectra for a large sample. Statistical studies of these properties and their relation to other integral properties and to each other should then be carried out. Such surveys may provide other examples of explosive activity as in NGC 4258. Outstanding questions are also whether the radio disks have an exponential dependence on the distance from the center and whether the scale lengths correlate with those of the optical-light distribution. Further work on the question of the compression strength and extensive measurements of that property are highly desirable.

We note further that observations of extragalactic H II regions and their luminosity function related to the morphological type are essential because of the importance of these objects for the determination of the distance scale in the universe and the Hubble constant. Also we urge that coordinated radio and optical observations of newly discovered supernovae be made in order to study further examples like SN 1970g in M lOl, although the sensitivity requirements for the radio observations are severe. An extensive search for radio halos is desirable ; obviously the relation to the disk brightness is a major topic to be studied.

The question of the origin of cosmic rays in galaxy disks has not yet been resolved satisfactorily. The lines of research outlined above and the subsequent analyses and interpretations are expected to shed light on this important question.

Literature Cited

Albada, G. D. van, Shane, W. W. 1975. Astron. Astrophys. 42 : 433

Allen, R. 1. 1975a. La Dynamique des Galaxies Spirales, ed. L. Weliachew, p. 1 57. Paris : CNRS

Allen, R. J. 1975b. See Allen 1 975a, p. 205 Allen, R. J., Goss, W. M., Ekers, R. D.,

Bruyn, A. G. de 1 976. A stroll. Astrophys. 48 : 253

Allen, R. J., Ekers, R. D., Burke, B. F., Miley, G. K. 1973. Nature 241 : 260

Allen, R. 1., Hartsuiker, 1. W. 1 972. Nature 239 : 324

Allen, R. 1., Raimond, E. 1972. Astron. A strophys. 19 : 3 1 7

Allen, R. J., Sullivan, W . T . 1973. Astron. Astrophys. 25 : 187

Arp, H. C. 1 969. Astroll. Astrophys. 3 : 41 8 Baldwin, J . E . 1 954. Nature 1 74 : 320 Baldwin, J. E. 1 955. MNRAS 1 1 5 : 690 Baldwin, J. E. 1967. I A U Symp. 3 1 , Radio

Astronomy and the Galactic System, ed. H. van Woerden, p. 337. London : Academic

Baldwin, J. E., Costain, C. H. 1960. MNRAS 1 2 1 : 413

Baldwin, J. E . , Pooley, G. G. 1973. M N RAS 161 : 127

Becklin, E. E., Fomalont, E. 8. , Neugebauer, G. 1973. Ap. J. 181 : L27

Berkhuijsen, E. M., Wielebinski, R. 1 973. Astrophys. Lett. 13 : 169

Berkhuijsen, E. M., Wielebinski, R. 1 974. Astron. Astrophys. 34 : 1 73

Brosche, P., Einasto, J., Rummel, U. 1974. Veroff. Astroll. Rech. I nst. Heidelberg. No. 26

Bruyn, A. G. de. 1973. Astron. Astrophys. 26 : 105

Bruyn, A. G. de, Willis, A. G. 1974. Astron. Astrophys. 33 : 351

Burke, B. F. 1967. See Baldwin 1967, p. 361 Burke, B. F., Miley, G. K. 1973. Astrol!.

Astrophys. 28 : 379 Cameron, M. J. 197 1a. MNRAS 1 52 : 403 Cameron, M. J. 1971b. MNRAS 152 : 429 Cameron, M. J. 1971c. MNRAS 152 : 439 Cooley, R . c., Roberts, M. S., Swenson,

G. W. 1967. Science 1 56 : 1087 de Jong, M. L. 1965. Ap. J. 142 : 1333 de Jong, M. L 1 967. Ap. J. 1 50 : 1 Dennison, B., Balonek, T. J., Terzian, Y.,

Balick, B. 1975. Pub!. Astron. Soc. Pac. 87 : 83

Durdin, J. M., Terzian, Y. 1972. Astron. J.

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.


