The Persistent Problem of Spiral Galaxies

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<ul><li><p>IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. PS-14. NO. 6, DECEMBER 1986</p><p>The Persistent Problem of Spiral GalaxiesHALTON ARP</p><p>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.</p><p>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.</p><p>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.</p><p>I. INTRODUCTIONr HE Earl of Rosse was one of the first observers to turn</p><p>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-</p><p>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?</p><p>Manuscript received April 10, 1986; revised August 1, 1986.The author is with the Max-Planck Institut fur Physik und Astrophysik,</p><p>Institut fur Astrophysik, 8046 Garching b., Munich, FRG.IEEE Log Number 8610904.</p><p>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.</p><p>II. THE ROOTS OF DENSITY WAVE THEORY</p><p>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!</p><p>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</p><p>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</p><p>0093-3813/86/1200-0748$01 .00 1986 IEEE</p><p>748</p></li><li><p>ARP: THE PERSISTENT PROBLEM OF SPIRAL GALAXIES</p><p>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.</p><p>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</p><p>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.</p><p>III. WHAT WE ACTUALLY OBSERVE IN GALAXIESIs there an alternative approach to understanding spiral</p><p>galaxies? 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</p><p>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-</p><p>ciated with luminous matter was first brought to the atten-</p><p>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</p><p>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</p><p>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.</p><p>IV. CONFRONTATION OF THE THEORIES WITH THEOBSERVATIONS</p><p>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.</p><p>A. Observed Width of Spiral ArmsBecause the density wave's raison d'etre is to keep the</p><p>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.</p><p>749</p></li><li><p>IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. PS-14, NO. 6, DECEMBER 1986</p><p>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 ArmsAny large-scale magnetic field in the underlying disk</p><p>would 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.</p><p>C. Narrowing ofArms at EndsOne of the most certain properties that one can gener-</p><p>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.</p><p>'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.</p><p>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.</p><p>(a)</p><p>(b)Fig. 1. (a) Excited hydrogen emission in M51 shows narrow tapering arms.</p><p>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...</p></li></ul>