The ACS Virgo and Fornax Cluster Surveys

Refereed Publications from the ACS Virgo Cluster Survey (2004-2009):

 

"The ACS Virgo Cluster Survey. I. Introduction to the Survey", Cote, P., Blakeslee, J.P., Ferrarese, L., Jordan, A., Mei, S., Merritt, D., Milosavljevic, M., Peng, E.W., Tonry, J.L., & West, M.J. 2004, Astrophysical Journal Supplement Series, 153, 223.

 

The Virgo Cluster is the dominant mass concentration in the Local Supercluster and the largest     collection of elliptical and lenticular galaxies in the nearby universe. In this paper, we present an introduction to the ACS Virgo Cluster Survey: a program to image, in the F475W and F850LP bandpasses (equivalent to SDSS g and z), 100 early-type galaxies in the Virgo Cluster using the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. We describe the selection of the program galaxies and their ensemble properties, the choice of filters, the field placement and orientation, the limiting magnitudes of the survey, coordinated parallel observations of 100 "intergalactic" fields with WFPC2, and supporting ground-based spectroscopic observations of the progam galaxies. In terms of depth, spatial resolution, sample size and homogeneity, this represents the most comprehensive imaging survey to date of early-type galaxies in a cluster environment. We briefly describe the main scientific goals of the survey which include the measurement of luminosities, metallicities, ages, and structural parameters for the many thousands of globular clusters associated with these galaxies, a high-resolution isophotal analysis of galaxies spanning a factor of ~ 450 in luminosity and sharing a common environment, the measurement of accurate distances for the full sample of galaxies using the method of surface brightness fluctuations, and a determination of the three-dimensional structure of Virgo itself.

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"The ACS Virgo Cluster Survey. II. Data Reduction Procedures", Jordan, A., Blakeslee, J.P., Peng, E.W., Mei, S., Cote, P., Ferrarese, L., Tonry, J.L., Merritt, D., Milosavljevic, M., & West, M.J. 2004, Astrophysical Journal Supplement Series, 154, 509.

 

The ACS Virgo Cluster Survey is a large program to carry out multi-color imaging of 100 early-type members of the Virgo Cluster using the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. Deep F475W and F850LP images (equivalent to SDSSg and z) are being used to study the central regions of the program galaxies, their globular cluster systems, and the three-dimensional structure of Virgo itself. In this paper, we describe in detail the data reduction procedures used for the survey, including image registration, drizzling strategies, the computation of weight images, object detection, the identification of globular cluster candidates, and the measurement of their photometric and structural parameters.

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"The ACS Virgo Cluster Survey. III. Chandra and HST Observations of Low-Mass X-ray Binaries and Globular Clusters in M87", Jordan, A., Cote, P., Ferarese, L., Blakeslee, J.P., Mei, S., Merritt, D., Milosavljevic, M., Peng, E.W., Tonry, J.L., & West, M.J. 2004, Astrophysical Journal, 613, 279.

 

The ACIS instrument on board the Chandra X-ray Observatory has been used to carry out the first systematic study of low-mass X-ray binaries (LMXBs) in M87, the giant elliptical galaxy near the dynamical center of the Virgo Cluster. These images --- having a total exposure time of 154 ks --- are the deepest X-ray observations yet obtained of M87. We identify 174 X-ray point-sources, of which ~ 150 are likely LMXBs. This LMXB catalog is combined with deep F475W and F850LP images taken with ACS on HST (as part of the ACS Virgo Cluster Survey) to examine the connection between LMXBs and globular clusters in M87. Of the 1688 globular clusters in our catalog, fX = 3.6 +/- 0.5% contain a LMXB. Dividing the globular cluster sample by metallicity, we find that the metal-rich clusters are 3 +/- 1 times more likely to harbor a LMXB than their metal-poor counterparts. In agreement with previous findings for other galaxies based on smaller LXMB samples, we find the efficiency of LMXB formation to scale with both cluster metallicity, Z, and luminosity, in the sense that brighter, more metal-rich clusters are more likely to contain a LXMB. For the first time, however, we are able to demonstrate that the probability, px, that a given cluster will contain a LMXB depends sensitively on the dynamical properties of the host cluster. Specifically, we use the HST images to measure half-light radii, concentration indices and central densities, ρ0, for each globular, and define a parameter, Γ, which is related to the tidal capture and binary-neutron star exchange rate. Our preferred form for px is then

                                                    px ∝ Γρ0-0.42±0.11 Z/Zsun0.33±0.1.

We argue that if the form of px is determined by dynamical processes, then the observed metallicity dependence is a consequence of an increased number of neutron stars per unit mass in metal-rich globular clusters. Finally, we present a critical examination of the LXMB luminosity function in M87 and re-examine the LXMB luminosity functions for M49 and NGC 4697. We find no compelling evidence for a break in the luminosity distribution of resolved X-ray point sources in any of these galaxies. Instead, the LMXB luminosity function in all three galaxies is well described by a power law with an upper cutoff at LX ~ 1039 erg/s.

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"The ACS Virgo Cluster Survey. IV. Data Reduction Procedures for Surface Brightness Fluctuation Measurements with the Advanced Camera for Surveys", Mei, S., Blakeslee, J.P., Tonry, J.L., Jordan, A., Peng, E.W., Cote, P., Ferrarese, L., Merritt, D., Milosavljevic, M., & West, M.J. 2005, Astrophysical Journal Supplement Series, 156, 113.
 

The Advanced Camera for Surveys (ACS) Virgo Cluster Survey is a large program to image 100 early-type members Virgo galaxies using the F475W and F850LP bandpasses of the Wide Field Channel of the ACS instrument on the Hubble Space Telescope (HST). The scientific goals of this survey include an exploration of the three-dimensional structure of the Virgo Cluster and a critical examination of the usefulness of the globular cluster luminosity function as a distance indicator. Both of these issues require accurate distances for the full sample of 100 program galaxies. In this paper, we describe our data reduction procedures and examine the feasibility of accurate distance measurements using the method of surface brightness fluctuation (SBF) applied to the ACS Virgo Cluster Survey F850LP imaging. The ACS exhibits significant geometrical distortions due to its off-axis location in the HST focal plane; correcting for these distortions by resampling the pixel values onto an undistorted frame results in pixel correlations that depend on the nature of the interpolation kernel used for the resampling. This poses a major challenge for the SBF technique, which normally assumes a flat power spectrum for the noise. We investigate a number of different interpolation kernels and show through an analysis of simulated galaxy imageshaving realistic noise properties that it is possible, depending on the kernel, to measure SBF distances using distortion-corrected ACS images without introducing significant additional error from the resampling. We conclude by showing examples of real power spectra from our survey.

