the persistent problem of spiral galaxies

IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. PS-14. NO. 6, DECEMBER 1986

The Persistent Problem of Spiral Galaxies

HALTON ARP

Abstract-The current explanation for spiral galaxies is that densitywaves in a spiral form rotate through the disks of these galaxies, con-tinually forming new arms of hot bright stars and excited gas. Thediscussion here shows that many observed properties of spiral galaxiescontradict this assumed density wave mechanism.

Alternatively, it has been clear since the early 1950's that galaxiescharacteristically eject material from their nuclei. A number of as-tronomers have presented evidence that it is those ejections from thecentral regions of rotating galaxies that are responsible for the spiralarms.

The evidence is reviewed and evaluated here, and it is concludedthat the form and nature of the arms, their magnetic fields and rota-tional velocity characteristics, can best be explained by ejections ofmaterial, including plasma, from which the spiral arm stars areformed. This conclusion furnishes an answer to the long-standingproblem of how the magnetic fields arise in the outer regions of spirals.Perhaps most importantly, the formation and renewal of spiral armsby ejection of plasma does not require them to be in rotation only un-der the pull of gravitational forces. If rotational energy is transferredto outer regions by ejections, the current interpretation of rotationcurves may overestimate masses of spiral galaxies. If the problem of"'missing mass" is diminished, so is the necessity for exotic suggestionsto account for this undetected matter.

I. INTRODUCTIONr HE Earl of Rosse was one of the first observers to turn

a big telescope on galaxies. Around 1850 he sketchedthe symmetrical spiral pattern of the "Whirlpool Ne-bula," now known as Messier 51. Astronomical photog-raphy later showed how ubiquitous this spiral form wasin our universe of galaxies. But still today, it may be thatlittle more is understood about the cause of spiral struc-ture than when the first of these forms were glimpsed solong ago.Upon photographic study with large telescopes, it be-

came apparent that the spiral arms were composed prin-cipally of bright young stars. These hot stars also excitedradiation from associated clouds of gas. As Baade char-acterized it, the luminous stars and nebulosities outlinethe arms like beads on a string. But there arose a difficultparadox. It became known in the 1940's from work byOort, Lindblad, and others that galaxies rotated differen-tially, with the inner parts going faster than the outer. Thismeant that in a few rotations a spiral arm should be woundup in a circle. With the assumed ages of the galaxies,however, spirals should have performed almost 100 rev-olutions! Why was not the universe filled with ring gal-axies?

Manuscript received April 10, 1986; revised August 1, 1986.The author is with the Max-Planck Institut fur Physik und Astrophysik,

Institut fur Astrophysik, 8046 Garching b., Munich, FRG.IEEE Log Number 8610904.

In 1964 a mathematician, C. C. Lin, proposed a solu-tion to this dilemma. He proposed that the spiral armswere density enhancements in a smooth disk of stars andgas. This wave of higher density was attributed to a grav-itational perturbation which traveled through the disk in aspiral form [1]. In this density enhancement new starswere born and the arm moved through the disk continuallydecaying and reforming but never being sheared into aring by differential rotation of the underlying disk. Thistheory has captivated almost a generation of astronomers,enormously complex calculations and analyses have beenmade on the basis of it, and this terminology in one formor another is accepted by many astronomers today. Yet,in the following pages, I will argue that the observationsexclude the possibility of the theory's being a major factorin the creation of spiral arms or an explanation for theexistence of spiral galaxies.

II. THE ROOTS OF DENSITY WAVE THEORY

The underlying assumption of this theory is that spiralgalaxies are in overall equilibrium. First, it assumes thatthere is a reasonably smooth disk of mixed stars and gasthrough which the waves may propagate. Second, it as-sumes that the spiral galaxies have remained in essentiallytheir present form for the age of the universe, - 2 x 1010years. Given a material disk in differential rotation, onecannot preserve the same embedded spiral arm for 2 x1010 years. Hence, continually reforming the arm is theonly solution!

For nearby well-studied galaxies like M31, M81, andM33 a reasonable case can be made for the existence ofconsiderable interarm density of gas and older stars rela-tive to the arms. But for many classical spirals with con-spicuous arms, there is no evidence that interarm regionscontain appreciable material. In fact, for some extremecases of spirals, the arms have so much contrast to theirbackground, are so straight and long, and project so farout in space that it should be surprising to find interarmmaterial in the formn of a smooth disk around them.The other assumption, that all spirals are 2 x 1010 years

old springs from the deduction that all galaxies condensedat the same epoch from the primeval hot medium of theBig Bang. For most individual galaxies there is no obser-vational proof of this age. Some could be very recentlyformed or very recently have become spirals. For thenearby spirals that we have mentioned, even if we makethe assumption that their oldest, type II, stars are all thesame 2 x 1010 years of age, this by no means precludes

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ARP: THE PERSISTENT PROBLEM OF SPIRAL GALAXIES

the spiral arms we see from being relatively recent events.In fact, the density wave solution requires the old arm tofade away before the new arm has moved very far in thedirection of the pattern rotation. As far as we know, thereal situation could be that the arms we see in spirals couldfade in a fraction of a revolution and entirely new arms,in a completely different direction, could be set up. Thereare many real spirals that exist which have more arms thanthe two-arm grand-design spirals that are taken as proto-types for the density wave theory.

Perhaps the fundamental drawback in the density wavetheory is that it is only a mathematical solution to an equa-tion involving an axisymmetric gravitational potential. Itnaturally permits a radial density enhancement, but thereis no physical cause for this radial density enhancementto occur in a galaxy. Even a spontaneous density enhance-ment would only be transferred inward or outward in ra-dius by local gravitational forces which would rapidlyweaken and diffuse.When the only formal solution to the problem of the

existence of spiral arms is so lacking in physical causa-tion, it would seem worthwhile to reexamine our assump-tions in the light of all the observational evidence avail-able.

