M31 Globular Cluster System Research Articles

Reddening, colour and metallicity of the M31 globular cluster system

Using metallicities from the literature, combined with the Revised Bologna Catalogue of photometric data for M31 clusters and cluster candidates [the latter of which is the most comprehensive catalogue of M31 clusters currently available, including 337 confirmed globular clusters (GCs) and 688 GC candidates], we determine 443 reddening values and intrinsic colours, and 209 metallicities for individual clusters without spectroscopic observations. This, the largest sample of M31 GCs presently available, is then used to analyse the metallicity distribution of M31 GCs, which is bimodal with peaks at [Fe/H] ≈− 1.7 and −0.7 dex. An exploration of metallicities as a function of radius from the M31 centre shows a metallicity gradient for the metal-poor GCs, but no such gradient for the metal-rich GCs. Our results show that the metal-rich clusters appear as a centrally concentrated spatial distribution; however, the metal-poor clusters tend to be less spatially concentrated. There is no correlation between luminosity and metallicity among the M31 sample clusters, which indicates that self-enrichment is indeed unimportant for cluster formation in M31. The reddening distribution shows that slightly more than half of the GCs are affected by a reddening of E(B − V) � 0.2 mag; the mean reddening value is E(B − V) = 0.28 +0.23 −0.14 mag. The spatial distribution of the reddening values indicates that the reddening on the north-western side of the M31 disc is more significant than that on the south-eastern side, which is consistent with the conclusion that the north-western side is nearer to us.

Metal Abundance Properties of M81 Globular Cluster System

ABSTRACTThis paper is the third in a series of papers on M81 globular clusters. In this paper, we present spatial and metal abundance properties of 95 M81 globular clusters, which comprise nearly half the entire M81 globular cluster system. These globular clusters are divided into two M81 metallicity groups by a KMM test. Our results show that the metal‐rich clusters did not demonstrate a centrally concentrated spatial distribution the way the clusters in M31 do, and metal‐poor clusters tend to be less spatially concentrated. In other words, the distribution of the metal‐rich clusters in M81 is not very similar to that of M31. The quick histogram shows that most of the metal‐rich clusters are distributed at projected radii of 4–8 kpc. Note also that the metal‐rich clusters are distributed within the inner 20 kpc, and the metal‐poor ones, out to radii of ∼40 kpc. Like our Galaxy and M31, the metallicity distribution of globular clusters in M81 along a galactocentric radius suggests that some dissipation occurred during the formation of the globular cluster system; i.e., smooth, pressure‐supported collapse models of galaxies are unlikely to produce the type of radial distribution of metallicity presented in this paper. There is no evident correlation between globular cluster luminosity and metallicity in M81 globular clusters. The overwhelming conclusion of this paper seems to be that a more complete and thorough cluster search is needed in M81.

Observational Constraints on the Formation and Evolution of Globular Cluster Systems

AbstractThis paper reviews some of the observational properties of globular cluster systems, with a particular focus on those that constrain and inform models of the formation and dynamical evolution of globular cluster systems. I first discuss the observational determination of the globular cluster luminosity and mass function. I show results from new very deep HST data on the M87 globular cluster system, and discuss how these constrain models of evaporation and the dynamical evolution of globular clusters. The second subject of this review is the question of how to account for the observed constancy of the globular cluster mass function with distance from the center of the host galaxy. The problem is that a radial trend is expected for isotropic cluster orbits, and while the orbits are observed to be roughly isotropic, no radial trend in the globular cluster system is observed. I review three extant proposals to account for this, and discuss observations and calculations that might determine which of these is most correct. The final subject is the origin of the very weak mass-radius relation observed for globular clusters. I discuss how this strongly constrains how globular clusters form and evolve. I also note that the only viable current proposal to account for the observed weak mass-radius relation naturally effects the globular cluster mass function, and that these two problems may be closely related.