77 : 637 Ekers, R. D. 1974. IAU Symp. 58, The

Formation and Dynamics of Galaxies, ed. J. R. Shakeshaft, p. 257. Dordrecht : Reidel

Ekers, R. D. 1975. Structure and Evolution afGalaxies, ed. G. Setti, p. 2 1 7. Dardrecht : Reidel

Fanti, R., Gioia, I., Lari, c., Lequeux, J., Lucas, R. 1973. Astron. Astrophys. 24 : 69

Gordon, M. A. 1974. IA U Symp. 60, Galactic Radio AstrOllOmy, ed. F. J. Kerr, S. C. Simonson, p. 477. Dordrecht : Reidel

Goss, W. M., Allen, R. J., Ekers, R. D., Bruyn, A. G. de. 1973. Nature Phys. Sci. 243 : 42

Gottesman, S. T., Broderick, 1. J., Brown, R. L., Balick, B., Palmer, P. 1972. Ap, J, 1 74 ' 383

Gull, S. F. 1973. MNRAS 161 : 47 Hanbury Brown, R., Hazard, C. 1951 .

MNRAS I I I : 357 Hanbury Brown, R., Hazard, C. 1959,

MNRAS 1 1 9 : 298 Haynes, R. F., Huchtmeier, W. K. H.,

Siegman, B. C., Wright, A. E. 1975. A Compendium of Radio Measurements of Bright Galaxies. CSIRO : Div. Radiophys.

Hodge, P. 1974. Publ. Astron. Soc. Pac. 86 : 845

Huchtmeier, W. 1972. Astron. Astrophys. Suppl. 7 : 397

Ilovaisky, S. A., Lequeux, J. 1972. Astron. Astrophys. 20 : 347

Israel, F. P. 1975. Mittelberg Conference on H II-Regions and Related Topics. Bonn. Germany. ed, T. L. Wilson, D. Downes, p. 288. Heidelberg : Springer

Israel, F. P., Gass, W. M., Allen, R. J. 1975. Astron. Astraphys. 4{): 421

Israel, F. P., Kruit, P. C. van der. 1974. Astron, Astrophys. 32 : 363

Jaffe, W. 1972. Astron. Astrophys. 20 : 461 Jansky, K. G. 1933. Nature 1 32 : 66 Jansky, K. G. 1935. Proc. IRE 23 : 1 1 58 Kap-Herr, A. von, Jones, B. B., Wielebinski,

R. 1975. Astron. Astrophys. 41 : 1 1 5 Kazes, I., Le Squeren, A . M., Nguyen

Quang-Rieu. 1970. Astrophys. Lett. 6 : 193 Kenderdinc, S., Baldwin, J. E. 1965. Observa

tory 85 : 24 Kraus, J. D. 1%4. Nature 202 : 1202 Kruil, P. C. van der. 1971 . Astron. Astrophys.

1 5 : 1 10 Kruit, P. C. van der. 1972. Astrophys. Lett.

1 1 : 1 73 Kruit, P. C. van der. 1973a. Astron.

Astrophys. 29 : 231 Kruit, P. C. van der. 1973b. Astron.

Astrophys. 29 : 249 Kruit, P. C. van der. 1973c. Astron.

, 1strophys. 29 : 263 Kruit, P. C. van der. 1973d. Nature Phys.

Sci. 243 : 127 Kruit, P. C. van der. 1973e. Ap. J. 1 86 : 807 Kruit, P. C. van der. 1974a. Ap. J. 1 88 : 3 Kruit, P. C. van der. 1974b. Ap. 1. 192 : 1 Kruit, P. C. van der, Oort, 1. H., Mathewson,

D. S. 1972. Astron. Astrophys. 21 : 169 Landecker, T. L., Wielebinski, R. 1970.

Aust. J. Phys. Suppl. 1 6 Large, M . I., Mathewson, D . S., Haslam,

C. G. T. 1959. Nature 183 : 1250 Leibacker, J. 1964. Astron. J. 69 : 374 Le Marne, A. 1968. MNRAS 1 39 : 461 Lequeux, J . 1971 a. Astron. Astrophys. 15 : 30 Lequeux, 1. 1 97 1b. Astron. Astrophys. 1 5 : 42 Le Squeren, A. M., Crovisier, 1. 1 974.

Astran. Astraphys. 3 1 : 447 MacLeod, J. M. 1964. Science 145 : 389 Marscher, A. P. 1974. The evolution of radio

emission from young supernova remnants. PhD thesis. Univ. Virginia, Charlottesville

M athewson, D. S" Clarke, J. N. 1972. Ap. J. 1 78 : LI05

Mathewson, D. S., Clarke, J. N. 1973a. Ap. J. 179 : 89

Mathewson, D. S., Clarke, J. N. 1973b. Ap. J. 1 80 : 725

Mathewson, D. S., Clarke, J. N. 1973c. Ap. J. 1 82 : 697

Mathewson, D. S., Healey, J. R. 1 964. IAU/ U RSI Symp. 20, The Galaxy and the Magellanic Clouds. ed. F. J. Kerr, A. W. Rodgers, p. 283. Canberra : Aust. Acad. Sci.