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"The ACS Virgo Cluster Survey. V. SBF Calibration for Giant and Dwarf Early-Type Galaxies", Mei, S., Blakeslee, J.P., Tonry, J.L., Jordan, A., Peng, E.W., Cote, P., Ferrarese, L., West, M.J., Merritt, D., & Milosavljevic, M. 2005, Astrophysical Journal, 625, 121

 

As part of the Advanced Camera for Survey (ACS) Virgo Cluster Survey, we have measured Surface Brightness Fluctuations (SBF) in a sample of 100 early-type Virgo galaxies. Distances derived from these measurements are needed to explore the three-dimensional structure of the Virgo Cluster, study the intrinsic parameters of globular clusters associated with the program galaxies, and compare with the galaxy distances derived from globular cluster luminosity functions. Our SBF measurements have been performed in the F850LP bandpass of the Wide Field Channel of the ACS on the Hubble Space Telescope. These are the first measurements of this kind, and we present the first SBF calibration for this bandpass. The measured fluctuations depend on galaxy stellar population variations, which we quantify by galaxy color (g475 - z850)0, where g475 and z850 are the galaxy magnitudes, respectively, in the F475W and F850LP ACS filters. We derive the following calibration for the absolute SBF magnitude M850:

                    M850 = (-2.06  0.04) + (2.0  0.2)[ (g475 - z850)0 - 1.3 ]

in the range 1.3 < (g475-z850)0< 1.6, and

                    M850 = (-2.06  0.04) + (0.9  0.2)[ (g475 - z850)0 - 1.3 ]

in the range 1.0 < (g475 - z850)0 < 1.3.The quoted zero-point uncertainty here includes all sources of internal error; there is an additional systematic uncertainty of ~ 0.15 mag due to the uncertainty in the distance scale calibration. Physically, the two different color regimes correspond to different galaxy types: giant ellipticals and S0s at the red end, and early-type dwarfs at the blue end. For the first time in SBF studies, we are able to provide a firm empirical calibration of SBF in early-type dwarf galaxies. Our results agree with stellar population model predictions from Bruzual & Charlot (2003) in the range 1.3 < (g475 −z850)0< 1.6, while our empirical slope is somewhat steeper than the theoretical prediction in the range 0.9 < (g475 - z850)0 < 1.3.

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"The ACS Virgo Cluster Survey. VI. Isophotal Analysis and the Structure of Early-Type Galaxies", Ferrarese, L., Cote, P., Jordan, A., Peng, E.W., Blakeslee, J.P., Piatek, S., Mei, S., Merritt, D., Milosavljevic, M., Tonry, J.L., & West, M.J. 2006, Astrophysical Journal Supplement Series, 164, 334

 

We present a detailed analysis of the morphology, isophotal parameters and surface brightness profiles for 100 early-type members of the Virgo Cluster, from dwarfs (MB = −15.1 mag) to giants (MB = −21.8 mag). Each galaxy has been imaged in two filters, closely resembling the Sloan g and z passbands, using the Advanced Camera for Surveys on board the Hubble Space Telescope.

Dust and complex morphological structures are common. Dust is detected in as many as 18, preferentially bright, galaxies. The incidence rate in the 26 galaxies brighter than BT = 12.15 mag, which form a magnitude limited sample, is 42%. The amount and distribution of dust show no obvious correlations with galaxy morphology; dust features range from faint wisps and patches on tens of parsec scales, to regular, highly organized kpc-scale dust disks. Blue star clusters are interspersed within the larger, clumpier dust disks, while thin, dynamically cold stellar disks are seen in association with the smaller, uniform nuclear dust disks.

Kiloparsec-scale stellar disks, bars, and nuclear stellar disks are seen in 60% of galaxies with intermediate luminosity (−20 < MB < −17). In at least one case (VCC 1938 = NGC 4638), the large-scale stellar disk has a sharp inner edge, possibly produced when disk instabilities led to the formation of a (now dissolved) bar. This process might indeed be seen unfolding in one galaxy, VCC 1537 (=NGC 4528). A spiral structure might be present in VCC 1199, an elliptical companion of M49. In dwarf galaxies, spiral structures are confirmed in VCC 856 and detected for the first time in VCC 1695.

Surface brightness profiles, ellipticities, major axis position angles, and isophotal shapes are derived typically within 8 kpc from the center for the brightest galaxies, and 1.5 kpc for the faintest systems, with a resolution (FWHM) of 7 pc. For all but 10 of the galaxies, the surface brightness profiles are well described by a Sersic model with index $n$ which increases steadily from the fainter to the brightest galaxies. In agreement with previous claims, the inner profiles (typically within 100 pc of the center) of eight of the 10 brightest galaxies, to which we will refer as "core'' galaxies, are lower than expected based on an extrapolation of the outer Sersic model, and are better described by a single power-law function. Core galaxies are clearly distinct in having fainter central surface brightness, m0, and shallower logarithmic slope of the inner surface brightness profile, g, than expected based on the extrapolation of the trend followed by the rest of the sample, for which both m0 and g increase steadily with galaxy magnitude. Large-scale, global properties also set core galaxies apart: the effective radius in particular is found to be almost one order of magnitude larger than for only slightly less luminous non-core galaxies. Contrary to previous claims, we find no evidence in support of a strong bimodal behavior of the inner profile slope, g; in particular the g distribution for galaxies which do not show evidence of multiple morphological components (disks or bars) is unimodal across the entire magnitude range (a factor 460 in B-band luminosity) spanned by the ACSVCS galaxies. Although core galaxies have shallow inner profiles, the shallowest profiles (lowest g values) are found in faint dwarf systems. The widely adopted separation of early-type galaxies between "core" and "power-law" types, which had originally been prompted by the claim of a clear bimodal distribution of g values, is therefore untenable based on the present study.

