Star Clusters in M31: Old Clusters with Bar Kinematics
H. Morrison, N. Caldwell, R. Schiavon, E. Athanassoula, A. Romanowsky, P. Harding
aa r X i v : . [ a s t r o - ph . C O ] N ov Draft version November 2, 2018
Preprint typeset using L A TEX style emulateapj v. 11/10/09
STAR CLUSTERS IN M31: OLD CLUSTERS WITH BAR KINEMATICS
Heather Morrison
Department of Astronomy, Case Western Reserve University, Cleveland OH 44106-7215electronic mail: [email protected]
Nelson Caldwell
Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USAelectronic mail: [email protected]
Ricardo P. Schiavon
Gemini Observatory, 670 N. A’ohoku Place , Hilo, HI 96720, USAelectronic mail: [email protected]
E. Athanassoula
LAM/OAMP, UMR6110, CNRS/Univ. de Provence, 38 rue Joliot Curie, 13388 Marseille 13, Franceelectronic mail: [email protected]
Aaron J. Romanowsky
UCO/Lick Observatory, University of California, Santa Cruz, CA 95064, USAelectronic mail: [email protected]
Paul Harding
Department of Astronomy, Case Western Reserve University, Cleveland OH 44106-7215electronic mail: [email protected]
Draft version November 2, 2018
ABSTRACTWe analyze our accurate kinematical data for the old clusters in the inner regions of M31. Thesevelocities are based on high S/N Hectospec data (Caldwell et al 2010). The data are well suitedfor analysis of M31’s inner regions because we took particular care to correct for contamination byunresolved field stars from the disk and bulge in the fibers. The metal poor clusters show kinematicswhich are compatible with a pressure-supported spheroid. The kinematics of metal-rich clusters,however, argue for a disk population. In particular the innermost region (inside 2 kpc) shows thekinematics of the x family of bar periodic orbits, arguing for the existence of an inner Lindbladresonance in M31. Subject headings: catalogs – galaxies: individual (M31) – galaxies: star clusters – globular clusters:general – star clusters: general INTRODUCTION
Globular clusters can provide simultaneous estimatesof velocity, metallicity and age: a powerful trio withwhich to study the history of a galaxy. They are par-ticularly helpful to complement integrated light studies,which average over all stellar populations along a lineof sight. In this paper we discuss the kinematics of oldclusters projected on the inner 10 kpc of M31. Roughlyone third of our sample of over 300 old clusters in M31(presented in Caldwell et al. 2009 and 2010, Papers 1 &2 hereafter) are located within 3 kpc of its center. Be-cause of our careful treatment of the effect of field starcontamination from the bright bulge and inner disk re-gion in our fibers, our dataset is particularly well suitedfor study of the central regions of M31.Early work on bulge kinematics (Davies et al. 1983)showed that bulges resemble low-luminosity ellipticals inbeing kinematically hot with a high degree of rotationalsupport (
V /σ ∼ R / bulges – whichare kinematically hot and formed rapidly from mergersand collapses – and bulges formed via secular evolutionof disks, which have a lower Sersic index (Kormendy& Kennicutt 2004). In this second category Athanas-soula ( 2005) distinguished the boxy/peanut bulges –which are parts of bars seen edge-on – and the disk-likebulges, which have a disk shape. Boxy/peanut bulgescan be distinguished in near-edge-on galaxies from pho-tometry or via kinematics (eg Kuijken & Merrifield 1995;Bureau & Athanassoula 1999).Evidence from isophotal twists and kinematics wasused to argue that M31 might have a triaxial bulgeor a bar (Lindblad 1956, Stark 1977, Stark & Binney1994). More recently, Athanassoula & Beaton (2006) andBeaton et al. (2007), using deep 2MASS observations,considerably strengthened the case for a bar and sug-gested that M31 also has a centrally concentrated classi- Morrison et al.cal bulge, which dominates the light in the inner 200 pc.We note here that this is a considerably smaller and lessdominant classical bulge than the one suggested by pre-vious authors: de Vaucouleurs (1958) found an effectiveradius of 3.5 kpc, and Walterbos & Kennicutt (1988) de-rived an effective radius of 2 kpc and found that the bulgecontributed 40% of the light of the galaxy. Our globularcluster kinematical data allow us to further explore thisshift in our view of M31’s bulge, since we have [Fe/H]and velocity measurements for 98 old clusters projectedwithin 3 kpc of M31’s center.We assume a distance of 770 kpc throughout(Freedman & Madore 1990) and a PA of 37.7 degrees.The XY coordinate system we use in this paper has unitsof kpc, with positive X along the major axis towards theNE. CLUSTER KINEMATICS