Mathewson, D. S., Healey, J. R., Westerlund, B. E. 1963. Nature 199 : 681

M athewson, D, S . , Kruit, P. C. van der, Brouw, W. N. 1972. Astron. Astrophys. 17 : 468

McCutcheon, W. M. 1973. Astron. J. 78 : 1 8 McGee, R . X., Newton, L . M. 1972. Aust. J,

Phys. 25 : 619 Mills, B. Y. 1955. Aust. J. Phys. 8: 368 Mills, B. Y., Aller, L. H. 1971. Aust, J. Phys.

24 : 609 Mouschovias, T. C., Shu, F. H., Woodward,

P. R. 1974. Astran. Astrophys. 33 : 73 Oort, J. H. 1974. See Ekers 1974, p. 375 Oort, J. H, 1975. See Allen 1 975a, p. 5 1 7 Pooley, G. G. 1%9a. MNRAS 144 : 101 Pooley, G. G. 1969b. MNRAS 144 : 143 Pooley, G. G., Kenderdine, S. 1%7. Nature

214 : 1 1 90 Price, R. M. 1974. Astran. Astrophys. 33 : 33 Purton, C. R., Wright, A. E. 1972. MNRAS

1 59 : 1 5P Reber, G. 194{). Ap. J. 91 : 62 1 Reber, G. 1 944. Ap. J. 1 00 : 279 Rieke, G. H., Low, F. J. 1972. Ap. J. 1 76 : L95 Rieke, G. H., Low, F. 1. 1975. Ap, J. 197 : 1 7 Roberts, W . W . 1969. Ap. J . 158 : 123

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.


Roberts, W. w., Roberts. M . S., Shu, F. H. 1975. Ap. J. 1 96 : 381

Roberts, W. W., Yuan, C. 1970. Ap. J. 1 6 1 : 877

Ryle, M., Smith, F. G., Elsmore, B. 1950. MNRAS 1 10 : 508

Sancisi, R., Allen, R. 1. , Albada, T. S. van. 1 975. See Allen 1 975a, p. 295

Sandage, A. R., Tammann, G. A. 1974. Ap. J. 1 90 : 525

Schweizer, F. 1 974. Structure and populations of spiral galaxies from surface photometry. PhD thesis. Univ. Calif., Berkeley

Seeger, C. L., Westerhout, G., Conway, R. G. 1 957. Ap. J. 1 26 : 585

Shain, C. A. 1959. I A U Symp. 9, Paris Symposium 011 Radio Astronomy, ed. R. N. Bracewell, p. 328 . Stanford : Stanford Univ. Press

Slee, O. B. 1972a. Proc. Astrol1. Soc. Aust. 2 : 159

Slee, O. B. 1972b. Astrophys. Lett. 12: 75

Spencer, 1. H. 1 973. Interferometric observations of M31 and M33. PhD thesis. Mass. Inst. Techno!., Cambridge, Mass.

Spencer, J. H., Burke, B. F. 1 972. Ap. J. 1 76 : LlOl

Spencer, J. H., Burke, B. F. 1 973. Ap. J. 185: L83

Terzian, Y., Pankonin, V. 1972. Ap. J. 1 74 : 293

Toomre, A., Toomre, 1. 1972. Ap. J. 1 78 : 623 Tovmassian, H. M., Terzian, Y. 1 973.

Astrophys. Lett. 1 5 : 97 Wade, C. M. 1 968. Astrol1. J. 73 : 876 Webster, A. 1 975. MNRAS 1 7 1 : 243 Westerhout, G., Oort, J. H. 1 95 1 . Bull. Astron.

Inst. Neth. 1 1 : 323 Westerlund, B. E., Mathewson, D. S. 1 966.

M N R A S 1 3 1 : 37 1 Wiele bin ski, R. 1 976. Astron. Astrophys.

48 : 1 5 5 Wright, M. C . H. 1 9 7 1 . Astrophys. Lett.

7 : 209

Ann

u. R

ev. A

stro

. Ast

roph

ys. 1

976.

14:4

17-4

45. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by D

uke

Uni

vers

ity o

n 10

/13/

13. F

or p

erso

nal u

se o

nly.

the radio continuum morphology of spiral galaxies

Documents