Once core galaxies are removed, dwarf and bright ellipticals display a continuum in their morphological parameters, contradicting some previous beliefs that the two belong to structurally distinct classes. However, dwarfs span a wider range in morphological characteristics than brighter systems: their surface brightness profiles vary from exponential to almost r1/4 laws, they comprise both nucleated and non-nucleated varieties, and several systems display evidence of disks, spiral structures and recent star formation. This is taken as evidence that dwarf galaxies, as currently classified, form an heterogeneous class.

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"The ACS Virgo Cluster Survey. VII. Resolving the Connection Between Globular Clusters and Ultra-Compact Dwarf Galaxies”, Hasegan, M., Jordan, A., Cote, P., Djorgovski, S.G., McLaughlin, D.E., Blakeslee, J.P., Mei, S., West, M., Peng, E.W., Ferrarese, L., Milosavljevic, M., Tonry, J.L., & Merritt, D. 2005, Astrophysical Journal, 627, 203.
 

We investigate the connection between globular clusters and ultra-compact dwarf galaxies (UCDs) by examining the properties of ten compact, high-luminosity (-11.8 < MV < -10.8) objects associated with M87 (NGC4486, VCC1316), the cD galaxy in the Virgo Cluster. These objects, most of which were previously classified as M87 globular clusters, were selected from a combination of ground- and space-based imaging surveys. Our observational database for these objects --- which we term DGTOs or "dwarf-globular transition objects" --- includes Advanced Camera for Survey (ACS) F475W and F850LP imaging from ACS Virgo Cluster Survey, integrated-light spectroscopy from Keck/ESI, and archival F606W WFPC2 imaging. We also present a search for DGTOs associated with other galaxies based on ACS imaging for 100 early-type galaxies in Virgo. Our main findings can be summarized as follows:

1. Out of the six DGTOs in M87 with both ground-based spectroscopy and HST imaging, we find two objects to have half-light radii, velocity dispersions and mass-to-light ratios that are consistent with the predictions of population synthesis models for old, metal-rich, high-luminosity globular clusters.
 

2. Three other DGTOs are much larger, with half-light radii rh ~ 20 pc, and have V-band mass-to-light ratios in the range 6 < M/LV < 9. These objects, which we consider to be UCDs, resemble the nuclei of nucleated dwarf elliptical galaxies in the Virgo Cluster, having similar mass-to-light ratios, luminosities, and colors.
 

3. The classification of the sixth object is more uncertain, but it bears a strong resemblance to simulated "stellar superclusters" which are presumed to form through the amalgamation of multiple young massive clusters.
 

4. In general, the UCDs in M87 are found to follow the extrapolated scaling relations of galaxies more closely than those of globular clusters. There appears to be a transition between the two types of stellar systems at a mass of ~ 2x106 solar masses. We suggest that the presence of dark matter may be the fundamental property distinguishing globular clusters from UCDs.
 

5. We identify a sample of 13 DGTO candidates from the complete ACS Virgo Cluster Survey, selecting on the basis of half-light radius, magnitude and color. For a number of these objects, membership in Virgo can be established through radial velocities or surface brightness fluctuation measurements with our ACS images. Three of these DGTO candidates are embedded in low-surface brightness envelopes.
 

6. Five of the 13 DGTOs in Virgo are associated with a single galaxy: M87. This finding suggests that proximity to the Virgo center may be of critical importance for the formation of these objects, although we find M87 to be more abundant in DGTOs than would be expected on the basis of its luminosity, the size of its globular cluster system, or the local galaxy density.
 

These results show that distinguishing bonafide UCDs from high-luminosity globular clusters requires a careful analysis of their detailed structural and dynamical properties, particularly their mass-to-light ratios. In general, the properties of the UCDs in our sample are consistent with models in which these objects form through tidal stripping of nucleated dwarf galaxies.

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"The ACS Virgo Cluster Survey. VIII. The Nuclei of Early-Type Galaxies", Cote, P., Piatek, S., Ferrarese, L., Jordan, A., Merritt, D., Peng, E.W., Hasegan, M., Blakeslee, J.P., Mei, S., West, M.J., Milosavljevic, M., & Tonry, J.L., 2005, Astrophysical Journal Supplement Series, 165, 57.
 

The ACS Virgo Cluster Survey is a Hubble Space Telescope program to obtain high-resolution imaging, in widely separated bandpasses (F475W ~g and F850LP ~z), for 100 early-type members of the Virgo Cluster, spanning a range of ~460 in blue luminosity. We use this large, homogenous dataset to examine the innermost structure of these galaxies and to characterize the properties of their compact central nuclei. We present a sharp upward revision in the frequency of nucleation in early-type galaxies brighter than MB ~ −15 (66 ≤ fn ≤ 82%) and show that ground-based surveys underestimated the number of nuclei due to surface brightness selection effects, limited sensitivity and poor spatial resolution. We speculate that previously reported claims that nucleated dwarfs are more concentrated to the center of Virgo than their non-nucleated counterparts may be an artifact of these selection effects. There is no clear evidence from the properties of the nuclei, or from the overall incidence of nucleation, for a change at MB ~ −17.6, the traditional dividing point between dwarf and giant galaxies. There does, however, appear to be a fundamental transition at MB ~ −20.5, in the sense that the brighter, "core-Sersic" galaxies lack resolved (stellar) nuclei. A search for nuclei which may be offset from the photocenters of their host galaxies reveals only five candidates with displacements of more than 0.5 arcsec, all of which are in dwarf galaxies. In each case, though, the evidence suggests that these "nuclei" are, in fact, globular clusters projected close to the galaxy photocenter. Working from a sample of 51 galaxies with prominent nuclei, we find a median half-light radius of <rh> = 4.2 pc, with the sizes of individual nuclei ranging from 62 pc down to ≤ 2 pc (i.e., unresolved in our images) in about a half dozen cases. Excluding these unresolved objects, the nuclei sizes are found to depend on nuclear luminosity according to the relation rh ∝ L0.500.03. Because the large majority of nuclei are resolved, we can rule out low-level AGN as an explanation for the central luminosity excess in almost all cases. On average, the nuclei are ~3.5 mag brighter than a typical globular cluster. Based on their broadband colors, the nuclei appear to have old to intermediate-age stellar populations. The colors of the nuclei in galaxies fainter than MB ~ −17.6 are tightly correlated with their luminosities, and less so with the luminosities of their host galaxies, suggesting that their chemical enrichment histories were governed by local or internal factors. Comparing the nuclei to the "nuclear clusters" found in late-type spiral galaxies reveals a close match in terms of size, luminosity and overall frequency. A formation mechanism that is rather insensitive to the detailed properties of the host galaxy is required to explain this ubiquity and homogeneity. The mean of the frequency function for the nucleus-to-galaxy luminosity ratio in our nucleated galaxies, <log10η> = −2.490.09 dex (σ = 0.590.10), is indistinguishable from that of the SBH-to-bulge mass ratio, <log10M•/Mgal> = −2.610.07 dex (σ = 0.450.09), calculated in 23 early-type galaxies with detected supermassive black holes (SBHs). We argue that the compact stellar nuclei found in many of our program galaxies are the low-mass counterparts of the SBHs detected in the bright galaxies. If this interpretation is correct, then one should think in terms of Central Massive Objects − either SBHs or compact stellar nuclei −that accompany the formation of almost all early-type galaxies and contain a mean fraction ~0.3% of the total bulge mass. In this view, SBHs would be the dominant formation mode above MB ~ −20.5.