III. WHAT WE ACTUALLY OBSERVE IN GALAXIES

Is there an alternative approach to understanding spiralgalaxies? Curiously enough, as early as 1938 a model wasproposed in which the spiral arms represented massesstreaming out of two diametrically opposed points on arotating disk [2]. In 1956 Th. Schmidt-Kaler discussedthis model and suggested, essentially, that gaseous matterwas supplied from the center of the galaxy [3]. The plasmacomponent of this material was "stabilized" on the armsby a magnetic field, presumably using the Chandrasek-har-Fermi model calculated in 1953 [4]. But there was nophysical reason, or observational precedent, in those timesto expect material to flow out of the center of galaxies.Subsequently, however, it became clear that one of themost general properties that we observe in galaxies is thatthey have eruptive nuclei. Galaxies of all types, not justspirals, show nonthermal radiation, variability, and ex-plosions as well as radio, optical, and X-ray jets emergingfrom their nuclei. It would be strange indeed to postulate,in the face of modern evidence, that there was a class ofgalaxy which did not have energetic events occurring inits nucleus from time to time.The actual observed activity most often takes the form

of ejections of material outward from the nucleus of a gal-axy in two opposite directions. The numerous maps ofradio jets and counter-jets are strong testimony to thisphenomenon. Here we do have a physical mechanism fordistributing material in a different state linearly throughthe disk of a galaxy in opposite directions from the center.In fact it is unavoidable.The fact that ejections in galaxies were actually asso-

ciated with luminous matter was first brought to the atten-

tion of the astronomical world by the discussions of theArmenian astronomer V. A. Ambartsumian in the late1950's [5]. He simply perceived from careful study of thePalomar Sky Survey that many galaxies were visiblyejecting matter. Among other consequences of this ob-served ejection he suggested that spiral arms also re-sulted. In 1960, P. Pismis and S. S. Huang proposed amore specific model for spiral arms as ejection trajectories[6]. Through the subsequent years Pismis has continuedto develop these models [7]. In 1963 Arp discussed ejec-tion from a nucleus with an imbedded magnetic field [8]and in 1969 published a series of large telescope photo-graphs of special types of spiral galaxies aimed at dem-onstrating that ejection from central regions was the onlypossible explanation of spiral arms [9]. Through the yearsthere have been many researchers who published argu-ments or models that demonstrated that ejection could bethe cause of spiral arms (for example: Bonometto andLucchin [9], Barrecelli [10], and Schmidt-Kaler [11]).Some models of interacting magnetic plasmoids have

also been proposed as possible starting points for spiralstructure by Peratt et al. [54]. In this connection, the ideasof Alfven and Birkeland that electric currents can flowalong magnetic field lines in astrophysical situationsshould be evaluated for possible effects in spiral galaxies[55].However, we will concentrate here on the ability of the

ejection models to explain the phenomenon of spiral gal-axies. In order to understand more clearly how the ejec-tion mechanism works, it is most efficient to discuss it incomparison to the alternative density wave theory and tosee how each fares in explaining the observations of spiralgalaxies.

IV. CONFRONTATION OF THE THEORIES WITH THEOBSERVATIONS

What is usually meant by confrontation of density wavetheory with observations is whether, for example, the cal-culations on the shock front in the gas in the arm impartvelocities to newly formed stars that are in agreement withobserved velocities. The kind of observations we will dis-cuss here, however, are not reproducible by adjusting pa-rameters in a calculation. They are the qualitative large-scale phenomena which we can survey in many spiral gal-axies and which must be generally explained, in at leastan order-of-magnitude way, by a theory of spiral struc-ture.

A. Observed Width of Spiral ArmsBecause the density wave's raison d'etre is to keep the

spiral arms from winding up in the central regions of thegalaxies relative to the outer regions, the density wavemust travel more slowly than the underlying disk in thecentral regions. In the prototype spiral M51, this patternspeed at 1-kpc radial distance from the center is supposedto be 33 km/s [12]; the underlying disk, however, is ro-tating about 100 km/s faster.

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IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. PS-14, NO. 6, DECEMBER 1986

At 100-km/s relative velocity the density wave formsstars out of 2 kpc of underlying disk gas in only 2 x 107years. In 2 x 107 years newly formed stars would onlyhave time to have aged to spectral class B4 [13], still veryhot bright stars conspicuously defining a spiral arm.Therefore, the spiral arm should be at least 2 kpc wide inthe direction of rotation (see Fig. 1). But no spirals, M51included, have arms anywhere near this width. We cannotescape the fact that based on the density wave hypothesis,arms in spiral galaxies should be much wider than ob-served with a marked gradient of brighter hotter stars onone edge of the arm and fainter cooler stars on the other.This would be very conspicuous and is obviously not ob-served.'

B. Magnetic Fields Running Along Arms

Any large-scale magnetic field in the underlying diskwould be drawn into a circular orientation by differentialrotation after only a few rotations. Magnetic field linesare generally observed to run along the spiral arms at theconsiderable inclination of the arms to circular orienta-tion. This is a decisive point against the density wave the-ory and will be discussed in more detail in Section V.

C. Narrowing ofArms at Ends

One of the most certain properties that one can gener-alize from studying photographs of many spiral galaxiesis that the arms are generally thicker near the center ofthe galaxy and taper down to narrow endings (e.g.,NGC7616 pictured in Fig. 2). They almost never widentoward their outer termination as is the result of densitywave geometry. 2 This property of the density wave spiralsis fundamental to the fact that two mathematical spiralloci, say the leading and trailing edge of a shock front,can be close together near the center of the galaxy butincreasingly diverge as one moves outward. This is sche-matically illustrated in Fig. 3. On the other hand, ejectionmechanisms can, in one conceptual form, actually ejectsmall compact bodies outward. These form the smallcross-section termini. On any form of ejection theory,however, the track of the ejection is most likely to haveits strongest interaction with the interior material in thegalaxy and/or widen at its base as time develops. So wewould expect either tubular ejection tracks of constantcross section or tapering ones. This is what we observe.I consider this another decisive observational discrimi-nation between the two theories.

'Lin realized this problem in 1968 114]. He tried to hold the stars on thearms by the gravitational attraction of a 5-percent mass density enhance-ment in the arms. But this mechanism was only appreciable over time scalesof the order of 108 years, much larger than time scales needed. In any case,if they held the stars onto the arms, the density waves would very quicklysweep all the supposed material in the disk into the arms.

2Some photographic processing tends to make spiral arms seem widerat their ends [56]. This is a result of dividing their intensity by a vanishingamount of background surface brightness. A more proper gradient empha-sis technique of image processing [57] shows that the total luminosity andwidth diminishes at the end of the arms.

(a)

(b)Fig. 1. (a) Excited hydrogen emission in M51 shows narrow tapering arms.