Virgo’s Intracluster Globular Clusters as Seen by the Advanced Camera for Surveys

We report the discovery of four candidate intracluster globular clusters (IGCs) in a single deep HST ACS field of the Virgo Cluster. We show that each cluster is roughly spherical, has a magnitude near the peak of the Virgo globular cluster luminosity function, has a radial profile that is best fitted by a King model, and is surrounded by an excess of point sources that have the colors and magnitudes of cluster red giant stars. Despite the fact that two of our IGC candidates have integrated colors redder than the mean of the M87 globular cluster system, we propose that all of the objects are metal-poor, with [M/H] < -1. We show that the tidal radii of our intracluster globular clusters are all larger than the mean for Milky Way clusters and suggest that the clusters have undergone less tidal stress than their Galactic counterparts. Finally, we normalize our globular cluster observations to the luminosity of intracluster stars and derive a value of SN ~ 6 for the specific frequency of Virgo intracluster globular clusters. We use these data to constrain the origins of Virgo's intracluster population and suggest that globular clusters in our intracluster field have a different origin than globular clusters in the vicinity of M87. In particular, we argue that dwarf elliptical galaxies may be an important source of intracluster stars.

Massive Young Clusters in the Disk of M31

We have studied the properties of a sample of 67 very blue and likely young massive clusters in M31 extracted from the Bologna Revised Catalog of globular clusters, selected according to their color [(B - V)0 ≤ 0.45] and/or the strength of their Hβ spectral index (Hβ ≥ 3.5 A). Their existence in M31 has been noted by several authors in the past; we show here that these blue luminous compact clusters (BLCCs) are a significant fraction (15%) of the whole globular cluster system of M31. Compared to the global properties of the M31 globular cluster system, they appear to be intrinsically fainter and morphologically less concentrated, with a shallower Balmer jump and enhanced Hβ absorption in their spectra. Empirical comparison with integrated properties of clusters with known ages, as well as with theoretical simple stellar population models, consistently indicates that their typical age is less than ~2 Gyr, so they are probably not as metal-poor as would be deduced if they were older. When selecting BLCCs by either their (B - V)0 colors or the strength of their Hβ index, the cluster sample turns out to be distributed on the outskirts of the M31 disk, sharing the kinematic properties of the thin, rapidly rotating disk component. If confirmed to be young and not metal-poor, these clusters indicate the occurrence of significant recent star formation in the thin disk of M31, although they do not set constraints on the epoch of its early formation.

The Ages of Dwarf Elliptical Galaxies

We present narrow band photometry of 91 dwarf ellipticals in the Coma and Fornax clusters taken through the Stromgren (uvby) filter system. Dividing the sample by dwarf morphology into nucleated (dEN) and non-nucleated (dE) dwarfs reveals two distinct populations of early-type systems based on integrated colors. The class of dEN galaxies are redder in their continuum colors as compared to bright cluster ellipticals and dE type dwarfs, and their position in multi-color diagrams can only be explained by an older mean age for their underlying stellar populations. By comparison with the narrow band photometry of the M87 globular cluster system (Jordan et al. 2002), we find that dENs are a higher metallicity continuation of the old, metal-poor color sequence of galactic globulars and the blue population of M87 globulars. Bright ellipticals and dE dwarfs, on the other hand, follow the color sequence of the metal-rich, red population of M87 globulars. A comparison to SED models, convolved to a simple metallicity model, finds that dENs and blue globulars are 3 to 4 Gyrs older than cluster ellipticals and 5 Gyrs older than dE type galaxies. The implication is that globulars and dEN galaxies are primordial and have metallicities set by external constraints such as the enrichment of their formation clouds. Bright ellipticals and dE galaxies have metallicities and ages that suggest an extended phase of initial star formation to produce a younger mean age, even if their formation epoch is similar to that of dENs and blue globulars, and an internally driven chemical evolutionary history.