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"The ACS Virgo Cluster Survey. IX. The Color Distribributions of Globular Cluster Systems in Early-Type Galaxies", Peng, E.W., Jordan, A., Cote, P., Blakeslee, J.P., Ferrarese, L., Mei, S., West, M.J., Merritt, D., Milosavljevic, M., & Tonry, J.L. 2005, Astrophysical Journal, 639, 95
 

We present the color distributions of globular clusters (GC) systems for 100 Virgo cluster early-type galaxies observed in the ACS Virgo Cluster Survey, the deepest and most homogeneous survey of this kind to date. We find that while the color distributions of individual GC systems can show significant variations from one another, their general properties are consistent with continuous trends across galaxu luminosity, color, and stellar mass. On average, galaxies at all luminosities in our study (−22 < MB < −15) appear to have bimodal or asymmetric GC color distributions. Almost all galaxies possess a component of metal-poor GCs, with the average fraction of metal-rich GCs ranging from 15 to 60% as a function of galaxy luminosity and color. The colors of both subpopulations correlate with host galaxy luminosity and color, with the red GCs having a steeper slope. The steeper correlation seen in the mean color of the entire GC system is driven by the increasing fraction of metal-rich GCs for more luminous galaxies.

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To convert color to metallicity, we also introduce a preliminary (g-z)-[Fe/H] relation calibrated to Galactic, M49 and M87 GCs. This relation is nonlinear with a steeper slope for [Fe/H] < −0.8. As a result, the metallicities of the metal-poor and metal-rich GCs vary similarly with respect to galaxy luminosity and stellar mass, with relations of [Fe/H]MP ~L0.160.04~M*0.170.04 and [Fe/H]MR ~ L0.260.03 ~ M*0.220.03, respectively. While these relations are shallower than the mass-metallicity relation predicted by wind models and observed for dwarf galaxies, they are very similar to the mass-metallicity relation for star forming galaxies in the same mass range. The offset between the two GC populations varies slowly (~ M*0.05) and is approximately 1 dex across three orders of magnitude in mass, suggesting a nearly universal amount of enrichment between the formation of the two populations of GCs. We also find that although the metal-rich GCs show a larger dispersion in color, it is the metal-poor GCs that have an equal or larger dispersion in metallicity. The similarity in the M*-[Fe/H] relations for the two populations, however, imply that the conditions of GC formation for the metal-poor and metal-rich GCs could not have been too different. Like the color-magnitude relation, these relations derived globular clusters present stringent constraints on the formation and evolution of early-type galaxies.

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"The ACS Virgo Cluster Survey. X. Half-light Radii of Globular Clusters in Early-Type Galaxies: Environmental Dependencies and a Standard Ruler for Distance Estimation", Jordan, A., Cote, P., Blakeslee, J.P., Ferrarese, L., McLaughlin, D., Mei, S., Peng, E.W., Tonry, J.L., Merritt, D., Milosavljevic, M., Sarazin, C.L., Sivakoff, & West, M. 2005, Astrophysical Journal, 634, 1002
 

We have measured half-light radii, rh, for thousands of globular clusters (GCs) belonging to the one hundred early-type galaxies observed in the ACS Virgo Cluster Survey and the elliptical galaxy NGC4697. An analysis of the dependencies of the measured half-light radii on both the properties of the GCs themselves and their host galaxies reveals that, in analogy with GCs in the Galaxy but in a milder fashion,  the average half-light radius increases with increasing galactocentric distance or, alternatively, with decreasing galaxy surface brightness. For the first time, we find that the average half-light radius decreases with the host galaxy color. We also show that there is no evidence for a variation of rh with the luminosity of the GCs. Finally, we find in agreement with previous observations that the average rh depends on the color of GCs, with red GCs being ~ 17 % smaller than their blue counterparts. We show that this difference is probably a consequence of an intrinsic mechanism, rather than projection effects, and that it is in good agreement with the mechanism proposed in Jordan (2004). We discuss these findings in light of two simple pictures for the origin of the rh of GCs and show that both lead to a behavior in rough agreement with the observations. After accounting for the dependencies on galaxy color, galactocentric radius and underlying surface brightness, we show that the average GC half-light radii <rh>can be successfully used as a standard ruler for distance estimation. We outline the methodology, provide a calibration for its use, and discuss the prospects for this distance estimator with future observing facilities. We find  <rh> = 2.70.35 pc for GCs with (g-z) = 1.2 mag in a galaxy with color (g-z)gal = 1.5 mag and at an underlying surface z-band brightness of mz = 21 mag arcsec-2. Using this technique, we place an upper limit of 3.4 Mpc on the 1-s line-of-sight depth of the Virgo Cluster. Finally, we examine the form of the rh distribution for our sample galaxies and provide an analytic expression which successfully describes this distribution.