Photograph by Allan Sandage. (b) A smoothed schematic representationof the spiral arms in M51 is shown. The filled circles represent newlyformed 0 stars at sample radii. In the density wave theory, the star con-densation moves slower than the disk in the inner portions, and faster inthe outer. The corotation of disk and density wave is adopted to be nearthe middle of the galaxy as in Shu et al. [12]. After 2 x I0C years theO stars would have aged to B4 and moved off the arms to the positionindicated by the open circles. It is seen that in a small fraction of arotation period, the older, but still very conspicuous spiral arm stars,should have moved into the interarm region ahead of the arrm in the innerregion, and far behind the arm in the outer regions.

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Fig. 2. NGC7616, showing a third spiral arm crossing perpendicularly overone of a pair of spiral arms. Also shown are straight tangential segmentscoming off a curved arm (see [9]).

Fig. 3. A logarithmic spiral is fitted to the M51 spiral arms. Since a den-sity wave has a finite angular width, the leading and trailing edge of thisdensity wave must diverge as one goes out along the arms. This is asillustrated in the above diagram but is opposite to what is observed inspiral galaxies.

D. Companions on the Ends of Spiral ArmsThere exists a whole class of spiral galaxies with com-

panions at the end of one arn. M51, the Whirlpool Ne-bula, shown in Fig. 4 is the most famous example. Insurveys, about 2 percent of all peculiar galaxies fall intothis class, and there are hundreds of examples in the Cat-alogue ofSouthern Peculiar Galaxies and Associations byArp and Madore [15]. It has been argued that these com-panions actually originate as compact bodies ejected fromthe nuclei of their umbilically attached galaxies, and thatthe arm is the track of their ejection [16]. Regardless ofwhether one accepts that explanation for their origin, theirvery existence is disasterous for the density wave theory.The reason is simply that the density wave spiral arm must

Fig. 4. M51, the whirlpool nebula. It is used in the present paper as anexample of a two-arm grand-design spiral. Photograph taken by AllanSandage with the Palomer 200-in telescope.

continuously dissolve and reform as its pattern rotatesaround the disk. These companion galaxies, obviously at-tached to the arm by visible interaction in many cases,cannot continuously dematerialize and reform themselves

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Fig. 5. No. 188 in the Atlas of Peculiar Galaxies [20]. This is a demon-stration of a long ejected spiral arm from an unsymmetrical central spi-ral.

in different locations. If these companions are attached tothe spiral arms this is a reductio ad absurdum for the den-sity wave mechanism.

In fact, the class of spirals with companions attached toarms was such a vivid proof of the material nature of spi-ral arms that it led to the famous and much admired grav-

itational encounter calculations of Toomre and Toomre[17]. There they showed that a close encounter betweena companion and a larger galaxy could gravitationallydraw out a curved plume of stars which, in certain ori-entations, would closely resemble M51. My criticisms ofthis as a general explanation for symmetrical two-arm spi-rals are: 1) the arm to the companion has to have a specialmechanism to light up the stars; 2) the calculated coun-

terarm has, except in special projections, a broad splayedappearance unlike the observed symmetrical arms in mostspiral galaxies; and 3) in observational reality there is a

continuum of sizes of companions on spiral arms, goingdown to very small objects which are obviously burrow-ing out from the center of the parent galaxy (see [16]).Even if we supposed for a moment that the MS1 class

of spirals could be generated by encounters, there is an-

other class of spirals, identical in symmetrical spiral-armform, which is completely isolated. Since we have arguedthat the density wave does not work in the case of spiralswith companions on ends of arms, then for similar spiralswithout companions, the only mechanism remaining isejection activity from the nucleus.

E. Spirals with Odd Numbers ofArmsMost of the so-called grand-design spirals which are

observed have two rather symmetrical arms. The mathe-matical density-wave solutions should have even numbersof symmetrical arms because that mode dominates [18].

In the real universe, however, sometimes spirals haveeither one or three arms! For example, a surprising num-ber (about 0.5 percent of all peculiar galaxies) have threearms [15].

In the ejection mechanism two-armed spirals are naturalbecause we typically observe ejection in opposite direc-tions from nuclei in jets and counterjets. But we alsosometimes observe one-sided optical or radio jets. Thiswould naturally account for one-armed spirals, an ob-served phenomenon which is difficult for the density wavetheory because it represents an asymmetrical nuclear per-turbation in an equation assumed to be symmetric. Three-armed spirals are somewhat more difficult for ejectionmechanisms, but ejections can be oriented to conservemomentum and there are some examples of multiple jetsfrom active nuclei (e.g., NGC1097 [19]). Again, how-ever, spirals that are three-armed all the way from theouter to inner regions are an observational difficulty to thedensity wave theory.

F. Anomalous Arms

Most directly, in extreme cases we can show arms thatare clearly due to ejection phenomena. One case is No.188 in the Atlas of Peculiar Galaxies [20]. This galaxyshown here in Fig. 5 has a long filamentary arm reachingalmost straight away from disturbed inner regions. Thewidth and surface brightness as well as the condensationswithin it mark it as a typical spiral arm. Yet it could onlyhave been ejected. The inner spiral has also definitely beendeformed, presumably by-the energetic ejection, yet theintegrity of its high-surface-brightness arms has beenmostly preserved, a difficult feat for nonmaterial densitywave arms.

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Fig. 6. NGC7743 showing spiral arns which have stopped forming starsand are diffusing and fading away.

Other examples of arms responding as physical entitiesto a physical force are No. 108 through No. 112 in theAtlas ofPeculiar Galaxies. In No. 111, one arm of a two-armed spiral has been bent at its base where it enters thecentral galaxy, obviously by the effect of a nearby com-panion. Finally, there are cases where we can actually seenew arms cutting across old arms. NGC7616 (Fig. 2 here)shows a two-armed spiral with a clearly delineated thirdarm emerging perpendicularly through one of the grand-design inner spiral arms. One can even see signs of erup-tion where this third arm cuts across the inner arm. Clearlythis has been ejected through the inner arm and curvedaround later by rotation of the galaxy.

In this picture there is also clearly visible a long straightportion of arm which takes off tangentially from this sameinner arm. Such tangential arm segments, in both emis-sion and dust absorption, are very characteristic of a largenumber of spiral galaxies. For example, see the "spurs"coming off the west side of M51 in Figs. 1 and 4. Theyare inexplicable on density wave concepts but, as we shallsee later they perform.a critical role in the ejection pictureof spiral arms.