Modeling the Dynamical Evolution of the M87 Globular Cluster System

We study the dynamical evolution of the M87 globular cluster system (GCS) with a number of numerical simulations. We explore a range of different initial conditions for the GCS mass function (GCMF), for the GCS spatial distribution and for the GCS velocity distribution. We confirm that an initial power-law GCMF like that observed in young cluster systems can be readily transformed through dynamical processes into a bell-shaped GCMF. However,only models with initial velocity distributions characterized by a strong radial anisotropy increasing with the galactocentric distance are able to reproduce the observed constancy of the GCMF at all radii.We show that such strongly radial orbital distributions are inconsistent with the observed kinematics of the M87 GCS. The evolution of models with a bell-shaped GCMF with a turnover similar to that currently observed in old GCS is also investigated. We show that models with this initial GCMF can satisfy all the observational constraints currently available on the GCS spatial distribution,the GCS velocity distribution and on the GCMF properties.In particular these models successfully reproduce both the lack of a radial gradient of the GCS mean mass recently found in an analysis of HST images of M87 at multiple locations, and the observed kinematics of the M87 GCS.Our simulations also show that evolutionary processes significantly affect the initial GCS properties by leading to the disruption of many clusters and changing the masses of those which survive.The preferential disruption of inner clusters flattens the initial GCS number density profile and it can explain the rising specific frequency with radius; we show that the inner flattening observed in the M87 GCS spatial distribution can be the result of the effects of dynamical evolution on an initially steep density profile. (abridged)

M31’s Undisturbed Thin Disk of Globular Clusters

We show that there is a subsystem of the M31 globular clusters with thin-disk kinematics. These clusters span the entire metallicity range of the M31 globular cluster system, in contrast to the (thick) disk globular clusters in the Milky Way, which are predominantly metal-rich. Disk globular clusters are found across the entire disk of M31 and form ~40% of the clusters projected on its disk. The existence of such a disk system suggests that there was a relatively large thin disk in place very early in M31's history. Accurate measures of the ages of these clusters will constrain the epoch of disk formation in M31. There is currently no strong evidence for differences in age between Milky Way and M31 globular clusters. While age differences are subtle for old populations, it is unlikely that disk clusters with [Fe/H] around -2.0 were formed after significant star formation began in the galaxy, as the protocluster gas would be enriched by supernova ejecta. Thus it is likely that M31 had a rather large disk in place at early epochs. The very existence of such a cold disk means that M31 has suffered no mergers with an object of 10% or more of the disk mass since the clusters were formed. This makes the suggestion of Brown et al. that M31 could have suffered an equal-mass merger 6-8 Gyr ago less viable.

The Kinematics and Metallicity of the M31 Globular Cluster System

With the ultimate aim of distinguishing between various models describing the formation of galaxy halos (e.g., radial or multiphase collapse and random mergers), we have completed a spectroscopic study of the globular cluster system of M31. We present the results of deep intermediate-resolution fiber-optic spectroscopy of several hundred of the M31 globular clusters using the Wide Field Fibre Optic Spectrograph at the William Herschel Telescope in La Palma, Canary Islands. These observations have yielded precise radial velocities (±12 km s-1) and metallicities (±0.26 dex) for over 200 members of the M31 globular cluster population out to a radius of 15 from the galaxy center. Many of these clusters have no previous published radial velocity or [Fe/H] estimates, and the remainder typically represent significant improvements over earlier determinations. We present analyses of the spatial, kinematic, and metal abundance properties of the M31 globular clusters. We find that the abundance distribution of the cluster system is consistent with a bimodal distribution with peaks at [Fe/H] ~ -1.4 and -0.5. The metal-rich clusters demonstrate a centrally concentrated spatial distribution with a high rotation amplitude, although this population does not appear significantly flattened and is consistent with a bulge population. The metal-poor clusters tend to be less spatially concentrated and are also found to have a strong rotation signature.