Paper 2 presented [Fe/H], age and velocity measure-ments based on high S/N Hectospec (Fabricant et al.2005) spectra (a median S/N of 75 per ˚ A ) for over 300M31 clusters with ages greater than 6 Gyr. (In fact thegreat majority of these clusters have ages greater than 10Gyr.) Here we discuss the old clusters from this paperwhich are within 2 kpc of M31’s major axis. Repeat Hec-tospec observations showed a median velocity error of 6km s − . Our study contains 17 entirely new cluster ve-locities and is the first fiber study to use offset exposuresnear each cluster in the bright inner regions to correct forthe contamination from field stars there. Caldwell et al.(2010) showed that ignoring this effect can lead to ve-locity errors of more than 100 km s − . In the smallnumber of cases where our velocities differed significantlyfrom the Hectochelle velocities of Strader and Caldwell(2010, in preparation), we have used the more accurateHectochelle data. Our [Fe/H] values are in good agree-ment with the recent results of Beasley et al. (2005) andColucci et al. (2009) and the HST color-magnitude de-rived values. We found that the old cluster metallicitydistribution was neither unimodal nor simply bimodal,showing a median [Fe/H] around –1.0 and possible peaksat [Fe/H] = –0.3, –0.8 and –1.4.Previous work on M31 globulars suggested a largersystemic rotation for the metal-richer clusters (egHuchra et al. 1991; Barmby et al. 2000). Since all butone of the clusters with [Fe/H] > − . mean velocity of all the stars alongthat line of sight. Figure 1 compares cluster velocities(with different colors for clusters with different metallic-ities: red for most metal rich through blue for the most metal poor) with these mean velocity estimates, shown inblack. The top panel shows clusters with [Fe/H] > − . Fig. 1.—
Velocity of clusters projected less than 2 kpc fromthe major axis. The upper panel shows the metal-rich clusters([Fe/H] > –0.6) and the lower panel the more metal-poor clusterswhich dominate M31’s old clusters. In both panels, black symbolsshow the mean velocity of stars at that position, integrated alongthe line of sight. Open symbols denote ages less than 10 Gyr,closed symbols greater than 10 Gyr (note that we are unable tomeasure ages for clusters with [Fe/H] less than –1.0, and we useclosed symbols for these clusters). The vertical grey lines show theend of the thick bar (dashed lines) and the thin bar (solid), fromBeaton et al. (2007) and Athanassoula & Beaton (2006), and thesolid black line is the rotation curve from Kent (1989). Note thatmeasurements of the dimensions of the thin and particularly thethick bar are approximate only. Metal poor clusters (lower panel) show little sign ofrotation and occupy the four quadrants of the plot simi-larly. On the other hand, the metal-rich clusters (upperpanel) show a distinct and quite cold kinematical signa-ture. There are almost no clusters in the forbidden quad-rants (occupancy here corresponds to rotation in the op-posite direction to the disk) and most of those more than2 kpc from the center (ie | X | >
2) have velocities whichclosely follow the disk velocity at that position. However,in the inner 2 kpc the signature differs from the usualone for a disk composed of stars on near-circular orbits.Although all except one cluster occupy the same quad-rant as the disk, thus respecting the same direction ofrotation, their velocities can deviate from the local meanvelocity of the integrated light by up to 350 km s − (re-call that M31’s rotation velocity is 250 km s − ). We notethat very high velocities are also observed in the HI gasin this region (Brinks & Shane 1984). In the followingsection we will describe expectations for the kinematicsof thin disk, bar and classical bulge objects, and showthat this signature is expected for bar orbits.ar Globular Clusters in M31 3 Kinematics: expectation from disk, bar and bulge