Completely different kinds of spiral arms are shown inFig. 6. Here, obviously, star formation has stopped in thearms, and they are fading and dispersing. They are notwound up, nor is there a gradient of brightness acrossthem. Such examples are rare so we must conclude armsare generally replenished or reestablished.

G. Energy Loss in Density WavesA number of dissipative processes reduce the lifetime

of the postulated density waves. Toomre [21], Kaljnas[22], and Roberts and Shu [23] calculated density wavelifetimes of from 108 to 109 years. Finally, Schmidt-Kaler

[24] showed that a density wave model would require apowerful energy source to sustain or renew it on the orderof time of a single rotation of a spiral galaxy.

H. Rotation CurvesThis is so crucial a consideration that we will devote a

later section to it. The conclusion will be that for some ofthe extreme forms of best delineated spiral galaxies, grav-ity may not have very much influence at all on the rota-tion. Naturally, this is not a favorable environment fordensity wave theory. We shall also investigate whetherthese extreme cases have some component of relevance inthe more familiar nearby spiral galaxies, which we knowto be predominantly rotating under the influence of grav-ity.

V. THE GEOMETRY OF THE MAGNETIC FIELDS

If a jet were to punch out through a resisting medium,of an inner disk in solid body rotation, material wouldcontinue to flow out of this channel until it closed upagain. So there is no necessity to require a magnetic fieldto guide ionized particles down the barrel of this rotatinggun, at least for short bursts. On the other hand, fromwhat is known about extragalactic radio jets, it is clearthat they contain magnetic fields and that on the outsideof the jets the magnetic fields lie generally parallel to thesides of the jets [25]. This is as would be expected if ma-terial with a magnetic field imbedded in it were to bestretched out in the direction of ejection (see [8]). Sincethere is considerable ionized material in the nuclei of gal-axies, particularly in galaxies in an active phase, it wouldseem to be almost a necessity to observe magnetic fieldsalong the spiral arms under the hypothesis that they arecaused by ejections from central regions.

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What then is actually observed of the structure of mag-netic fields in the disks of spiral galaxies? In our ownMilky Way Galaxy, where we have the best opportunityto measure the magnetic field in the nearby spiral arm, themeasures are difficult to interpret because the rotations ofpolarized optical and radio vectors by the magnetic fieldare integrated throughout our entire line of sight to distantsources. Moreover, the properties of nearby arm seg-ments may not be representative of our whole galaxywhich may have numerous, possibly quite fragmented,spiral arms. It is clear, however, that spiral arms in ourgalaxy do have magnetic fields. Measures of interstellarpolarization of starlight by Hiltner in 1951 [26] enabledhim to conclude that there was a large-scale field of theorder of a microgauss in the plane of the galaxy. As earlyas 1959, Mills [27] showed with radio observations thatnonthermal (synchrotron) radiation was observed tangen-tially along the arns interior to the sun. Later polarizationmeasures by Morris and Berge (1964) [28], Gardner andDavies (1966) [29], and Mathewson (1968) [30] estab-lished the presence of a magnetic field of a few micro-gauss running along the local arm (a so-called longitudi-nal or bisymmetric field).The latest measures have also confirmed at least one

reversal, possibly more, in the magnetic fields in the armsin our sector of the Milky Way Galaxy [31]. This is par-ticularly important because ejected filaments which camethrough from an interior region possessing an orderedmagnetic field would be expected to result in field lineson one side of the ejected tube opposite to the field lineson the other side. No other method of accounting for suchmultiple reversals in the disk of the Milky Way seemsplausible.The grand-design spirals, being much more distant,

were thought to represent a more difficult problem in de-termining magnetic field geometry. However, extensiveradio polarization measures on the prototype M51 by Se-galovitz et al. in 1976 enabled the field configuration tobe determined by Tosa and Fujimoto in 1978 [32]. Thefield turned out to a spiral, lying along the spiral arms(twisted and bisymmetric in the terminology of the latterreference) .3Of course the place of the magnetic field lines really

could have been deduced as soon as continuum emissionwas observed to outline the arms of spiral galaxies (forexample, the synchrotron emission outlining the arms ofM51 measured by Mathewson et al. in 1971 [33]). Thereason is simply that the continuum radio emission hasalways been identified as synchrotron emission. The ra-

diating electrons must spiral around the magnetic forcelines which therefore must be confined to the arms. Irreg-ular magnetic lines or lines directed in directions otherthan along the arms would lead to lifetime and originproblems and lack of confinement.

3See also Beck et al., Astron. Astrophys., vol. 152, p. 237, 1985, forrecent measures of the magnetic field aligned along arms of M81, and alsoa review by Beck in these TRANSACTIONS, pp. 740-746.

It has been clear for some time, therefore, that the or-dered component of the magnetic field lies along the arms.However, it must also be clear that the density wave the-ory cannot move these field lines through the disk, evap-orating them in back and forming them ahead as it is sup-posed to do for the young stars. Even if a spiral field layahead of the density wave waiting to be compressed andintensified, that field in the disk should have been woundup long ago. In my opinion, this is conclusive refutationof the density wave mechanism for spiral-arm formation.

In fact these observed longitudinal spiral magnetic fieldsexclude the two extant theories for magnetic field originin disk galaxies. In either the dynamo theory or the pri-mordial field theory, any fields which were present atsome time in the past should be sheared out into nearlycircular orientations now (for the latest discussion of thesetheories and an opposite conclusion see Ruzmaiken et al.[34]). But observationally, a "bisymmetric spiral field"with a pitch angle of 140 is measured in our own galaxy[31] and steeper pitch angles are observed for M51 andsimilar spirals. As Manchester [35] states " * * the ob-served large scale of the longitudinal component appearsto be a problem with all proposed models for the originof the galactic field." As Tosa and Fujimoto [29] state

*. ..we are confronted with a serious problem of whythe magnetic lines of force are not tightly wound up."Valee [36] states, "The distribution of magnetic fields canbest be explained by recent creation of magnetic fields

How then are these large-scale tubular magnetic fieldsrecently created? It would seem clear that only jets andstreams of magnetized plasma ejected out from the centralregions can explain the longitudinal tilted magnetic fieldsthat are observed now in outer regions of spiral galaxies.We will explore in the following sections how long suchspiral fields can persist with appreciable pitch angles inthe outer regions of spiral galaxies.