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"The ACS Virgo Cluster Survey. XI. The Nature of Diffuse Star Clusters in Early-Type Galaxies", Peng, E.W., Cote, P., Jordan, A., Blakeslee, J.P., Ferrarese, L., Mei, S., West, M.J., Merritt, D., Milosavljevic, M., Tonry, J.T. 2005, Astrophysical Journal, 639, 838
 

We use HST/ACS imaging of 100 early-type galaxies in the ACS Virgo Cluster Survey to investigate the nature of diffuse star clusters (DSCs). Compared to globular clusters (GCs), these star clusters have moderately low luminosities (MV > -8) and a broad distribution of sizes (3 < rh < 30 pc), but they are principally characterized by their low mean surface brightnesses which can be more than three magnitudes fainter than a typical GC (mug> 20 mag arcsec-2). The median colors of diffuse star cluster systems are red, 1.1 < g-z < 1.6, which is redder than metal-rich GCs and often as red as the galaxy itself. Most DSC systems thus have mean ages older than 5 Gyr or else have super-solar metallicities implying that diffuse star clusters are likely to be long-lived, surviving for significant fraction of a Hubble time. We find that 12 galaxies in our sample contain a significant excess of diffuse star cluster candidates. Nine of them are morphologically classified as lenticulars (S0s), and five of them visibly contain dust. We also find a substantial population of DSCs in the halo of the giant elliptical M49, associated with the companion galaxy VCC 1199. Most DSC systems appear to be both aligned with the galaxy light and associated with galactic disks, but at the same time many lenticular galaxies do not host substantial DSC populations, and environment and clustercentric radius do not appear to be good predictors of their existence. Diffuse star clusters in our sample share similar characteristics to those identified in other nearby lenticular, spiral, and dwarf galaxies. Unlike luminous GCs, whose sizes are constant with luminiosity, DSCs are bounded at the bright end by an envelope of nearly constant surface brightness. We suggest that populations of diffuse star clusters preferentially form, survive, and coevolve with galactic disks. Their properties are broadly consistent with those of merged star cluster complexes, and we note that despite being 3-5 magnitudes brighter than DSCs, ultra-compact dwarfs have similar surface brightnesses. The closest Galactic analogs to the DSCs are the old open clusters. We suggest that if a diffuse star cluster population did exist in the disk of the Milky Way, it would be very difficult to find.

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"A Fundamental Relation Between Compact Stellar Nuclei, Supermassive Black Holes, and Their Host Galaxies", Ferrarese, L., Cote, P., Dalla Bonta, E., Peng, E.W., Merritt, D., Jordan, A., Blakeslee, J.P., Hasegan, M., Mei, S., Piatek, S., Tonry, J.L., & West, M.J. 2006, Astrophysical Journal Letters, 644, L21

Imaging surveys with the Hubble Space Telescope (HST) have shown that roughly 50-80% of low- and intermediate-luminosity galaxies contain a compact stellar nucleus at their center, regardless of host galaxy morphological type. We combine HST imaging for early-type galaxies from the ACS Virgo Cluster Survey with ground-based long-slit spectra from KPNO to show that the masses of compact stellar nuclei in Virgo Cluster galaxies obey a tight correlation with the masses of the host galaxies. The same correlation is obeyed by the supermassive black holes (SBHs) found in predominantly massive galaxies. The compact stellar nuclei in the Local Group galaxies M33 and NGC205 are also found to fall along this same scaling relation. These results indicate that a generic by-product of galaxy formation is the creation of a Central Massive Object (CMO) -- either a SBH or a compact stellar nucleus -- that contains a mean fraction, ~ 0.2%, of the total galactic mass. In galaxies with masses greater than a few times 1010 solar masses, SBHs appear to be the dominant mode of CMO formation.

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"The ACS Virgo Cluster Survey. XII. The Luminosity Function of Globular Clusters in Early-Type Galaxies", Jordan, A., McLaughlin, D.E., Cote, P., Ferrarese, L., Peng, E.W., Mei. S., Villegas, D., Merritt, D., Tonry, J.L., & West, M.J. 2007, Astrophysical Journal Supplement Series, 171, 101

We analyze the luminosity function of the globular clusters (GCs) belonging to the early-type galaxies observed in the ACS Virgo Cluster Survey. We have obtained maximum likelihood estimates for a Gaussian representation of the globular cluster luminosity function (GCLF) for 89 galaxies. We have also fit the luminosity functions with an “evolved Schechter function”, which is meant to reflect the preferential depletion of low-mass GCs, primarily by evaporation due to two-body relaxation, from an initial Schechter mass function similar to that of young massive clusters in local starbursts and mergers. We find a highly significant trend of the GCLF dispersion s with galaxy luminosity, in the sense that the GC systems in smaller galaxies have narrower luminosity functions. The GCLF dispersions of our Galaxy and M31 are quantitatively in keeping with this trend, and thus the correlation between s and galaxy luminosity would seem more fundamental than older notions that the GCLF dispersion depends on Hubble type. We show that this narrowing of the GCLF in a Gaussian description is driven by a steepening of the cluster mass function above the classic turnover mass, as one moves to lower-luminosity host galaxies. In a Schechter-function description, this is reflected by a steady decrease in the value of the exponential cut-off mass scale. We argue that this behavior at the high-mass end of the GC mass function is most likely a consequence of systematic variations of the initial cluster mass function rather than long term dynamical evolution. The GCLF turnover mass MTO is roughly constant at MTO ~ (2.2 ± 0.4)105 solar masses in bright galaxies but it decreases slightly (by ~ 35% on average, with significant scatter) in dwarf galaxies with MB > -18. It could be important to allow for this effect when using the GCLF as a distance indicator. We show that part, though perhaps not all, of the variation could arise from the shorter dynamical friction timescales in less massive galaxies. We probe the variation of the GCLF to projected galactocentric radii of 20–35 kpc in the Virgo giants M49 and M87, finding that the turnover point is essentially constant over these spatial scales. Our fits of evolved Schechter function simply average dynamical mass losses (D) over a Hubble time that vary more than MTO, and systematically but non-monotonically as a function of galaxy luminosity. If the initial GC mass distributions rose steeply toward slow masses as we assume, then these losses fall in the range 2x105 < D < 106 solar masses per GC for all of our galaxies. The trends in D are broadly consistent with observed, small variations of the mean GC half-light radius in ACSVCS galaxies, and with rough estimates of the expected scaling of average evaporation rates (galaxy densities) versus total luminosity. We agree with previous suggestions that if the full GCLF is to be understood in more detail, especially alongside other properties of GC systems, the next generation of GCLF models will have to include self-consistent treatments of dynamical evolution inside time-dependent galaxy potentials.

