CHINESE ASTRONOMY AND ASTROPHYSICS

PERGAMON Chinese Astronomy and Astrophysics 23 (1999) 463-470

Radio observations of nearby moderately

luminous IRAS galaxies t *

LI Yong-sheng SU Bu-mei

Yunnan Observatory, Chinese Academy of Sciences, Kunming 650011

Abstract Six nearby moderately luminous IRAS galaxies have been observed

at two wavelengths with the Australia Telescope Compact Array. Radio emis-

sion was detected in two of them, IRAS 20272-4738 and IRAS 23156-4238, and their parameters including flux, peak position, size and spectral index, obtained.

These sources were confirmed with infrared, radio and optical data. Combining

with previous results we discuss their emission characteristics.

Key words: IRAS galaxies-radio emission

1. INTRODUCTION

Mid- and far-infrared emission from a number of galaxies has been detected by Infrared

Astronomical Satellite (IRAS) at 12, 25, 60 and 100pm wavelengths. The IRAS observa-

tions help us to understand the infrared properties of these objects. Among them, there

is a subclass with far-infrared luminosities LFIR < 10” Lo and a similar spectral energy

distribution. The spectrum peaks between 50 to 200pm, which is the characteristic heat

emission by dust and the heating is generally attributed to be done by young stars This is

why we take the infrared luminosity as a measure of the degree of star formation in recent

times. In an effort to understand the radio properties of moderately luminous IRAS galaxies,

we selected 6 galaxies from nearby (Z < 0.1) southern IRAS galaxies for observation with

the Australia Telescope Compact Array (ATCA). Table 1 gives their infrared positions,

cosmological distances (He = 75 km-’ Mpc-‘, qo = 0.5), flux densities at 60 and lOOpm!

t The Chinese Academy ofSciences S. S. Huang Foundation for Astrophysical F&sear&

Received 1996-03-18; revised version 1998-09JX

* A translation of Acta Astron. Sin. Vol. 40, No. 2, pp. 206-212, 1999

0275-1062/99/s - see front matter @ 1999 Elsevier Science B. V. Ail rights reserved.

PII: SO275-1062(99)00078-8

464 LI Yang-dheng, su Bu-mei / Chineae A8tronomy and A8trophysics 23 (1999) 463-470

the far-infrared (42.5-122.5pm) fluxes, S~rn, as defined in (2), and the corresponding far-

infrared luminosities L. The FIR in (2) is defined by Helou et al ~1 as

FIR = 1.26~10-~*[2.58Seo~m + SICMJ,~] (WrK2) (11

where Ssopm and Sioopm are taken from the IRAS catalog. FIR estimates the power pass-

ing through a unit area in an ideal rectangular bandwidth of 80 pmat the central frequency

ve = 3.75 x 10” Hz, and S~rn is the flux density at this band:

SFIR = FIR/uo = 0.336[2.58&opm + Sloopm] (JY) (2)

2. RADIO OBSERVATIONS AND DATA REDUCTION

The Australia Telescope Compact Array (ATCA) is located near Narrabri, Australia at

latitude 30“s. It consists of 6 22-meter diameter radio telescopes set up in the east-west

direction. The longest baseline is 6 km. We used the longest baseline in our two observations

to obtain the best resolution. The observations at 3 and 6cm shared the same feed, and

were carried out simultaneously, and similarly for the observations at 13 and 21 cm. The

switch between the 6 cm/3 cm and 13 cm/21 cm modes could be made in 20 seconds. The

rms deviations with a 100 MHz bandwidth and one hour integration time are 0.09, 0.11, 0.10

and O.lOmJy for 21, 13, 6 and 3cm, respectively.

