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eaa.iop.org DOI: 10.1888/0333750888/2626 Peculiar Galaxies James L Higdon From Encyclopedia of Astronomy & Astrophysics P. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888 Downloaded on Thu Mar 02 23:33:12 GMT 2006 [131.215.103.76] Institute of Physics Publishing Bristol and Philadelphia Terms and Conditions

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Page 1: Peculiar Galaxies - Caltech Astronomygeorge/ay21/eaa/eaa... · 2006-03-02 · Peculiar Galaxies ENCYCLOPEDIA OF ASTRONOMY AND STROPHYSICS Peculiar Galaxies A peculiar galaxy is an

eaa.iop.orgDOI: 10.1888/0333750888/2626

Peculiar GalaxiesJames L Higdon

From

Encyclopedia of Astronomy & AstrophysicsP. Murdin

© IOP Publishing Ltd 2006

ISBN: 0333750888

Downloaded on Thu Mar 02 23:33:12 GMT 2006 [131.215.103.76]

Institute of Physics PublishingBristol and Philadelphia

Terms and Conditions

Page 2: Peculiar Galaxies - Caltech Astronomygeorge/ay21/eaa/eaa... · 2006-03-02 · Peculiar Galaxies ENCYCLOPEDIA OF ASTRONOMY AND STROPHYSICS Peculiar Galaxies A peculiar galaxy is an

Peculiar Galaxies E N C Y C L O P E D I A O F A S T R O N O M Y AN D A S T R O P H Y S I C S

Peculiar GalaxiesA peculiar galaxy is an object that cannot be easilyclassified as a SPIRAL, ELLIPTICAL, or IRREGULAR GALAXY basedon its optical morphology. They constitute between 5%and 10% of the known galaxy population, although most‘normal’ galaxies will show peculiar features if examinedcarefully. Peculiar galaxies show a great diversity ofform. The vast majority can be attributed to stronggravitational tides generated in the close passage of twogalaxies, to the extent that the terms ‘peculiar galaxy’and ‘interacting galaxy’ are now virtually synonymous.These objects provided the first clear evidence that agalaxy’s environment can profoundly affect its evolution(see GALAXY EVOLUTION).

Many peculiar galaxies are experiencing episodes ofenhanced star formation, called starbursts. They alsoshow a greater tendency to host ACTIVE GALACTIC NUCLEI

(AGN) compared with the normal galaxy population.Conversely, when one examines the most luminousobjects in the universe—quasars, radio galaxies andinfrared galaxies—one often finds peculiar morphologies,suggesting a direct link between interactions and extremelevels of emission. Tidally induced mergers of separategalaxies almost certainly account for a substantial fractionof the local elliptical population. This process has probablyplayed a key role in GALAXY FORMATION.

Peculiar galaxies can be used to deduce the structureof normal galaxies and in a few cases probe large-scalemass distributions. Observational and numerical studiesof these objects offer important insights into phenomenathat have shaped the formation and evolution of galaxies.

Galaxies that will not fitEDWIN HUBBLE introduced his ‘tuning fork’ classificationscheme in 1929, in which galaxies were grouped intospiral, elliptical and irregular families based on theirappearance in photographic plates. However, even atthis early stage a small number of galaxies defied thesebroad categories. Deep exposures of NGC 4038/39 (aka‘the Antennae’) and NGC 5216/18 revealed long and faintfilaments of light (see figure 1). Equally perplexing wasthe elliptical galaxy Messier 87 (M87, figure 1), whichshowed a remarkable blue linear ‘jet’ emanating from itscore. As intriguing as these objects were, they seemedto be rare exceptions and attracted little attention. Theirapparent scarcity was due to difficulties in recordingextended and low-surface-brightness emission with thetelescopes and photographic plates then in use. Thissituation changed dramatically with the advent of wide-field Schmidt cameras. On completion of the NationalGeographic–Palomar Sky Survey in 1956, the numberof peculiar galaxies grew from a mere handful to manythousands—roughly 10% of the galaxies visible in theSky Survey plates. This new population displayed abewildering array of morphologies: luminous bridges andtails, often extending many galaxy diameters, ellipticalswith faint ripples or dust lanes, ring-shaped galaxies, even

systems that appeared to be dissolving. Figure 1 showsseveral examples taken from the Digitized Sky Surveyand gives some idea of their varied (and beautiful) forms.Many of these were included in the photographic atlasesof peculiar galaxies by Vorontsov–Vel’yaminov (1956) andArp (1963), which are still widely used reference works.