Thin disks in galaxies have “cold” kinematics domi-nated by rotation and show a low velocity dispersion.We showed in Paper 1 (see Figure 13) that the youngM31 clusters (with ages less than 2 Gyr) have such kine-matics: the young clusters all follow the same narrowlocus in position vs velocity. We also showed (see Figure12) the mean velocity field across the face of the disk, ob-tained from our “sky” fibers. The mean velocity changessmoothly and slowly as we look from the receding side ofthe disk through the center to the approaching side, asexpected for a thin disk.It is particularly simple to follow the kinematic signa-ture of a cold, thin disk by examining velocities of objectsseen close to the major axis. In a galaxy close to edge-onsuch as M31, such a star in a circular orbit will have all itsvelocity in the line of sight, giving a clean measure of V φ ,the azimuthal component, from the line-of-sight velocity.For disk stars observed at larger distances from the ma-jor axis, less of their azimuthal velocity will be projectedonto the line of sight and so the change in mean velocityfrom one side of the disk to the other will be smaller.Most orbits in bars follow the two main families ofclosed periodic orbits (Binney and Tremaine 2008): the x orbits, which are aligned along the long axis of the bar(close to the major axis in M31, see Beaton et al (2007))and x orbits which are aligned along its short axis (closeto M31’s minor axis). For x orbits, velocities can reachvery high values close to the center of the galaxy. Thisis due to the fact that they are observed near-end-on, sothat the line-of-sight component is nearly along the orbitat its pericenter (Bureau & Athanassoula 1999). Figure2 (from Binney et al. 1991) illustrates the spatial andvelocity signatures of x and x orbits. It can be seenthat in this example, the x orbits reach velocities muchhigher than the circular velocity. (A similar position-velocity diagram for a M31-like system can be seen in themiddle top panel of Figure 11 of Athanassoula & Beaton2006). Fig. 2.—
Illustration of regions occupied both spatially andkinematically by x and x orbits in a barred potential, fromBinney et al. (1991). Left panel shows their spatial location in aface-on view, while the right panel shows longitude-velocity plots. x orbits are aligned along the bar major axis and shown withsolid lines, while x orbits align perpendicular to the bar majoraxis and are shown with dotted lines. Note that in this exam-ple, the x orbits can reach velocities significantly higher than thecircular velocity, which is v=1 in this model. Lastly, we would expect any classical bulge component to show
V /σ ∼
1: some rotational support but a roughlyequivalent amount of random motion. Kent (1989) fittedthe M31 bulge using an oblate rotator model with major-axis velocity of around 90 km s − and velocity dispersionof 130 km s − at 1.5 kpc from the center. DISCUSSION
We saw in Figure 1 that the kinematics of old M31clusters with [Fe/H] > − . x orbits in M31’sbar. This is in very good agreement with orbital struc-ture in bars since the x orbits are always confined tothe innermost regions, in the region interior to the innerLindblad resonance (ILR). Most of the rest of the metal-rich clusters have orbits consistent with disk objects.We see little or no indication in the kinematics in theupper panel of Figure 1 for a kinematically hot popu-lation such as the classical bulge of Kent (1989). How-ever, we note again that the classical bulge identified byBeaton et al. (2007) was quite small, only dominatingthe inner 200 pc. We have only one cluster within 200pc of M31’s center in our sample, so we cannot probe thekinematics of this region in M31. Only in the lower panel,with the more metal-poor clusters, do we see a signaturelike that of a kinematically hot classical bulge: there areroughly equal numbers of clusters in each