IRAS name

(1)

04134-5611

19360-1900

20272-4738

23156-4238

23166-4231

23552-3252

Table 1 Infrared Data for Six IRAS Galaxies

RA D.X z %“n %lorm SFlR ML/L@)

(1950) (1950) (JY) (JY) (JY)

(2) (3) (4) (5) (6) (7) (8)

4h 13” 25’. 0 - 56’11’41” 0.004 7.17 22.7 13.84 9.58

19h 36” 21’.0 - 19’00’44” 0.036 0.78 2.06 1.37 10.52

20h 27” 15’. 4 - 47’38’33” 0.006 9.46 13.53 12.75 9.94

23h 15” 38”. 1 - 42’38’41” 0.005 48.01 72.76 66.07 10.49

23h 16” 36’. 4 - 42’31’49” 0.006 6.13 18.28 11.46 9.84

23h 55” 14’. 2 - 32’52’08” 0.001 5.67 33.39 16.13 8.26

We observed IRAS 19360-1900 and IRAS 231564238 simultaneously at 4.8 and 8.6 GHz

in the June of 1993, and IRAS 04134-5611, IRAS 20272-4738, IRAS 231664231 and IRAS

23552-3252 at 1.4 and 2.49GHz in the February of 1997. The parameters for these two

observations are given in Table 2.

The source 1934-638 was used as a flux calibrator during these two observations. It

has flux densities of 2.48, 5.83, 11.59 and 14.94 Jy at 8.4, 4.8, 2.4 and 1.4 GHz, respectively.

The source was observed once or twice a day, and each time lasted 5 minutes. The phase

calibrators are compact sources with flux variations less than 2% and are located very close

LI Yang-sheng, SU Bum& / Chinese Astronomy and Astrophysics 23 (1999) 463-470 465

to the program sources in position, chosen from the VLA Calibrator Catalog. Before and

after the observation of the program sources, the phase calibrators were observed for several

minutes. A snapshot mode was adopted for the observation of the program sources. A total

of 6 scans were made for each source, and each scan lasted 8 to 10 minutes. To improve the u-v coverage of the sources, the hour-angle coverage of each source was spread over more

than 6 hours.

Table 2 Parameters of Radio Observations

IRAS Name Other Name Date Freq. (GHz) Flux Cal. Phase Cd.

04134-5611 NGC1546 1997.2 1.384:2.368 1934 - 638 0437 - 454

19360-1900 1993.6 4.800;8.636 1934 - 638 2126 - 185

20272-4738 NGC6918 1997.2 1.384;2.368 1934-638 2004 - 447

23156-4238 NGC7582 1993.6 4.800;8.636 1934-638 2215-508

23166-4231 NGC7599 1997.2 1.384;2.368 1934 - 638 2254 - 367

23552-3252 NGC7793 1997.2 1.384;2.368 1934 - 638 2254-367

The observational data were first recorded in the PFITS format and transferred to the standard FITS at Epping. The data were carefully inspected and some bad data points

deleted before mapping, especially for the 13 and 21 cm data since they are more easily

affected by radio interference. The data were finally processed on the Pentium-133 com- puter in Yunnan Observatory with the Astronomical Image Processing System (AIPS) of the

American National Radio Astronomical Observatory (NRAO), and mapping was made by

the task “IMAGR”.

Since the ATCA has only 15 baselines and our observing time was relatively short, the

u-v coverage in our observations was not very good. This may result in high sidelobes in

the aperture synthesis pattern. If we take deep cleaning in the mapping process, then the

resulting flux density may be lower than its actual value. After repeated tests we adopted

the following strategy: when the peak flux on the residual map falls below 0.5mJy/beam,

we stop the clean processing. The peak flux on the residual map will be obviously higher

than the expected theoretical rms noise level, and the isolated weak components among the

cleaned components may be due to effects of sidelobes. Therefore, we add the flux of the

components at the same position together and clean out the components with flux sums less

than O.l5mJy/beam, and then put the remaining components back into the residual map

to obtain the cleaned map. This method avoids deep cleaning on a map as well as reduces

the effects of the sidelobes. The rms noise level near the program sources on the maps at

these four wavelengths by the above method is in the range 0.1 to 0.2mJy/beam.