To the early investigators, these striking objectssuggested gigantic explosions, galaxies in birth, galaxiesin fragmentation, material ejections from nuclei, galaxiescompelled by strong magnetic forces—or entirely newforces, even the creation of matter. However, the earliestexplanation has proven to be the most successful, namelypeculiar galaxies are the result of strong gravitational tidesgenerated by near collisions of ordinary galaxies. This wasoriginally suggested by the fact that peculiar galaxies areusually found in close pairs, and often with symmetricallyplaced features. This collisional interpretation had beenrejected initially because galaxy encounters were thoughtto be exceedingly rare and because tides were consideredincapable of creating long and narrow plumes like thoseseen in ‘the Antennae’. However, it was eventuallyrealized that galaxies exist in groups, sometimes separatedby only a few disk diameters. Galaxies should thereforeexperience several close passages over their lifetimes (seeGALAXIES: INTERACTIONS AND MERGERS). It was Alar and JuriToomre’s landmark 1972 paper that conclusively showedhow gravitational tides could produce systems like the‘the Antennae’ through a near collision of two spirals.A wide range of peculiar morphologies could in factresult depending on the relative masses, inclinations,orbital speed and collision geometry of the two galaxies.Gravitational tides vary with distance like R−3, so theyare most significant over a brief interval near closestapproach. It is here that large-scale peculiarities suchas plumes and bridges are abruptly ‘launched’ into theirtrajectories. The Toomres also extended the work ofHolmberg by suggesting that sufficient orbital angularmomentum could be lost in an encounter for two galaxiesto coalesce, producing something resembling an ellipticalgalaxy. Simple statistical considerations implied that mostellipticals in the NGC catalog could have arisen this way.The idea that galaxies were isolated island universes hadbeen dramatically overthrown.

Peculiar galaxies have been studied extensively overthe past 30 yr, both observationally and theoretically.During this period the tidal interpretation has provento be so successful that the terms ‘peculiar galaxy’ and‘interacting galaxy’ are often used interchangeably.

Peculiar galaxy morphologiesThere is a great diversity of form among peculiar galaxies,as even a quick examination of the Vorontsov–Vel’yaminovor Arp atlases will show. Even so, a number of clearcategories can be recognized:

Interacting spirals with tails and/or bridges‘Antennae’-like systems result from close passages ofsimilarly massive spirals. Bridges (e.g. NGC 2535/36)

Copyright © Nature Publishing Group 2001Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998and Institute of Physics Publishing 2001Dirac House, Temple Back, Bristol, BS1 6BE, UK 1

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require lower-mass companions to prevent quick captureand are thus less common. Both may extend 100–200 kpcfrom the host galaxies. Bridges and plumes originate inthe outer disks, and are therefore often blue in color andgas rich. Some contain dwarf-galaxy-sized condensationsof stars and gas that appear to be gravitationally bound.

Interacting spiral–elliptical pairsBecause their internal motions are not dominated byrotation, interacting ellipticals produce diffuse sprays ofstars rather than tails and bridges. In spiral–ellipticalencounters, therefore, a single tail or bridge is seen, andthe spiral typically displays the most visible damage (e.g.NGC 274/5).

Shell galaxiesRoughly 40% of ellipticals show faint interleaved shells orripples in their outer regions (e.g. Arp 230). These may bethe result of weak tidal interactions or the cannibalizationof another galaxy. The peculiar elliptical Centaurus A, forexample, has both shells and a dust lane, suggesting theconsumption of a gas-rich spiral.

Galaxies with optical jetsRemarkably linear filaments are sometimes found nearpeculiar spirals. Most are tidal tails and bridgesthat only appear linear owing to edge-brightening andprojection effects (e.g. NGC 4676). The much rarer opticalsynchrotron jets are manifestations of AGN. M87 and3C 273 are the best known examples.

cD and ’dumb-bell’ galaxiesThe centers of rich galaxy clusters are often occupied bygiant ellipticals called cD galaxies. These frequently pos-sess multiple nuclei, most likely from the cannibalizationof another cluster galaxy. An extreme form of this class are‘dumb-bell’ galaxies, which are binary ellipticals embed-ded in a common stellar envelope (e.g. NGC 750). Theseare expected to coalesce after a few orbits (i.e. ∼5×108 yr).

Merger remnantsBoth the Vorontsov–Vel’yaminov and Arp atlases includeobjects now believed to represent the late stages in themerger of two galaxies (e.g. Arp 220). At this point they areroughly elliptical in appearance, although their vestigialtidal tails, large gas masses and double nuclei—generallyvisible only at radio and infrared wavelengths—betraytheir true origin.

Polar-ring galaxiesRoughly 0.5% of known S0 galaxies possess rings orbitinga plane perpendicular to the disk. NGC 4650A (figure 2)is the prototype polar-ring galaxy. The rings are oftenrich in gas and young stars and are believed to representmaterial captured during an interaction that has settled insemistable polar orbits. Spiral–spiral mergers, in whichone is transformed into an S0, have also been proposed asa formation mechanism.