quadrant, andwe see that the velocity dispersion rises sharply close tothe center, as we would expect for a centrally concen-trated classical bulge. However, as we shall show below,the starlight in this region is dominated by old metal-rich stars of near solar abundance, so these metal-poorclusters are not tracing the dominant component here.To summarize: we see strong evidence from the kine-matics of the metal-rich old clusters ([Fe/H] > –0.6) forboth disk and bar kinematics. A number of the clus-ters within 2 kpc of the center of M31 show the kine-matic signature of x orbits in a barred potential. Therest of these clusters (plus the other metal-rich clusterswithin 2 kpc of the major axis) show the cold kinematicsof the disk. These kinematics strongly confirm the re-sult of Beaton et al. (2007) and Athanassoula & Beaton(2006) that M31 has a bar whose inner parts constitutethe boxy bulge which dominates its light in the inner fewkpc. To our knowledge, this is the first clear detectionof globular clusters with bar kinematics in any galaxy.However, there is one massive cluster (the Arches clus-ter) in the Milky Way which has a large space velocity(232 km s − ) and is currently at a projected distance ofonly 26 pc from the galactic center (Stolte et al. 2008).These authors note that the cluster could be on a transi-tional trajectory between x and x orbits in the MilkyWay’s barred potential, and may have been formed in astarburst triggered when a massive molecular cloud “col-lided on the boundary between x and x orbits in theinner bar”. Relations between clusters and bulge/disk field stars
We now consider the relationship between field starsand globular clusters in the inner regions of M31.Trager et al. (2000) studied the integrated light of M31’sbulge, in a circular aperture of diameter 250 pc. Theyfound a mean metallicity of +0.2 dex, and a mean age ofaround 6 Gyr. More recently, Saglia et al. (2010) havemade a detailed study of the M31 bulge region using a Morrison et al.number of long-slit exposures with the HET. They finda mean metallicity around solar, and an age of around12 Gyr in the inner 1-2 kpc. (Note that they do seea metallicity gradient, reaching up to [Z/H]=+0.4, overthe inner 200 pc, the region dominated by the classicalbulge.)Sarajedini & Jablonka (2005) used HST/WFPC2 ob-servations to produce a color-magnitude diagram forM31’s bulge at 1.6 kpc from its center, and inferreda metallicity distribution which peaked near solar.Olsen et al. (2006) summarized near IR color-magnitudediagrams from high spatial resolution studies of M31 tofind that the stellar population in the inner few kpc wasdominated by old, nearly solar-metallicity stars. Inter-estingly, by comparing fields in the bulge with an innerdisk field, they found no evidence for an age differencebetween bulge and disk. This is unsurprising if M31’sbulge is dominated by a bar, since bar stars are merelyinner disk stars which have become part of the bar pat-tern.The mean metallicity of the integrated light from fieldstars thus exceeds the mean metallicity of the globularsin the inner few kpc; it is closer to the mean of thosewith [Fe/H] > –0.6, which show either disk or bar kine-matics. (It has been suggested before that globular clus-ters are formed less efficiently in metal-rich populations:Strader et al. (2005) calculate that the efficiencies differby more than a factor of 10 in the Milky Way, by compar-ing metal-rich globular clusters to the bulge luminosityand metal-poor numbers to the halo luminosity. Thisnumber will not be changed radically if we substitutethe thick disk luminosity for the bulge luminosity in thiscalculation.)Thus a simple picture can explain the existence of themetal-rich globular clusters in M31: they merely partic-ipated in the early formation of the inner disk and theonset of the bar instability. SUMMARY