VI. NATURE OF ARMS CAUSED BY EJECTION

What are the consequences of having a magnetic fieldrun down the arm? The most immediate consequence isthat ionized material flowing out from the center of thegalaxy will, to some extent at least, be channeled downthe arm. Although they were principally interested incosmic rays. Chandrasekhar and Fermi as long ago as1953 [4] computed the magnetic field needed to balancethe gravitational attraction to the spiral arm. It turned outto be between 1 and 10 x 10-6 G. It is interesting to notethat values in this range have always been measured forthe central magnetic field in the spiral arms near the sun,in M31 and M81.

If we suppose that the magnetic field is strong enoughto guide ionized material, at least in the inner thicker seg-ments of the arms, then we have a means of transferringangular momentum and rotational energy from the innerparts of the galaxy out to larger radii. The outflowing ma-

terial simply pushes against the spiral magnetic field andapplies a force in its direction of rotation. Since the ge-

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ometry of the spiral magnetic field opens outward, theflow of gas from the center acts in some respects like anejection-driven Fourth-of-July pinwheel. The ejectioncould be more or less steady over an appreciable time orthrough bursts or explosions.The important aspect for this section is that wherever

sufficient magnetic field is present, outflowing ionizedmaterial must run down the arms. When this materialcomes inevitably to a part of the arm where the magneticfield is weak or broken, the material will continue straighton, tangential to the arm at this point. This is a naturalexplanation for a very puzzling phenomenon which isknown to anyone who has looked closely at pictures of anumber of spiral galaxies. There are straight segmentswhich tend to come off in short pieces, tangential to thearm. These arm pieces have no explanation by other the-ories. In the ejection theory they are simply leakage outof the arm of material that is flowing along it after themain ejection.Sometimes a spiral arm is observed to bifurcate con-

spicuously. Here, at a certain point, a spiral arm turns intotwo spiral arms. An ejection could behave this way eitherthrough continued ejection at different speeds, secondaryejection, or fissioning of an ejected body. As a solutionto an axisymmetric differential equation, however, thisbrings the density wave solution to an impossible point.Of course we can explain multiple arms in the ejectionhypothesis as successive ejections and therefore also ex-plain arms that cut across previously existing arms.

But what about the physical nature of these ejectedarms? These are now real material arms, not transientcondensations as in the density wave theory. First of all,there is ionized material flowing along the longitudinalmagnetic field as evidenced by the synchrotron continuumradiation. If neutral hydrogen accompanies this plasma,or the plasma recombines into neutral hydrogen, we havea tube containing hydrogen which is now more or less theconventional picture of a spiral arm. In order to condensestars out of this hydrogen distribution, we could shock itby interactions with an underlying disk of different veloc-ity. This is the standard density wave method of formingstars, and apparently everyone is happy with it.4 Or per-haps there is some other way of triggering star formationwhich has not yet been suggested. In the ejection modelwith material flowing down the arms there may be alter-nate mechanisms for cooling and condensing the flow intoprestellar cloudlets. There is even the possibility that con-densations which already represent protostars are carriedout in the hydrogen stream.

4When the synchrotron radiation outlining the arms in MS1 was mea-sured [33], a great deal was made of the fact that the radiation was said tocome from slightly inside the arm where the underlying disk was supposedto overtake the arm and first compress the material in the arm. The trouble,as we see in Fig. 1, is that at radii outside corotation, based on the densitywave theory, this situation should have been reversed, which it was not.On the other hand, based on the current ejection theory, the armn is rotatingfaster than the underlying disk throughout, and any interaction with thedisk would pull plasma condensations to the trailing (inside) edge of thearm all along the arm.

Once the stars light up in the arm we obtain the well-known H II regions which are excited by the hot 0 and Bstars, supernova explosions, and stellar winds that blowhuge bubbles in the gas and magnetic field. Clearly thisis the mechanism responsible for the aforementioned rup-tures, breaks, and irregularities that abound in spiral arms.But if new material flows along in the original directionfrom this broken stem, the arm may be regrown and per-sist for many rotations of the galaxy. It is interesting,nevertheless, to calculate the expected behavior of a sin-gle ejected arm from the standpoint of persistence in timeagainst falling back under gravity and winding up underdifferential rotation.

VII. DURATION OF ARMS AND GRAVITATIONAL ORBITSIN THE EJECTION THEORY

It is surprising to note that there have been well-worked-out ejection theories for spiral arms published throughoutthe literature for many years. One series of models waspublished by Pismis and collaborators [6], [7]. She wasable to show that ejection and rotation in the central re-gions of a galaxy accounted very well for the small tightlywound spirals often found there. With the observed radialmotions and short rotation periods in the interior regionsof these galaxies there could be hardly any doubt that thisis the correct explanation for these spiral forms. Of coursethese spirals are obviously wound up, dissipated, and re-established during any appreciable interval of time duringwhich the nucleus is subject to intermittent ejections.There is no problem of duration of any given set of armsin this case. What is a problem is the ejection trajectoryin the outer regions of spiral disks. There, under the in-fluence of gravity, the ejected arm segments should fallback in the order of a galactic rotation period, of the orderof a few x 108 years. If the arms started to fall back theywould obviously violate the observed generally outward-opening spiral morphology.The solution to this difficulty relies on the same prin-

ciple in both the density wave and ejection theory, but theapplication is much different. The principle rests on thefact that the bright stars that constitute the spiral armshave ages of the order of 106 107 years (the brightest,spectral class 0 and B stars.) After a time of 3 X 108 yearsthese brightest hottest stars have burned away into faintermore evolved stars (about spectral class AO). The smalldispersive velocities with which the stars in the arm havebeen born then slowly carry away these fainter stars in thegeneral field and the arm fades and disperses into the gen-eral disk. The density wave mechanism, however, is sup-posed to form new stars at the rate at which the conden-sation wave moves through the disk. As we noted inSection IV, however, the rear edge of this very broad armcould then only be a few x 107 years old. This fails, byan order of magnitude, to be sufficient time for the armstars to fade and dissipate. We will now proceed to seethat the problem is much more tractable for ejected spiralarms because they have a much longer time to gracefully