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"Trends in the Globular Cluster Luminosity Function of Early-Type Galaxies", Jordan, A., McLaughlin, D.E., Cote, P., Ferrarese, L., Peng, E.W., Blakeslee, J.P., Mei. S., Villegas, D., Merritt, D., Tonry, J.L., & West, M.J. 2006, Astrophysical Journal Letters, 651, L25
 

We present results from a study of the globular cluster luminosity function (GCLF) in a sample of 89 early-type galaxies observed as part of the ACS Virgo Cluster Survey. Using a Gaussian parametrization of the GCLF, we find a highly significant correlation between the GCLF dispersion, sigma, and the galaxy luminosity, M_B, in the sense that the GC systems in fainter galaxies have narrower luminosity functions. The GCLF dispersions in the Milky Way and M31 are fully consistent with this trend, implying that the correlation between sigma and galaxy luminosity is more fundamental than older suggestions that GCLF shape is a function of galaxy Hubble type. We show that the sigma - MB relation results from a bonafide narrowing of the distribution of (logarithmic) cluster masses in fainter galaxies. We further show that this behavior is mirrored by a steepening of the GC mass function for relatively high masses, M >~ 3 x 105 Msun, a mass regime in which the shape of the GCLF is not strongly affected by dynamical evolution over a Hubble time. We argue that this trend arises from variations in initial conditions and requires explanation by theories of cluster formation. Finally, we confirm that in bright galaxies, the GCLF "turns over" at the canonical mass scale of MTO ~ 2 x 105 Msun. However, we find that MTO scatters to lower values (~1-2 x 105 Msun) in galaxies fainter than MB >~ -18.5, an important consideration if the GCLF is to be used as a distance indicator for dwarf ellipticals.

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"The ACS Virgo Cluster Survey. XIII. SBF Distance Catalog and the Three-Dimensional Structure of the Virgo Cluster", Mei, S., Blakeslee, J.P., Cote, P., Tonry, J.L., West, M.J., Ferrarese, L., Jordan, A., Peng, E.W., Anthony, A., & Merritt, D. 2007, Astrophysical Journal, 655, 144
 

The ACS Virgo Cluster Survey consists of HST ACS imaging for 100 early-type galaxies in the Virgo Cluster, observed in the F475W (SDSS g) and F850LP (SDSS z) filters. We derive distances for 84 of these galaxies using the method of surface brightness fluctuations (SBFs), present the SBF distance catalog, and use this database to examine the three-dimensional distribution of early-type galaxies in the Virgo Cluster. The SBF distance moduli have a mean (random) measurement error of 0.07 mag (0.5 Mpc), or roughly 3 times better than previous SBF measurements for Virgo Cluster galaxies. Five galaxies lie at a distance of d ~ 23 Mpc and are members of the W' cloud. The remaining 79 galaxies have a narrow distribution around our adopted distance of <d> = 16.5 +/- 0.1 (random mean error) +/- 1.1 Mpc (systematic). The rms distance scatter of this sample is s(d) = 0.6 +/- 0.1 Mpc, with little or no dependence on morphological type or luminosity class (i.e., 0.7 +/- 0.1 and 0.5 +/- 0.1 Mpc for the giants and dwarfs, respectively). The back-to-front depth of the cluster measured from our sample of early-type galaxies is 2.4 +/- 0.4 Mpc (i.e., 2s of the intrinsic distance distribution). The M87 (cluster A) and M49 (cluster B) subclusters are found to lie at distances of 16.7 +/- 0.2 and 16.4 +/- 0.2 Mpc, respectively. There may be a third subcluster associated with M86. A weak correlation between velocity and line-of-sight distance may be a faint echo of the cluster velocity distribution not having yet completely virialized. In three dimensions, Virgo's early-type galaxies appear to define a slightly triaxial distribution, with axis ratios of (1: 0.7 : 0.5). The principal axis of the best-fit ellipsoid is inclined 20 - 40 degrees from the line of sight, while the galaxies belonging to the W' cloud lie on an axis inclined by 10 - 15 degrees.

 

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"The ACS Virgo Cluster Survey. XIV. Analysis of Color-Magnitude Relations in Globular Cluster Systems" , Mieske, S., Jordan, A., Cote, P., Kissler-Patig, M., Peng, E.W., Ferrarese, L., Blakeslee, J.P., Mei. S., Merritt, D., Tonry, J.L., & West, M.J. 2006, Astrophysical Journal, 653, 193
 

We examine the correlation between globular cluster (GC) color and magnitude using HST/ACS imaging for a sample of 79 early-type galaxies (-21.7  <MB  <-15.2 mag) with accurate SBF distances from the ACS Virgo Cluster Survey. Using the KMM mixture modeling algorithm, we find a highly significant correlation, d(g-z)/dz = -0.037 +- 0.004, between color and magnitude for the subpopulation of blue GCs in the co-added GC color-magnitude diagram of the three brightest Virgo galaxies (M49, M87 and M60): brighter GCs are redder than their fainter counterparts. For the single GC systems of M87 and M60, we find similar correlations; M49 does not appear to show a significant trend. There is no correlation between (g-z) and M_z for GCs of the red subpopulation. The correlation d(g-z)/dg for the blue subpopulation is much weaker than d(g-z)/dz. Using Monte Carlo simulations, we attribute this to the fact that the blue subpopulation in M_g extends to higher luminosities than the red subpopulation, which biases the KMM fits. The correlation between color and M_z thus is a real effect. This conclusion is supported by biweight fits to the same color distributions. We identify two environmental dependencies of the color-magnitude relation: (1) the slope decreases in significance with decreasing galaxy luminosity; and (2) the slope is stronger for GCs at smaller galactocentric distances. We examine several mechanisms that might give rise to the observed color-magnitude relation: (1) presence of contaminators; (2) accretion of GCs from low-mass galaxies; (3) stochastic effects; (4) capture of field stars by individual GCs; and (5) GC self-enrichment. We conclude that self-enrichment and field-star capture, or a combination of these processes, offer the most promising means of explaining our observations.