Ring galaxiesRing galaxies such as the Cartwheel (figure 2) are createdby the passage of a companion through a spiral’s disk nearthe nucleus. This interaction crowds the disk’s orbits intoa large ring (Dring ∼ 10–35 kpc) that propagates outward atspeeds of ∼50–100 km s−1. Observations show that ∼90%of the spiral’s gas supply is concentrated in the ring, alongwith essentially all star formation activity.

The expected lifetimes of these peculiarities varygreatly. The multiple nuclei of cD and dumb-bell galaxiesevolve in a deep and rapidly changing gravitationalpotential, and so are believed to be very transient, perhapsneeding less than ∼108 yr to merge. Tidal tails, bridges andsprays experience a much shallower potential, and shouldpersist much longer, perhaps several billion years.

Most spirals show slightly distorted arms or minorasymmetries in their disks. These can be attributed toweak interactions with companions. Many otherwise‘normal’ galaxies show peculiar morphologies at lowlight levels. Particularly dramatic examples have beendiscovered by David Malin using special techniques (seeDETECTORS: PHOTOGRAPHY) to explore emission as faint as 0.5%of the night-sky glow (see figure 3). These structuresmay represent long-lived remnants of ancient interactions.There is in fact a wide range of morphological odditiesamong the general galaxy population. What we have beencalling peculiar galaxies throughout this article actuallyrepresent the extreme end of a continuum.

Star formation in peculiar galaxiesIt had been long noted that peculiar galaxies tended tobe very blue in color, and often with unusually luminousstar-forming complexes. Analysis of the UV excessByurakan Survey galaxies, for example, showed that 40%possessed double nuclei or obvious tidal features. Bythe 1970s sufficient observational data had accumulatedto examine possible links between unusual morphologiesand enhanced STAR FORMATION activity. The most influentialof these early investigations was that of Larson andTinsley (1978), who compared the optical UBV colors of anormal galaxy sample taken from the Hubble atlas withpeculiar galaxies from the Arp atlas. Figure 4, takenfrom that paper, shows that the Arp atlas galaxies are onaverage significantly bluer than the normal galaxy sample(i.e. smaller U -B for a given B-V ) with a much widerspread of values. This could be simply understood ifthe peculiar galaxy sample had recently experienced briefepisodes (∼107–108 yr duration) of elevated star formationactivity called a starburst. The large color dispersionprimarily reflects differences in the starburst’s age andstrength, and/or dust obscuration. Research over the lasttwo decades has established a clear connection betweenpeculiar morphology and the occurrence of starburstactivity. Theoretical studies show that tidal interactionscan be very effective in triggering starbursts by drivingstrong spiral or ring density waves or by transportinggas to the inner nucleus. Observations show that star

Copyright © Nature Publishing Group 2001Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998and Institute of Physics Publishing 2001Dirac House, Temple Back, Bristol, BS1 6BE, UK 2

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NGC 5216/18

NGC 4676The Antennae NGC 2535/36

M 87

NGC 274/5 Arp 230

NGC 750 Arp 220

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Figure 1. A montage of peculiar galaxies reproduced from the Digitized Sky Survey. They are presented in negative to make faintemission easier to see.

formation is increased only by modest factors of 2–3 over normal galaxies when averaged over the entirepeculiar galaxy population. However, there is a widerange in star formation enhancement among individualpeculiar galaxies, from essentially zero to several ordersof magnitude. The induced star formation may be globalin extent (e.g. the Cartwheel’s ring). However, themost intense starbursts occur within the galaxy’s innerkiloparsec. These nuclear starbursts tend to be found inthe most strongly interacting systems. Excellent examplesare the luminous infrared galaxies (LIRGs, e.g. Arp 220).LIRGs emit most of their total luminosity at infraredwavelengths (λ > 5 µm), in at least half of the cases owingto dust heated by young massive stars. Astonishing starformation rates (SFR∼(102–103)M� yr−1) and gas masses(Mgas ∼ (1010–1011)M�, i.e. a substantial fraction of thetotal gas mass) characterize their inner nuclear regions.LIRGs show a strong tendency to possess unusual opticalmorphologies. In the most luminous examples, essentially

all are peculiar.It should be emphasized that not all STARBURST GALAXIES

appear highly peculiar. Also, not all peculiar galaxiesare experiencing starbursts. This may reflect time delaysbetween interaction and peak star formation, weak andquickly damped tidal perturbations, details of the collisionor a lack of sufficient gas to fuel a starburst in the first place.Nevertheless, the frequency of starburst activity is muchhigher in peculiar galaxies. And, as the intensity of thestarburst increases, so does the likelihood that the galaxywill possess a highly disturbed optical morphology.