We have discussed accurate kinematical data for oldM31 clusters in its inner regions within 2 kpc of its ma-jor axis. The majority of the metal-rich clusters (thosewith [Fe/H] greater than –0.6) show disk kinematics, andmany of the clusters within the innermost bar region havethe signature of the x family. This clearly shows the ex-istence of an ILR and, to our knowledge, this is the firsttime it has been clearly shown using stellar kinematics.In the only other known example, Teuben et al. (1986)showed this using gas kinematics in the strongly barredgalaxy NGC 1365. Our result also gives an estimate ofthe ILR location, which provides useful constraints forfuture dynamical studies of M31 since it could be usedto set limits to the bar pattern speed. These metal-richclusters share the population properties (metallicity andage) of the integrated light in the inner few kpc, whichhas been studied both via spectroscopy and via deepcolor-magnitude diagrams from HST and adaptive op-tics imaging. By contrast, clusters with [Fe/H] less than–0.6 within 2 kpc of the major axis show little rotationalsupport and a velocity dispersion which increases as ra-dial distance to the center decreases.Our data do not probe the small region (200 pc) oc-cupied by M31’s classical bulge in the description ofBeaton et al. (2007), so we cannot comment on its kine-matics. However, we caution against simply interpret-ing a high velocity dispersion in M31’s inner few kpcas a bulge velocity dispersion and then using it to con-strain M31’s black hole mass (as done most recently bySaglia et al. 2010): the contribution of the bar, whichdominates the light there, needs to be assessed.HLM thanks the NSF for support under grantAST-0607518, AJR for grants AST-0808099 and AST-0909237, and EA the ANR for ANR-06-BLAN-0172.RPS is supported by Gemini Observatory, which is oper-ated by AURA, Inc, on behalf of the international Gem-ini partnership of Argentina, Australia, Brazil, Canada,Chile, the United Kingdom and the United States ofAmerica. We also thank John Wiley and Sons for per-mission to reproduce Figure 2. REFERENCESAthanassoula, E., & Beaton, R. L. 2006, MNRAS, 370, 1499Athanassoula, E. 2005, MNRAS, 358, 1477Barmby, P., Huchra, J. P., Brodie, J. P., Forbes, D. A., Schroder,L. L., & Grillmair, C. J. 2000, AJ, 119, 727Beasley, M. A., Brodie, J. P., Strader, J., Forbes, D. A., Proctor,R. N., Barmby, P., & Huchra, J. P. 2005, AJ, 129, 1412Beaton, R. L., et al. 2007, ApJ, 658, L91Binney, J., Gerhard, O. E., Stark, A. A., Bally, J., & Uchida,K. I. 1991, MNRAS, 252, 210Binney, J., Gerhard, O., & Spergel, D. 1997, MNRAS, 288, 365Binney, J. and Tremaine, S. 2008, Galactic Dynamics, PrincetonUniversity Press, Princeton NJ.Brinks, E., & Shane, W. W. 1984, A&AS, 55, 179Bureau, M., & Athanassoula, E. 1999, ApJ, 522, 686Bureau, M., & Athanassoula, E., 2005, ApJ, 626, 159Caldwell, N., Harding, P., Morrison, H., Rose, J. A., Schiavon, R.,& Kriessler, J. 2009, AJ, 137, 94Caldwell, N., Schiavon, R., Morrison, H., Rose, J. and Harding, P.2010, submitted to AJ.Colucci, J. E., Bernstein, R. A., Cameron, S., McWilliam, A., &Cohen, J. G. 2009, ApJ, 704, 385Davies, R. L., Efstathiou, G., Fall, S. M., Illingworth, G., &Schechter, P. L. 1983, ApJ, 266, 41de Vaucouleurs, G. 1958, ApJ, 128, 465 Fabricant, D., et al. 2005, PASP, 117, 1411Freedman, W. L., & Madore, B. F. 1990, ApJ, 365, 186Huchra, J. P., Brodie, J. P., & Kent, S. M. 1991, ApJ, 370, 495Kent, S. M. 1989, AJ, 97, 1614Kent, S. M. 1989, PASP, 101, 489Kormendy, J., & Kennicutt, R. C., Jr. 2004, ARA&A, 42, 603Kuijken, K., & Merrifield, M. R. 1995, ApJ, 443, L13Linblad, B. 1956, Stockholms Observatorium Annaler, Band 19,No 2.Minniti, D., Liebert, J., Olszewski, E. W., & White, S. D. M.1996, AJ, 112, 590Olsen, K. A. G., Blum, R. D., Stephens, A. W., Davidge, T. J.,Massey, P., Strom, S. E., & Rigaut, F. 2006, AJ, 132, 271Pritchet, C. J., & van den Bergh, S. 1994, AJ, 107, 1730Saglia, R. P., et al. 2010, A&A, 509, A61Sarajedini, A., & Jablonka, P. 2005, AJ, 130, 1627Stark, A. A. 1977, ApJ, 213, 368Stark, A. A., & Binney, J. 1994, ApJ, 426, L31Stolte, A., Ghez, A. M., Morris, M., Lu, J. R., Brandner, W., &Matthews, K. 2008, ApJ, 675, 1278Strader, J., Brodie, J. P., Cenarro, A. J., Beasley, M. A., &Forbes, D. A. 2005, AJ, 130, 1315Teuben, P. J., Sanders, R. H., Atherton, P. D., & van Albada,G. D. 1986, MNRAS, 221, 1 ar Globular Clusters in M31 5ar Globular Clusters in M31 5