755


,

EIE

S ITr

i~~~~~~~~~~~

E

I --

E

Fig. 7. Spiral arms as created in a burst of outward velocities from 0 to 2times tangential velocity. Computer calculations from Baracelli et al.[10]. Arms shown after times of l, 2, and one rotation period.

fade away before falling back or being wound up by dif-ferential rotation.A very clear and explicit model for forming spiral arms

by ejection was published by Barricelli et al. in 1972 [10].They assumed the arm material was emitted in a burst,with outward components which were various fractions ofthe tangential velocity of the orbit from which they were

emitted. They then computer-modeled the resulting spiralarms. They turned out to look exactly like the arms so

universally observed in spiral galaxies. The arms per-

sisted in grand design spirals for many rotations of thegalaxy (see Fig. 7).Of course, that part of the arm that was ejected with

escape velocity would never fall back into the galaxy. Buteven those portions ejected with less than escape velocitywould take of the order of a few x 108 years to fall back.This is much more time than available in the density wavetheory and allows the bright stars in the arm to fade anddisperse before the arm falls back into the central regionsto be sheared and absorbed by the underlying disk. Of

course, it is a matter of investigation as to whether thereis a leading edge to the arm which is continually suppliedwith new material from the center and increases the life-

time of the arm appreciably over a few x 108 years, or

whether the whole arm fades and is replaced by anotherarm at a later time.The periods calculated for fall back times, of course,

are presently based on the assumption that we are mea-

suring Keplerian velocities of rotation. But if the circular

velocity we observe at any given radius is to some extent

a larger velocity, unrelaxed in the gravitational field, then

the true mass of the galaxy will actually be less and thefall-back time greater. This will give even more time for

50000

c 4800

c 60000

0

O 5600

.2 6100

0

I

5500

120 80 40 0 40 80

Distance from nucleus (arc sec)

120

Fig. 8. A sampling of rotation curves from Rubin et al. [41]. The gradualincrease of rotational velocity with distance from the center as shownfor the small nearby galaxy NGC 3495 is contrasted to the flat discon-tinuous rotation curves for the peculiar Sc I spirals, which are usuallyassumed to be much more distant.

the persistence of spiral arms before these portions withless than escape velocity fall back.5As for the winding-up problem, it turns out that the ac-

tual observations in outer disk regions of spirals show thatthe problem is less severe than commonly believed. Forspirals with the best defined global patterns like NGC801,Rubin et al. [41] state, "Even in the absence of a spiraldensity wave a global pattem could persist over large re-

gions at large R for some 109 years." They further state,"Given only our present program Sc's the necessity for a

spiral density wave theory might not have arisen." Ofcourse if there is relatively little underlying disk in differ-ential rotation out at these radii the problem of windingup is reduced even further.The net result of these analyses is to show that the ob-

served forms of spiral arms can be explained by an ejec-tion model. Moreover, these arms do not need to be re-

formed as often as density wave spiral arms. The difficultywhich both the density wave and ejected spiral arms have,however, is that as we do go out in radius toward the edgeof the galaxy the observed velocity of rotation of the arms

tends to remain higher than expected in either theory. Wediscuss this problem in the following section.

VIII. THE FORMS OF ROTATION CURVES

In order to illustrate the problem presented by the ob-

served rotational velocities in spiral galaxies, we show

some extreme cases in Figs. 8 and 9. These examples be-

5It is interesting to note that at the radius of the sun in our own galaxy,the circular velocity is usually taken near 250 km/s. Comparison with Lo-

cal Group galaxies gives a smaller value [37], and a recent rotation curve

by Rohlfs et al. [38] gives only 184 km/s. Naturally, this would reduce the

derived mass of our galaxy and increase the fall-back time in the solar

vicinity.

NGC 753

-ScT - *NGC 801

UGC 2885 *

756

%-,

..%.O - 0 0 1

.


V(KM/SEC )

4UU

300

200

100

0

V

U 2885

00o o o

. .

.. . . . . NGC 801

0 20 40 60 81RADIUS (KPC)

RADIUS

Fig. 9. Flat rotation curves for two extreme examples of Sc I spirals [41].Below them is a schematic interpretation in terms of Keplerian rotationand observation. Arrows point to the direction that interaction with anunderlying Keplerian disk would pull the observed rotation down over a

period of time (density in disk increases toward center and therefore in-teraction does also).

long to a class of spirals with strong global patterns whichhave the narrowest best defined spiral arms. They arecalled Sc I galaxies (originally defined by van den Bergh[39]; see also Sandage and Tamman [40]). Surprisingly,they have the most unusual rotation curves of any galaxy.A sampling of these is shown in Fig. 8 from the work ofRubin et al. [41]. Their amazing property is that they are

almost exactly flat from the outermost regions measuredright up to the nucleus. Looking at the spectra, we seetwo perpendicular emission lines entering the nuclearcontinuum with no visible connecting gradient betweenthem. Fig. 9 shows a sample of these Sc I rotation curvesand a more ordinary spiral, an Sc III, for comparison.

It is astonishing that these plots have been routinely,unquestioningly interpreted as rotations in equilibriumwith the gravitational field of the galaxy. As we know,objects in orbit around a central mass should rotate more

slowly as we progress outward. There should be a Kep-lerian fall off in velocity as we go to larger radii. In orderto keep these rotation velocities constant as a function ofradius it is necessary to postulate a mass distribution inthe galaxy which varies directly as the radius (M(r) arwhere M(r) is mass inside radius r). In order to keep therotation plots perfectly straight, the mass has to vary ex-

actly proportional to r. There would have to be a doubleconspiracy of mass distribution in halos of spiral galaxies:just the right amount and distribution of mass taking con-

trol of the rotation at just the radius of maximum Keple-rian rotation. Such a mass has never been observationallydetected.What then could explain these peculiar rotation plots?