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"Parametric Representation of Surface Brightness Profiles: a Critical Comparison of Nuker and Core-Sersic/Sersic Models", Ferrarese, L., Cote, P. Blakeslee, J.P., Mei, S., Merritt, D., & West, M. 2006, A Response to Lauer et al. (2006), astro-ph/0609762

The parameterization of the surface brightness profiles of early-type galaxies has been instrumental in characterizing scaling relations and in defining the properties of these systems. In the study of the core properties (i.e. within the innermost few hundred parsecs), the most commonly used parameterization is given by the so called "Nuker" model (Lauer et al. 1995), described by an inner and outer power law joined at a "break" radius. In recent years, however, shortcoming of the Nuker model have started to become apparent (e.g. Graham et al. 2003). Indeed, Ferrarese et al. (2006) and Cote et al. (2006) found it necessary to adopt a different parameterization in their analysis of the surface brightness profiles of a sample of 100 early-type galaxies observed with the HST Advanced Camera for Surveys as part of the ACS Virgo Cluster Survey (ACSVCS). In the ACSVCS analysis, core-Sersic or Sersic models are claimed to provide good descriptions of the surface brightness profiles from parsec to kiloparsec scales, and are adopted in defining the properties of compact stellar nuclei. In this contribution, we present a more detailed comparison of Nuker and core-Sersic/Sersic models. This comparison is motivated by a recent astro-ph posting (Lauer et al., astro-ph/0609762) where, based on HST/WFPC1 or WFPC2 images of 22 of the ACSVCS galaxies, it is argued that the Sersic and core-Sersic models presented by Ferrarese et al. (2006) provide inadequate fits to the surface brightness profiles and that such models lead to the identification of spurious nuclear features. We show that the Lauer et al. criticisms are based on faulty assumptions and misrepresent the ACSVCS analysis. We further show that the Nuker model parameterization used by Lauer et al. not only fails to reproduce the surface brightness profiles on kiloparsec scales, but is also not a particularly good representation of the profiles of the ACSVCS galaxies on parsec scales. Indeed, we show that, for several of the galaxies in common with the ACSVCS sample, the Nuker model fits of Lauer et al. were likely biased by the lower signal-to-noise ratio and limited spatial extent of the WFPC1 or WFPC2 data used in their analysis. These shortcomings are probably responsible for the fact that Lauer et al. failed to recognize and characterize the properties of stellar nuclei in many early-type galaxies.

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"The Low-Mass X-ray Binary and Globular Cluster Connection in Virgo Cluster Early-type Galaxies: Optical Properties" , Sivakoff, G.R., Jordan, A., Sarazin, C.L., Blakeslee, J.P., Cote, P., Ferrarese, L., Juett, A.M., Mei. S., & Peng, E.W. 2007, Astrophysical Journal, 660, 1246
 

Low-mass X-ray binaries (LMXBs) form efficiently in globular clusters (GCs). By combining Chandra X-ray Observatory and Hubble Space Telescope-Advanced Camera for Surveys observations of early-type galaxies, we probe the LMXB-GC connection using the most accurate identification of LMXBs and GCs to date. We explore the optical properties of 270 GCs with LMXBs and 6,488 GCs without detectable X-ray emission from a sample of eleven massive early-type galaxies in the Virgo cluster. Globular clusters that are more massive, are redder, and have smaller radii are more likely to contain LMXBs. Using known structural scaling relations for GCs, the latter implies that denser GCs are more likely to hold LMXBs. Unlike Galactic GCs, a large number of GCs with LMXBs have half-mass relaxation times > 2.5 Gyr; GCs do not need to survive for more than five relaxation timescales to produce LMXBs. By fitting the dependence of the expected number of LMXBs per GC, lt, on the GC mass M, color (g-z), and half-mass radius rh,cor, we find that lt ∝ M1.24+/-0.08100.9(g-z) rh,cor-2.2 +/- 0.2. This rules out that the number of LMXBs per GC is linearly proportional to GC mass (99.89% confidence limit). We derive an expression to estimate the number of multiple LMXB sources in GCs and predict that most GCs with high X-ray luminosities contain a single LMXB. The detailed dependence of lt on GC properties appears mainly due to a dependence on a combination of mass and radius, and a dependence on color, that are essentially equivalent to a dependence on the encounter rate gh and the metallicity Z, lt ∝ gh0.82+/-0.05 Z0.39+/-0.07. Our analysis provides strong evidence that dynamical formation and metallicity play the primary roles in determining the presence of an LMXB in extragalactic GCs. The shallower than linear dependence of GC sources requires an explanation by theories of dynamical binary formation; however, we note that our use of gh as a proxy for the encounter rate, particularly if core-collapsed extragalactic GCs preferentially contain LMXBs, needs further testing in nearer galaxies. A metallicity-dependent variation in the number of neutron stars and black holes per unit GC mass, effects from irradiation induced winds, or suppression of magnetic braking in metal-poor stars may all be consistent with our derived abundance dependence; all three scenarios require further development.