3. OBSERVATIONAL RESULTS

Radio emission from IRAS 20272-4738 and IRAS 23156-4238 (NGC 7582), among these 6

IRAS galaxies, have been detected. Table 3 presents the radio parameters of these two galaxies. In the table “beam” is the restoring beam used in the cleaning process, SI and

466 LI Yang-aheng, SU Bu-mei / Chinese Astronomy and Astrophysics 23 (1999) 463-470

S, are the integrated and peak flux densities, “size” gives the FWHM values of the major

and minor axes in the Gaussian fitting. and “PA” is the position angle of the major axis.

Figs. la, lb, 2a and 2b show the contour maps of these two sources at different wavelengths,

and the “ + ” sign in the figures represents the IRA!3 source position.

20272-47 1388. OOOMHZ I I I IV I 20272-47 2364. OOOMHZ

I I I I I I -47 27 15

0 0 -47 28 05 -

30 - Q ,-. _.

45 - cs

10 -

29 00 - 0 35 - 0

a 15 -

/- > ._ ’

40 - I I I I I I I 1 I I

I I -I

10 303o52 50 48 46 44 4” _ 20 30 48.5 48.0 47. 5 47. 0 46.5 46.0

RIGHT ASCENSICN(J2000) RKHT ASCE?fSION(J’ZOOo)

Fig. la The contour plot of IRAS 202’72 Fig. lb The contour plot of IRAS 20272

-4738 at 21 cm. The contour levels are -10, -4738 at 13cm. The contour levels are -5,

-5, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, and -2, 2, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, and

100 per cent of the peak brightness 100 per cent of the peak brightness

Table 3 Radio Parameters of the IRAS Gal axies with Detected Radio Emission

IRAS name Radio peak position beam

RA DfX

WOOO)

20h 30” 47’. 1 - 47’28’24”

freq

GHZ s,ze

( “ x ‘* )

8.7X7.3

5.1x4.3

2.7x2.2

1.6X1.2

SI SP

+---I PA (miuf fmTY)

(‘1

size

( .’ x ” )

2.1x1.9

3.4x1.7

20272-4738

23156-4238 23h 18” 23’. 6 - 42’22’13”

1.384

2.368

4.800

8.636

-5 25.2kO.5 24.2

-11 lS.6+-0.5 18.6

-5 73.2+0.3 33.0

-11 45.1+0.3 11.0 I - PA

I ) r.

64

L66

LI Yang-sheng, SU Bu-mei / Chinese Astronomy and Astrophysics 93 (1999) 463-470 467

4. DISCUSSION

All these 6 IRAS galaxies, with the exception of IRAS 19360-1900, can be found in the

southern hemisphere sample of the IRAS Bright Galaxy Redshift Survey II (BGS-2) of

Sanders et a1.121. Their flux densities at 60 ,um wavelength are greater than 5.24 Jy. However,

IRAS 19360-1900 is relatively weak, and has a flux density of only 0.7 Jy at 60pm. Its

redshift is taken from the QDOD survey131.

4.1 Identification of the Radio Sources

Infrared, radio and optical data are used before mapping to check whether the detected

radio sources are the counterparts of the IRAS galaxies. By taking the difference between the

radio peak and infrared positions, we have the positional difference Aa = 3” and A6 = 2”

for IRAS 20272-4738, and Acr = 4.5” and A6 = 3” for IRAS 23156-4238. These two radio

sources are located within their IRAS positional uncertainty ellipse. With the radio peak

position as the center, we obtain the digitized sky maps of the sources from the DSS141. It can

be seen clearly from the maps that the radio sources lie at the center of their corresponding

spiral galaxies. Their infrared, radio and optical positions are in good mutual agreement.

These two detected radio sources are the counterpart of their corresponding IRAS sources.

23156-42 8636. ooobw I /- ‘, I I I I I I

23156-42 4796. OOOMRZ

I ’ 1 1 I I I , , _42 22 04 :.-! ,,-. t.,: /-\ , 1 _-.