Spectroscopy of Globular Clusters in M81

We present moderate-resolution spectroscopy of globular clusters (GCs) around the Sa/Sb spiral galaxy M81 (NGC 3031). Sixteen candidate clusters were observed with the Low Resolution Imaging Spectrograph on the Keck I telescope. All are confirmed as bona fide GCs, although one of the clusters appears to have been undergoing a transient event during our observations. In general, the M81 globular cluster system (GCS) is found to be very similar to the Milky Way (MW) and M31 systems, both chemically and kinematically. A kinematic analysis of the velocities of 44 M81 GCS, (the 16 presented here and 28 from previous work) strongly suggests that the red, metal-rich clusters are rotating in the same sense as the gas in the disk of M81. The blue, metal-poor clusters have halo-like kinematics, showing no evidence for rotation. The kinematics of clusters whose projected galactocentric radii lie between 4 and 8 kpc suggest that they are rotating much more than those which lie outside these bounds. We suggest that these rotating, intermediate-distance clusters are analogous to the kinematic sub-population in the metal-rich, disk GCs observed in the MW and we present evidence for the existence of a similar sub-population in the metal-rich clusters of M31. With one exception, all of the M81 clusters in our sample have ages that are consistent with MW and M31 GCs. One cluster may be as young as a few Gyrs. The correlations between absorption-line indices established for MW and M31 GCs also hold in the M81 cluster system, at least at the upper end of the metallicity distribution (which our sample probes). On the whole, the mean metallicity of the M81 GCS is similar to the metallicity of the MW and M31 GCSs. The projected mass of M81 is similar to the masses of the MW and M31. Its mass profile indicates the presence of a dark matter halo.

The Globular Clusters of M31: Keys to its Formation History

With the ultimate goal of distinguishing between various models describing the formation of galaxy halos (e.g., radial collapse, chaotic mergers), we present the results of a spectroscopic study of the globular cluster system of M31. We have obtained deep, intermediate-resolution spectra for several hundred of the M31 globular clusters using the WYFFOS fibre-fed spectrograph at the William Herschel Telescope. These observations have yielded precise radial velocities and metallicities for over 200 members of the M31 globular cluster population, the vast majority of which represent new data or significant improvements over pre-existing data.

Dynamics of the Globular Cluster System Associated with M87 (NGC 4486). II. Analysis

We present a dynamical analysis of the globular cluster system associated with M87 (=NGC 4486), the cD galaxy near the dynamical center of the Virgo Cluster. The analysis utilizes a new spectroscopic and photometric database, which is described in a companion paper. Using a sample of 278 globular clusters with measured radial velocities and metallicities and new surface density profiles based on wide-field Washington photometry, we study the dynamics of the M87 globular cluster system both globally (for the entire cluster sample) and separately (for the metal-rich and metal-poor globular cluster samples). This constitutes the largest sample of radial velocities for pure Population II tracers yet assembled for any external galaxy. Our principal findings are summarized as follows:

Dynamics of the Globular Cluster System Associated with M87 (NGC 4486). I. New CFHT MOS Spectroscopy and the Composite Database

We present a comprehensive database of kinematic, photometric, and positional information for 352 objects in the field of M87 (NGC 4486), the central giant elliptical galaxy in the Virgo Cluster; the majority of the tracers are globular clusters associated with that galaxy. New kinematic information comes from multislit observations with the Multi-Object Spectrograph (MOS) of the Canada-France-Hawaii Telescope (CFHT), an investigation that has added 96 new velocities to and confirmed many of the earlier values in a preexisting data set of 256 velocities published elsewhere. The photometry, consisting of magnitudes and colors in the Washington (T1, C-T1) system, is based on CCD observations made at the Cerro Tololo Inter-American Observatory (CTIO) and the Kitt Peak National Observatory (KPNO). The composite database represents the largest compilation of pure Population II dynamical tracers yet identified in any external galaxy; moreover, it extends to larger spatial scales than have earlier investigations. The inclusion of photometric information allows independent study of the distinct red and blue subpopulations of the bimodal globular cluster system of M87. In a companion paper we use this powerful data set to analyze the present dynamical state of the M87 globular cluster system and consider the question of its interaction and formation history.

The Kinematics of the Outer Halo of M87

Radial velocities are presented for a new sample of globular clusters in the outer halo of M87 at a distance of 300''–540'' (24–43 kpc) from the center of this galaxy. These are used to augment our previously published data, and an analysis of the rotation and velocity dispersion of the M87 globular cluster system is carried out. The rotation is ∼300 km s-1 at R = 32 kpc, at which point the velocity dispersion is also still quite high, ∼450 km s-1. The high rotation is interesting. The outer halo of M87 is, as was found in our previous kinematic analysis, very massive.