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"The ACS Virgo Cluster Survey. XV. The Formation Efficiencies of Globular Clusters in Early-Type Galaxies: The Effects of Mass and Environment" , Peng, E., S., Jordan, A., Cote, P., Takamiya, M., West, M.J., Blakeslee, J.P., Chen, C.-W., Ferrarese, L., Mei, S., Tonry, J.L., & West, A. 2008, Astrophysical Journal, 681, 197

The fraction of stellar mass contained in globular clusters (GCs), also measured by number as the specific frequency, is a fundamental quantity that reflects both a galaxy's early star formation and its entire merging history. We present specific frequencies, luminosities, and mass fractions for the globular cluster systems of 100 early-type galaxies in the ACS Virgo Cluster Survey. This catalog represents the largest homogeneous catalog of GC number and mass fractions across a wide range of galaxy luminosity (-22<M_B<-15). We find that 1) GC mass fractions can be high for both high and low luminosity early-type galaxies, but are universally low for galaxies with intermediate luminosities (-20<M_B<-17). 2) The fraction of red GCs increases with galaxy luminosity, but stays constant or decreases for galaxies brighter than M_z=-22. Specific frequencies for both blue and red GCs are higher in massive galaxies, but the behavior of specific frequency across galaxy mass is dominated by the blue GCs. 3) The GC mass fraction of low-mass galaxies exhibits a large intrinsic scatter that cannot be explained solely by a dependence on the galaxy's present-day stellar mass. We find that this scatter is due to an underlying dependence on environment, where dwarf galaxies closer to the cluster center have higher GC mass fractions. Nearly all the dwarfs with high GC fractions are within 1~Mpc of the cD galaxy M87, presenting the first strong evidence that GC formation in dwarf galaxies is biased toward dense environments. 4) Comparisons to early-type dwarf galaxies in the Millennium Simulation show that central dwarfs are likely to have older stellar populations and have formed more of their stars at higher star formation rates (SFRs) and star formation rate surface densities. This suggests that their GC formation is biased because their star formation happens earliest and most intensely. In addition, the average SFR surface density in the simulated dwarfs peaks at higher redshifts than the average SFR, naturally leading to GC populations that are older and more metal-poor than the field stars in their hosts. 5) Dwarfs within ~40 kpc of the giant ellipticals M87 and M49 are red for their luminosities and have few or no GCs, suggesting that they have been tidally stripped and have contributed their GCs to the halos of their giant neighbors. The central dwarfs with high GC mass fractions are thus likely to be the survivors most similar to the protogalaxies that assembled the rich M87 globular cluster system.

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"The ACS Virgo Cluster Survey. XVI. Selection Procedures and Catalogs of Globular Cluster Candidates" , Jordan, A., Peng, E., Blakeslee, J.P., Cote, P., Eyheramendy, S., Ferrarese, L., Mei, S., Tonry, J.L., & West, M.J. 2009, Astrophysical Journal Supplement Series, 180, 54

 

We present catalogs of globular cluster candidates for the 100 galaxies of the ACS Virgo Cluster Survey, a large program to carry out imaging of early-type members of the Virgo Cluster using the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. We describe the procedure used to select bona-fide globular cluster candidates out of the full list of detections based on model-based clustering methods with the use of expected contamination catalogs constructed using blank field observations and which are customized for each galaxy. We also present the catalogs of expected contaminants for each of our target galaxies. For each detected source we measure its position, magnitudes in the F475W (~ Sloan g) and F850LP (~ Sloan z) bandpasses and half-light radii by fitting point spread function convolved King models to the observed light distribution. These measurements are presented for 20375 sources, of which 12763 are likely globular clusters. Finally, we detail the calculation of the aperture corrections adopted for the globular cluster photometry.

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"The ACS Virgo Cluster Survey. XVII. The Spatial Alignment of Globular Cluster Systems with Early-type Host Galaxies", Wang, Q., Peng, E.W., Blakeslee, J.P., Cote, P., Ferrarese, L., Jordan, A., Mei, S., West, M.J. 2013, Astrophysical Journal, 769, 145.

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We study the azimuthal distribution of globular clusters (GCs) in early-type galaxies and compare them to their host galaxies using data from the ACS Virgo Cluster Survey. We find that in host galaxies with visible elongation (epsilon > 0.2) and intermediate to high luminosities (Mz < -19), the GCs are preferentially aligned along the major axis of the stellar light. The red (metal-rich) GC subpopulations show strong alignment with the major axis of the host galaxy, which supports the notion that these GCs are associated with metal-rich field stars. The metal-rich GCs in lenticular galaxies show signs of being more strongly associated with disks rather than bulges. Surprisingly, we also find that the blue (metal-poor) GCs can also show the same correlation. If the metal-poor GCs are part of the early formation of the halo and built up through mergers, then our results support a picture where halo formation and merging occur anisotropically, and that the present-day major axis is an indicator of the preferred merging axis.

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Virgo Redux: The Masses and Stellar Content of Nuclei in Early-type Galaxies from Multiband Photometry and Spectroscopy, Spengler, C., Côté, P., Roediger, J., Ferrarese, L., Sánchez-Janssen, R., Toloba, E., Liu, Y., Guhathakurta, P., Cuillandre, J.-C., Gwyn, S., Zirm, A., Muñoz, R., Puzia, T., Lançon, A., Peng, E.W., Mei, S., & Powalka, M. 2017, Astrophysical Journal, 849, 55.

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We present an analysis of 39 nuclei and their early-type hosts in the Virgo Cluster using 10 broadband filters: F300W, F475W, F850LP, F160W, {u}* {\text{}}{griz}, and K s . We describe the Virgo Redux program, which provides high-resolution UV and NIR imaging. Combining this data with optical and NIR imaging from the ACS Virgo Cluster Survey and the Next Generation Virgo Cluster Survey, we estimate masses, metallicities, and ages using simple stellar population (SSP) models. For 19 nuclei, we compare to SSP parameters derived from Keck and Gemini spectra and find reasonable agreement between the photometric and spectroscopic metallicity: the rms scatter is 0.3 dex. We reproduce the nucleus-galaxy mass fraction of {0.33}-0.07+0.09% for galaxy stellar masses {10}8.4{--}{10}10.3 {M}⊙ with a typical precision of ˜35% for the nuclei masses. Based on available model predictions, there is no single preferred formation scenario for nuclei, suggesting that nuclei are formed stochastically through a mix of processes. Nuclei metallicities are statistically identical to those of their hosts, appearing 0.07 ± 0.3 dex more metal-rich on average; however, omitting galaxies with unusual origins, nuclei are 0.20 ± 0.28 dex more metal-rich. Nuclei appear to be 0.56 ± 0.12 dex more metal-rich than ultracompact dwarf galaxies (UCDs) at fixed mass. We find no clear age difference between nuclei and their galaxies, with nuclei displaying a broad range of ages. Interestingly, we find that the most massive nuclei may be flatter and more closely aligned with the semimajor axes of their hosts, suggesting that they formed through predominantly dissipative processes.

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