‘.-A .;._,,

06

08

42 21 55-

2000-

05 - - s

g 8 5 lo-

6 ;: 15- I 2 x 20-

40 I I I , p , I I 2318 25.0 24. 5 24. 0 2;. 5 23.0 22. 5 22.0 23

RIGHT ASCEHSION(J2UOO)

Fig. 2a The contour plot of IRAS 23156

18 24.2 24.0 23.8 23. 6 23.4 23. 2 23. o RIGHT ASCDE~ION(J~OOO)

Fig. 2b The contour plot of IRAS 23156 -4238 at 6cm. The contour levels are -5, -1, -4238 at 3 cm. The contour levels are -5, -2, 1, 5, 10, 20, 30,40, 50, 60, 70,80, 90, and 100 2, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100

per cent of the peak brightness per cent of the peak brightness

468 LI Yong-sheng, SU Bu-mei / Chinese Astronomy and Astrophysics 23 (1999) 463-470

Some authors argue that NGC7582 is the optical counterpart of PKS23156-426. However, we note that the positional difference between PKS23156-426 (a=23hl9”07”, 6=-42’07’ 16.7” and NGC 7582 (~~~23~18, 6~42~22’) is 10’ in right ascension and 15’ in declination, whereas NGC 7582 is in good agreement in position with our detected radio source. Condon et a1.15i, with a 60” beam, also detected the radio source at a position very close to our detected radio source. With the 60” resolution the source should be weI1 separated from PKS 23156-426. Through the comparison of infrared, radio and optical po- sitions, we believe that the radio source coincident with NGC7582 is not PKS23156-426.

4.2 Discussion on Individual Sources IRAS 04134-5611 (NGC 1546) and IRAS 19360-1900 were observed at a radio wave-

length for the first time, but no radio emission was detected. IRAS 20272-4738 (NGC6918) is a spiral galaxy (Sab)I’i. It has a magnitude 14.4,

and redshift z = 0.0060I’i. Its continuum emission was detected at 1.3mm wavelengthIs]. Our ATCA observation of the galaxy was the first observation at centimeter and decimeter wavelengths. Its flux density is 25.2 mJy at 21 cm and 15.6 mJy at 13 cm. To estimate the 21cm/13cm spectral index, we use the 21cm cleaned beam to convolve the 13cm map, and obtain the integrated flux of the source, 19.7 mJy. The 21 cm/13 cm spectral index is ci = 0.46 (S c( v-~ ). The galaxy is a radio source with a flat continuum.

The logarithm of the radio luminosity (W/Hz) is 21.24 at 21 cm, and 21.03 at 13 cm. The source is marginaIIy resolved at about 5” resolution. It has a size of 2”, and the radio emission is concentrated in an area of 300~~. de Grijp et al.I’i suggest that the 60 pm/25 pm infrared colour is a very important parameter in distinguishing an AGN from a normal galaxy. Prom the definition air = log(Smpm/Sz5pm, we have air = 2.23 for IRAS 20272-4738, and the logarithm of the ratio of its far-infrared to radio (1.4 GHz) flux densities is q = 2.7.

IRAS 23156-4238 (NGC 7582) is a barred spiral galaxy, and a Seyfert-II according to its spectrum. The galaxy also is an X-ray source [loI . It was observed with the VLA A configuration at 6 and 20 cm wavelengths, with flux &cm = 60 k 5.OmJy and Ssocm = 160.0 f 9.OmJy Ill]. The detection at 6cm is not sensitive to the region beyond 7”, and at 20 cm it is not sensitive to emission beyond 25”. The VLA D+C configuration with a 60” restoring beam detected an integrated flux of 262.0 mJy at 20 cm15i. We detected the source using the ATCA at 6 and 3cm wavelengths, with a restoring beam about 2”.4 at 6 cm and 1”.4 at 3cm. It has an integrated flux of &cm = 73.2 * 0.3mJy at 6 cm and

S scm = 45.1 f 0.3 mJy at 3cm. The 6cm flux density is slightly higher than was obtained by the VLA A configuration. This may be due to the synthesized beam being wider in the ACTA than in the VLA A configuration.