The Globular Clusters and Halo of M31

This thesis presents the results of a study of the globular cluster (GC) system in the Andromeda galaxy (5M31 5 NGC 224). Deep Vand I-band images of 24 M31 GCs were taken with the High Resolution Camera (HRCam) at the CanadaFrance-Hawaii Telescope (CFHT) with typical seeing full widths at half-maxima of . These images are the FWHM ∼ 00.7 best ground-based data to date for determining structural parameters (such as core and tidal radii, ellipticities, and orientations) of M31’s GCs. Two-dimensional seeing-convolved Michie-King models were fitted to each GC, and the results were compared to the structures of the Galactic GCs. There is no evidence for any differences between the mean tidal radii and ellipticities of the GCs in the two galaxies. Core radii and half-mass radii are too strongly affected by seeing to allow such a comparison to be made with these images. Deep ( ) Vand I-band images of fields containing V ∼ 27 lim G302 and G312, two GCs in the M31 halo, were obtained using the Wide Field Planetary Camera 2 (WFPC2) aboard the Hubble Space Telescope (HST). These data were used to construct color-magnitude diagrams (CMDs) of the two GCs and of the M31 halo. Both GCs have old stellar populations similar to those found in Galactic GCs. G302 and G312 have iron abundances of [Fe/H] 5 21.85 5 0.12 and [Fe/H] 5 20.56 5 0.03, respectively. G302 shows no evidence for a color gradient, but the inner regions of G312 are bluer than the outer regions. The halo CMDs indicate that the M31 halo stars have a mean metallicity of with a spread of — [m/H] ∼ 20.6 2 . The shape of the halo [m/H] distribution 2.0 & [m/H] & 20.2 is similar to that in the halo of the Milky Way, but the M31 halo has times that of the Galactic halo. The mean — [m/H] ∼ 8 metallicity of stars in the halo of M31 is ∼4 times greater than that of the M31 GC system, unlike the situation in the halo of the Milky Way, where both the stars and the GC system have similar mean metallicities. Finally, no evidence is found for the presence of young stars, or of a second parameter problem, in either GC. Figure 1 shows the dereddened CMDs for the two GCs and the corresponding halo fields. Structural parameters for G302 and G312 were derived from the WFPC2 images and compared to those measured on the HRCam images. The tidal radii and ellipticities from the CFHT data agree with the more reliable results obtained using the HST data. Using the same methods, core radii are found to agree when the seeing FWHM is &4 times the core radius. Star counts reveal an extended elliptical stellar halo beyond the fitted King-Michie tidal radius of G302. The size of this extended halo is consistent with stellar evaporation rates that have been predicted for Galactic GCs. The orientation of this halo is combined with the GC’s published radial velocity to derive orbital parameters for G302.

The Distance to the M31 Globular Cluster System

The distance to the centroid of the M31 globular cluster system is determined by fitting theoretical isochrones to the observed red-giant branches of fourteen globular clusters in M31. The mean true distance modulus of the M31 globular clusters is found to be 24.47 +/- 0.07 mag. This is consistent with distance modulii for M31 that have been obtained using other distance indicators.

The Dynamics of the M87 Globular Cluster System

We present the results from a study of the dynamics of the system of globular clusters around M87. After eliminating foreground galactic stars and background galaxies, we end up with a sample of 205 bona fide M87 globular clusters for which we have radial velocities determined from multislit spectra taken with the Low Resolution Imaging Spectrograph on the Keck Telescope. We find that the mean radial velocity of the M87 globular clusters agrees well with that of M87 itself and that the velocity histogram is well represented by a Gaussian distribution. We find evidence for rotation in the globular cluster system. We find that the observed velocity dispersion of the M87 globular cluster system increases with radius from 270 km s-1 at r = 9 kpc to ≈ 400 km s-1 at r = 40 kpc. The inferred mass-to-light ratio in solar units increases from 5 at r = 9 kpc to ≈ 30 at r = 40 kpc with M(r) ~ r1.7. The long-slit optical spectroscopy near the center of M87 and the recent analysis of the ROSAT X-ray data are in good agreement with this analysis near the nucleus and in the outer parts of M87, respectively.