Peculiar galaxies and active galactic nucleiThe first securely identified optical counterparts ofpowerful extragalactic radio sources (Cygnus A andCentaurus A) were strikingly peculiar. This fact suggesteda direct connection between galaxy collisions and radioemission. However, subsequent observations foundluminous radio emission to be far more likely to be

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N

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CartwheelNGC 4650A

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Figure 2. Two examples of peculiar rings: NGC 4650A (polar-ring galaxy) and the Cartwheel (ring galaxy). Both images were takenwith the Hubble Space Telescope.

M 83 M 104 M 87

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Figure 3. Enhanced photographs of nearby ‘normal’ galaxies by David Malin. The spiral galaxies M83 and M104 both show faint loopsof starlight, while the elliptical galaxy M87 is embedded in an asymmetric spray of stars. More typical exposures have been insertedinto the over-exposed centers to give an idea of their relative scales.

associated with the nuclei of rather ordinary lookingellipticals. The issue was reopened in the 1970s withthe discovery that nuclear starbursts were common inpeculiar galaxies. Theoretical results also suggested thatinteractions could efficiently transport gas to small radiiand fuel an AGN’s central engine. Recent surveys showthat while the majority of galaxies with AGN in the localuniverse do not appear obviously peculiar, there is anincreased tendency for peculiar galaxies to possess activenuclei. For example, Seyfert nuclei—the most commonlocal type of AGN—occur more frequently in peculiarspirals than in normal spirals. The significance of thisassociation increases with the degree of the morphological

distortion. Further, one-third to one-half of the mostpowerful LIRGs appear to harbor heavily obscuredAGN. These objects are invariably peculiar at opticalwavelengths. In addition, ground-based imaging of low-redshift QUASARS shows that a significant fraction possesslarge-scale distortions, multiple nuclei or at least an excessof nearby peculiar companions. HST observations confirmthat many quasars are morphologically peculiar. Forexample, of the 20 z < 0.3 quasars studied by Bahcalland collaborators (figure 5), more than a third are clearlyinteracting tidally, with half of the remainder showingfaint extended structures resembling those of local mergerremnants.

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Figure 4. The UBV two-color distributions of normal (Hubble atlas) and peculiar (Arp atlas) galaxies reproduced from Larson andTinsley (1978). The solid curve represents the mean through the Hubble atlas sample. Typical uncertainties in the colors are shown ineach panel.

Figure 5. Optical images of low-redshift (z < 0.3) quasars taken with HST (Bahcall and collaborators, 1997). Note the closecompanions and/or obvious tidal features.

Again, most galaxies with AGN are not noticeably

peculiar, and most peculiar galaxies do not possess AGN.

Nevertheless, there is at least indirect evidence for a

connection between galaxy interactions and at least some

forms of powerful AGN.

Concluding remarksThe vast majority of peculiar galaxies represent highly per-turbed ordinary galaxies. Observational and numericalstudies of these systems can therefore provide insights intogalactic structure. The kinematics of polar rings and tidaltails, for example, can be used to probe dark halos on scalesmuch larger than ordinary optical or gas disks. Detailedcomputer simulations of interacting systems not only help

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refine the input galaxy models, they also allow one to bet-ter understand how galaxies might evolve in more com-plex environments such as compact groups or rich clusters.Further, several lines of evidence suggest that galaxies areassembled through the coalescense of dwarf galaxy sizedcomponents at high redshift, and there is evidence that atleast some nearby ellipticals have arisen through mergers.The ongoing mergers and merger remnants in the peculiargalaxy population can thus serve as templates for the studyof galaxy formation and evolution. Finally, the energy re-leased by starbursts and AGN can have a major impact onthe energy balance in a galaxy’s interstellar medium. Like-wise, studies of peculiar galaxies such asArp 220 show thatstarbursts can also supply substantial amounts of heavy el-ements. Both processes appear to have been much morecommon at higher redshifts. These few examples showthat peculiar galaxies are important for what they can tellus about the structure and evolution of galaxies and forproviding local examples of processes that have shapedthe chemical and luminosity evolution of the universe.

BibliographyThe peculiar galaxy literature is vast. Fortunately, there area number of excellent reviews with extensive references,e.g.

Kennicutt R, Schweizer F and Barnes J 1996 Galaxies:Interactions and Induced Star Formation (Saas-FeeAdvanced Course 26, Lecture Notes 1996) (Swiss Societyfor Astrophysics and Astronomy)

One would also benefit by reading such classic papers as

Holmberg E 1941 Astrophys. J. 94 385Larson R and Tinsley B 1978 Astrophys. J. 219 46

and of course

Toomre A and Toomre J 1972 Astrophys. J. 178 623

Perhaps the best introduction to this subject is simplyto browse through the Vorontsov–Vel’yaminov and Arpatlases. Electronic versions exist on the web (both atlasescan be found at nedwww.ipac.caltech.edu/level5/pi-galaxies.html), although the quality of Arp’s atlas is suchthat one should try to locate a large format copy in auniversity science library.

James L Higdon

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