The 3 cm/6 cm spectral index of the source is a: = 0.81, a typical steep spectrum source. The radio luminosities in W/Hz at 6 and 3cm are logPscm = 21.54 and log&cm = 21.33,

respectively, falling in the intermediate radio luminosity regime, when compared with the nearby Seyfert galaxy sample of Ulvestad Irzi. In addition, the flux density detected by the VLA D+C configuration is higher than that by the A configuration. It can be seen that much of its flux is from the structure larger than 25”, and the source consists of a nucleus and emission over an extended surrounding region. It is shown from our observations that

LI Yang-sheng, SU Bu-mei / Chinese Astronomy and Astrophysics 23 (1999) 463-470 469

the nuclear region of IRAS 23156-4238 is resolved at the resolution of l”, and the size of the

component is 3.4” x 1.7”. The radio emission is concentrated within an area of 300 pc, with

evidence of surrounding extended structure. The source has air = 2.37, and the logarithm

of the ratio far-infrared to radio (4.8GHz) flux densities is q = 2.96. This shows that the

far-infrared flux of the source is over 900 times its radio flux, the radio emission is much

weaker.

From a statistical analysis of the far-infrared and 6cm radio data of 7702 sources in

the IRAS FSC catalog, Condon et a1.1131 gave (q) = 2.75 f 0.03, a very narrow region.

Starbursting galaxies fall in the region with air > 1.25 and q > 2.25. IRAS23156-4238

just lies in the starburst region. Sadler et al. 11*1 observed the source using the PTI (Parkes-

Tidbinbilla Interferometer) with a 275-km baseline which is sensitive to scales < 0.1” and

brightness temperatures > 105. Their results show that the source has a flux density of

< 5 mJy on the PTI, that is, no compact nucleus with a scale < 0.1” has been detected for

the source at the current resolution.

IRAS 23166-4231 (NGC7599) is an SC spiral galaxy. There are strong emission lines

in its nuclear spectrum, and the line intensity ratio of Ho to [NII]X6584 is - 2.7. It is

classified as an HI1 regionlr51. The source was detected by the VLA with 60” resolution at

1.4GHz, and its integrated flux was 52mJy and the FWHM size 90” x 36”. However, we

did not detect the source at 21 and 13 cm wavelengths (Szi cm < 1.8 mJy, Siscm < 1.3 mJy,

(3a)). This may be due to the fact that the H II region is resolved at our resolution.

IRAS 23552-3252 (NGC 7793) is an Sa spiral galaxy. It was detected by the VLA with

60” resolution at 1.4 GHz151. Its integrated flux is 103 mJy, and peak flux 9.6 mJy. However,

no integrated flux densities and FWHM scales are given for the components. It may be

a very large extended source. At the resolution of our ATCA observations the source is

resolved and not detected at 21 and 13cm wavelengths (Szicm < 0.9 mJy, Srscm < 0.5 mJy,

(3fl)l.

5. CONCLUSIONS

We have observed 6 nearby IRAS galaxies simultaneously at two wavelengths using the Aus-

tralia Compact Array (ATCA). They have relatively strong emission at 6Opm wavelength,

Ssopm > 5.6 Jy, except for the weak source, IRAS 19360-1900. Their logarithmic far-

infrared luminosities (in units of La) are in the range 8.26 to 10.52, a moderate luminosity

regime. Radio emission from IRAS 20272-4738 and IRAS 23156-4238 has been detected

by the ATCA, and their flux densities, peak positions, sizes and spectral indices obtained.

Their counterparts have been identified by the use of infrared, radio, and optical data. From

our radio observations the radio emission is dominated by emission over an extended area,

and at a high resolution, the emission can be partially or completely resolved.

ACKNOWLEDGEMENT Australia Telescope is a National Facility funded by the Aus-

tralian Commonwealth and managed by CSIRO. We hank Dr Norris and Du Hong for

taking part in the observations.

470 LI Yang-sheng, SU Bu-mei / Chinese Astronomy and Astrophysics 23 (1999) 463-470

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