The conventional assumption is that all the material in thedisk of a spiral galaxy is in rotation around the centerunder only the influence of gravitational forces. Viewedin this way, the spiral galaxies are an experimental test ofNewtonian gravitational theory on the largest knownscale. This has led to the suggestion by Milgrom and Be-kenstein [42] and by Sanders [43] that Newtonian forcelaw is altered at the large scale of the outer regions ofspiral galaxies. This is perhaps a more attractive conclu-sion than the postulation of improbable distributions ofunseen matter.We should ask, however, whether the ejection theory

of spiral arm formation bears on this problem of flat ro-tation curves. At first sight it seems to make the problemworse because as the ejected segment of arm travels out-ward (ignoring gravity for the moment), the velocity inthe direction of circular rotation vo will vary with radiusas vo = constant/r. This is a sharper fall off with radiusthan expected from Keplerian orbits where vo = constant/r/2 . Therefore, there is a greater discrepancy with the ob-served vo = constant.So far we have ignored the possibility that plasma-mag-

netic forces could have an appreciable effect on the mo-tion of the spiral arms. It is interesting to note that in1965, Woltjer [44] made a comprehensive and detaileddiscussion of all the possible mechanisms that could beresponsible for spiral arms. He was able to point out ob-jections to all the possible types of theories; gravitational,gas-dynamical, and magnetohydrodynamic. But presum-ably one of them had to be the correct approach. Subse-quently, the gravitational-gas dynamic, density wave the-ory claimed the most attention but we have enumerated anumber of criticisms of it in the present paper. That leavesonly the plasma-magnetic theories. In recent years themost significant advance has been to show that the ge-ometry of the magnetic field follows the spiral arms (Sec-tion V). In fact this has led Sofue et al. [45] to suggestthat the concentration on "pan-gravitationalism" for un-derstanding spiral structure may have been due to lack ofobservational data on the large-scale configuration ofmagnetic fields. They now state "these fields must betaken into account for a full magneto-hydrodynamicaltreatment of the spiral arms." The view taken in the pre-sent paper is that the large-scale magnetic fields runningalong the arms are prima facie proof that differential ro-tation and mass motions of ejections do not destroy orseriously perturb these magnetic configurations and thatthis therefore proves they are able to guide ionized ma-terial in the arms against the forces of gravity and massmotions.

If this is true, as it appears to be, then there are twoways in which the rotation of ejected spiral arms might bespeeded up and make the rotation velocity greater than vo= constant/r or constant/r 1/2, that rotation velocity which

757


Fig. 10. Radio isophotes observed by van der Kruit et al. with the Wes-terbork Array are superposed on an optical photograph of the spiral gal-axy NGC4258 [48]. Note the ejected radio arms are more curved and lagbehind the thinner bright-star arms. The more slowly differentially ro-tating disk may be more interactive with the radio arms than the star armsin this spiral.

would be expected without magnetic forces. First, it isclear that ionized gas is ejected from the nuclei of manyspirals. If it flows out of the ends of curved spiral-armmagnetic fields it will add energy and velocity in the di-rection of rotation of the arms (the principle of a jet-drivenpinwheel). This is a simple, perhaps unavoidable, mech-anism to turn radial ejection energy of jets and explosionsinto rotation of the spiral arms. If the field lines do notcarry too large an amount of mass with them, then this is

a way to get spiral arms rotating faster than the underlyingdisk.Another factor which should be considered is that

plasma streaming out along the magnetic field is undoubt-edly stretching field lines that are attached to it. This is aforce in addition to gravity in the direction of rotation ofthe galaxy that can add rotational energy to ejected ma-terial.Of course, it is a priori unlikely that, even though these

758


Fig. 11. Photograph of the barred spiral NGC 1097 in the light of gaseoushydrogen alpha emission. Note the extreme thinness of arms as definedby the hot stars and emission. It is also important to see how a sectionof the spiral arm has been moved bodily outward by recent jets in thedirections indicated. Photograph by Arp with the 4-m CTIO telescope[51].

forces are in the right direction, they would give so ex-actly vo = constant as a function of radius. Still, the mostimportant next step that could be taken would be to com-pute detailed models testing various plausible values ofmass of gas, flows, and other pertinent parameters. Theflat rotation curves are clearly the outstanding challengefor theorists of spiral arms to explain. On the observa-tional side it would be most important to measure the ve-locity differences between gaseous spiral arms and under-lying stellar disks (where present) in a representativegroup of spiral galaxies.

IX. OBSERVATIONAL TESTSIf arms are to be ejected they should, at least initially,

have some radial motion outward. Or if arms are replen-ished by outward gaseous flow, the observation of radiallyexpanding motion would be of crucial importance in prov-ing or disproving this theory. We do see outward gas mo-tions in spiral galaxies. One of the most striking exampleswe see of this is in our own galaxy. As we look towardthe center we see the famous 3-kpc arm expanding towardus with a velocity of about -55 km/s and a symmetricallyplaced arm on the other side of our Milky Way nucleusexpanding radially away from the center with about 135km/s [46]. There could hardly be more direct observa-tional evidence that spiral arms are ejected from inner re-gions.6

61n an important analysis by S. V. M. Clube [47] it is suggested that thepopulation I (young stars) in our own galaxy are drifting outward withrespect to the older stars by about 40 km/s. Then the inner arms would beexpanding symmetrically away from the nucleus each with about 95-km/svelocity.

In the next nearest galaxy to us, M31, it was alreadydemonstrated in 1960 by Munch that the gas in the innerregions was expanding. To quote Minch, "The similaritybetween the expansion phenomena in the galactic systemand M3 1 is apparent in regard to the value of the velocity,the occurrence of large scale turbulence in the plane, andthe probable value of the mass flow."

In more distant spirals the available evidence is less di-rect. The place to look for radially expanding arms isalong the projected minor axis. In the real world, how-ever, if an observer found relative positive and negativeredshifts on either side of the minor axis, he or she wouldprobably conclude that the orientation of the true minoraxis had been slightly misjudged. It is also true that in theouter regions of many spirals the arm may have lost muchof its initial outward radial motion. In some cases it mayhave already started to fall back somewhat. Perhaps anoutward moving arm meets inward falling material fromprevious ejections. The redshifts involved could be smalland the situation could be confused, so it deserves to becarefully investigated without prior assumptions.

Streaming of material along the arms is natural, for themost part, in an ejection theory of spiral arms. Observa-tions in arms around the solar sector of our galaxy revealstreaming. The situation in our own galaxy, however, maybe somewhat confused because of lack of grand design.The kinematics of assumed density waves lead to smallstreaming motions, but again there is no physical reasonfor such streaming. In the ejection theories, however, ob-served phenomenon like magnetic fields and streaming areunavoidable.