Deep HST V- and I-Band Observations of the Halo of M31: Evidence for Multiple Stellar Populations

view Abstract Citations (74) References (54) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Deep HST V- and I-Band Observations of the Halo of M31: Evidence for Multiple Stellar Populations Holland, Stephen ; Fahlman, Gregory G. ; Richer, Harvey B. Abstract We present deep (V~27) V- and I-band stellar photometry obtained using the Hubble Space Telescope's WFPC2 in two fields in the M31 halo. These fields are located 32' and 50' from the center of M31 approximately along the SE minor axis at the locations of the M31 globular star clusters G302 and G3 12, respectively. The M31 halo luminosity functions are not consistent with a single high-metallicity population but are consistent with a mix of 50% to 75% metal-rich stars and 25% to 50% metal-poor stars. This is consistent with the observed red giant branch morphology, the luminosity of the horizontal branch, and the presence of RR Lyrae stars in the halo of M31. The morphology of the red giant branch indicates a spread in metallicity of -2 <~ [m/H]<~ -0.2 with the majority of stars having [m/H]~ -0.6, making the halo of M31 significantly more metal-rich than either the Galactic halo or the M31 globular cluster system. The horizontal branch is dominated by a red clump similar to the horizontal branch of the metal-rich Galactic globular cluster 47 Tuc but a small number of blue horizontal branch stars are visible, supporting the conclusion that there is a metal-poor component to the stellar population of the halo of M31. The number of blue horizontal branch stars is smaller than would be expected from the observed metallicity distribution but it is not clear if this is due to the photometric limits of our data or a second parameter effect in the halo of M31. We find a lower limit to the helium abundance of Y ~ 0.20 to 0.27, comparable with the Galactic value. Luminosity functions show weak evidence that the R_M31_ = 50' field contains a higher fraction of metal-poor stars than the R_M31_ = 32' field but the metallicity distributions of the RGB stars in each field strongly suggest that both fields have the same mix of stellar populations. Publication: The Astronomical Journal Pub Date: September 1996 DOI: 10.1086/118075 arXiv: arXiv:astro-ph/9606078 Bibcode: 1996AJ....112.1035H Keywords: GALAXIES: INDIVIDUAL: M31; GLOBULAR CLUSTERS: GENERAL; GALAXIES: NUCLEI; GALAXIES: STELLAR CONTENT; STARS: FORMATION; GALAXIES: FORMATION; Astrophysics E-Print: 31 pages AASTeX v4.0, 10 Figures avaliable at ftp://ftp.astro.ubc.ca/holland/M31_halo full text sources arXiv | ADS | data products SIMBAD (6) NED (1) MAST (1) ESA (1)

The M87 globular cluster system revisited

We present V and I photometry for a sample of 220 globular cluster candidates in the Virgo galaxy M87. The sample is essentially complete to V= 26. We confirm the bimodal colour distribution found by Elson & Santiago (Paper I), and find peaks at (V—I) = 0.92 and 1.23. A comparison with the colour distribution of globular clusters in the Milky Way shows that M87 contains a large population of red clusters that is virtually absent in our Galaxy. We also confirm the suggestion in Paper I that the luminosity functions of the red and blue clusters are different. The brighter clusters are predominantly blue, while the fainter clusters are roughly equally divided between red and blue. The presence of two distinct subpopulations with different mean luminosities casts doubt on the common assumption of a universal luminosity function for globular clusters.

Was the Compact Nucleus in M87 Formed by Destroyed Globular Clusters?

Imaging with the High Resolution Camera at the CFHT has been used to derive the radial distribution of globular clusters in M87 to within 10" (0.7 kpc) of its nucleus. The density profile of the globular cluster system shows a core radius of 1.0'+/- 0.1'(~4.6+/-0.5 kpc), ~30% smaller than inferred from earlier studies, but still an order of magnitude larger than the core radius of the inner galaxy light. This result is used to estimate an upper limit on the total mass which could have been removed from the M87 globular cluster system. This upper limit is only ~7.6 X 10^8^ M_sun_, which is less than 30% of the size of the compact nucleus (supermassive black hole) in M87.