Surprisingly enough, there are even spiral galaxies that

759


show clearly ejected jets, observed sometimes in radio andsometimes in optical wavelengths. In NGC4258, the well-known spiral discussed by van der Kruit et al. [48], a pairof radio (nonthermal) arms emerge from the center as aseparate pair of arms from the thinner optical arms (Fig.10). These radio arms are more wound up than the opticalarms suggesting that the magnetized plasma has stoppedflowing and diffused and reacted more than the youngerstar arms have with an underlying rotating disk. Thiswould be in agreement with the picture put forward in thepresent paper where the star arms are rotating faster thanthe underlying disk (where the disk is appreciable). Theseradio arms in NGC4258 are compressed along the leadingedge as if a more slowly rotating disk had been interactingwith them.7

In NGC1097 straight optical jets are observed emanat-ing from the "hot spot" nucleus of the galaxy. It is notclear whether these particular jets would ever lead to for-mation of spiral arms. However, where these jets cutthrough existing spiral arms they clearly physically dis-turb these arms [51], proving that the arms are real phys-ical entities. In the few x 107 years since the northernarm was punctured, the entire outer portion of the arm hasmoved both in the direction of rotation and outward, butmoved completely intact (Fig. 11). This is not all the ef-fect one would expect from giving a blow to a transientdensity condensation. We can also remark that NGC1097is a barred spiral. In many barred spirals one can con-spicuously see evidence of material flowing out a narrowchannel in the bar, to be liberated into the spiral arms thatcome off the end of the bar.8 This is precisely the picturewe initially involved to eject the spiral arms with an initialrotational velocity.

Finally, there is an almost laboratory example of a spi-ral arm that has been formed by ejection but not in a spiralgalaxy! The famous radio source Centaurus A (originat-ing from the E galaxy NGC5128) has an X-ray and radiojet emerging from its active nucleus. Further out there arelinear luminous markings rotated slightly (Fig. 12) andstill further out they feed into large ejected radio lobes.The crucially important aspect of these linear filaments isthat they consist of strings of 0 and B stars and excitedhydrogen gas [53], as do normal spiral arms in a spiralgalaxy. There can be no doubt that these features arise byejection from the center of NGC5128. There can be nodoubt that they demonstrate the formation of stars, gas,and H II regions along the line of ejection. The wholealignment is gently curved although the atmosphere of theE galaxy through which they have been ejected is proba-

7Material visible only in the red was discovered emerging from the in-terior regions of NGC4258 by Courtes and Cruvellier in 1961 [49]. Thismaterial appears to coincide with the beginning of the southern radio arm.Recent observations by Roy et al. [50] indicate there is no narrow emissionassociated with this optical filament. It would be of great importance toinvestigate observationally the nature of this material.

5Some authors have argued that the flow is inward along the bar, re-fueling the nucleus [52]. But, if refueling is necessary, it could come inalong the poles and out in the disk (Navier-Stokes flow). It would be im-portant to settle this question observationally.

Fig. 12. The giant elliptical galaxy NGC5128 (also known as radio sourceCentaurus A). Luminous gaseous emission filaments and hot stars leadback to center of galaxy where X-ray and radio jet emerge (insert below).Photograph by John Graham with CTIO 4-m telescope; X-ray and radioobservations by Feigelson and J. 0. Bums, respectively.

bly not in strong differential rotation. There does not seemto be any continuing ejection or renewal of this arm sothat it may fade away and become undetectable in 107_108 years. As it stands, it is a perfectly empirical, per-fectly rigorous demonstration of the formation of a spiralarm by an ejection event.

X. SUMMARYThere are a number of significant differences between

ejected spiral arms and density wave spiral arms. One isthat ejected arms are real physical arms consisting of gas,stars, and magnetic fields that persist for at least several,perhaps many, galactic rotations. In contrast, it is not clearhow continually reforming density wave arms-conden-sations progressing through a disk-could be twisted,bent, broken, bifurcated, and attached to companion gal-axies, all properties that real spiral arms are observed tohave.One kind of material that spiral arms contain is plasma

which is magnetized with field lines running along thearms. Such magnetic field orientations clearly could not

760

,---J6'I


have existed in differentially rotating disks without beingwound up into a circular geometry. Therefore, the onlyway to originate such a magnetic field is by ejectionthrough magnetized inner regions, which stretches thefield out along the ejected arms.The arms once ejected can endure as connected seg-

ments for a few x 108 years before falling back underpresent mass estimates of galaxies-and longer if galaxymasses are reduced. This is time enough for the arm tofade and dissipate. It is possible that they are either re-newed along the original locus or reestablished in newpositions. The density wave theory, on the other hand,requires very broad arms with a gradient of age across thearms which should be very conspicuous but which is notobserved.Another outstanding difference between the version of

ejection theory for spiral arms presented here and the priordensity wave theory is the rotational velocity of the spiralarms. Density waves are supposed to rotate much moreslowly than the differentially rotating disk in the inner re-gions of spiral galaxies. In the ejection theory for spiralarms, the arms could rotate faster than the underlying diskthroughout the galaxy (if indeed there is any appreciabledisk present in region of best defined arms). This in turncould lead to reduced values of spiral galaxy masses anddiminish the problem of missing mass and undetectedmatter. The quantitatively flat rotation curves observedare the major challenge for any theory of spiral arms toexplain.Arms observed in real galaxies are sometimes long and

straight, requiring on the face of it, ejection origin. One-armed and three-armed spirals are observed but are notnaturally explained by density wave theory because of thesymmetry inherent in the mathematical solutions of thattheory. Radio and optical jets are observed in galaxieswhich are unquestionably of ejection origin and yet whichincorporate some or all the properties of spiral arms. Thusthe possibility of ejection formation of spiral arms hasbeen empirically demonstrated.Most fundamental of all is the fact that there is no

known physical cause of the density waves that has beenpostulated in spiral galaxies. In contrast, there is a massof accumulated data that demonstrates the widespreadcharacteristic nature of ejections from the central regionsof galaxies. These ejections taking place in rotating gal-axies must create effects that lead to the features that havepuzzled astronomers for so long, the long filaments ofbrilliant stars and glowing gas that are arranged in suchgraceful spiral forms.

ACKNOWLEDGMENTThe author would like to thank U. Anzer, T. Schmidt-

Kaler, V. Clube, and J. Frank for important criticisms ofthis text while in preparation.

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the persistent problem of spiral galaxies

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