Patrick Cote

The remnants of galaxy formation from a panoramic survey of the region around M31.

In hierarchical cosmological models, galaxies grow in mass through the continual accretion of smaller ones. The tidal disruption of these systems is expected to result in loosely bound stars surrounding the galaxy, at distances that reach 10–100 times the radius of the central disk. The number, luminosity and morphology of the relics of this process provide significant clues to galaxy formation history, but obtaining a comprehensive survey of these components is difficult because of their intrinsic faintness and vast extent. Here we report a panoramic survey of the Andromeda galaxy (M31). We detect stars and coherent structures that are almost certainly remnants of dwarf galaxies destroyed by the tidal field of M31. An improved census of their surviving counterparts implies that three-quarters of M31’s satellites brighter than Mv = -6 await discovery. The brightest companion, Triangulum (M33), is surrounded by a stellar structure that provides persuasive evidence for a recent encounter with M31. This panorama of galaxy structure directly confirms the basic tenets of the hierarchical galaxy formation model and reveals the shared history of M31 and M33 in the unceasing build-up of galaxies.

Abundance Patterns in the Draco, Sextans, and Ursa Minor Dwarf Spheroidal Galaxies

The Keck I telescope and the High Resolution Echelle Spectrometer (HIRES) have been used to obtain spectra for red giant stars belonging to the Draco, Sextans, and Ursa Minor dwarf spheroidal (dSph) galaxies. An analysis of these spectra is presented, along with abundance ratios for more than 20 elements. The resulting database of element abundances for 17 dSph stars is the most extensive yet assembled for stars in such environments. Our principal findings are summarized as follows. (1) There is unambiguous evidence for a large internal dispersion in metallicity in all three galaxies: our program stars span a range of ?[Fe/H] = 1.53, 1.40, and 0.73 dex in Draco, Sextans, and Ursa Minor, respectively. (2) The abundance patterns among the dSph stars are remarkably uniform, suggesting that three galaxies have similar nucleosynthetic histories and, presumably, similar initial mass functions. (3) A comparison of the measured element abundance ratios for our sample of dSph stars with published values for Galactic halo and disk field stars suggests that the dSph galaxies have 0.02 [?/Fe] 0.13 dex, whereas the halo field star sample has [?/Fe] ~ 0.28 dex over the same range in metallicity. (4) The most metal-rich dSph stars in our sample have [Y/Fe] abundances that are significantly lower than those measured for halo field stars of similar metallicity, while the measured [Ba/Eu] ratios for the dSph stars suggest that the early chemical evolution of these galaxies was dominated by the r-process. Taken together, these results suggest that the Galactic halo is unlikely to have assembled, in its entirety, through the disruption of dwarf galaxies similar to the low-luminosity, LV = 3 ? 105LV,?, dSph galaxies studied here. We also note that the measured [Zn/Fe] abundance ratios for the dSph stars are lower than those of damped Ly? systems having low levels of dust depletion by roughly an order of magnitude. The first high-resolution abundance analysis for the distant Galactic globular cluster NGC 2419 is also presented. From a HIRES spectrum of a single red giant, we find a metallicity of [Fe/H] = -2.32 ? 0.11 dex. This is slightly lower than, but still consistent with, published estimates based on low-resolution spectroscopy. With the possible exception of a slight enhancement in the abundances of some heavy elements such as Ce, Nd, Y, and Ba, the observed abundance pattern closely resembles those exhibited by red giants in M92: a nearby, well-studied globular cluster of nearly identical metallicity.

The ACS Virgo Cluster Survey. VI. Isophotal Analysis and the Structure of Early-Type Galaxies*

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), imaged in the 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 42% of galaxies brighter than BT = 12.15 mag, while kiloparsec-scale stellar disk, bars, and nuclear stellar disks are seen in 60% of galaxies with intermediate luminosity. Isophotal parameters are derived typically within 8 kpc from the center for the brightest galaxies, and 1.5 kpc for the faintest systems, with a resolution of 7 pc. For most galaxies, the surface brightness profiles are well described by a Sersic model with index n that increases steadily with the galaxy luminosity; only for 8 of the 10 brightest galaxies are the inner profiles (typically within 100 pc of the center) lower than expected based on an extrapolation of the outer Sersic model, and are better described by a single power-law function. Contrary to previous claims, we find no evidence in support of a strong bimodal behavior of the logarithmic slope of the inner surface brightness profile, γ; in particular the γ distribution for galaxies that do not show evidence of multiple morphological components is unimodal across the entire magnitude range spanned by the ACSVCS galaxies. Although the brightest galaxies have shallow inner profiles, the shallowest profiles are found in faint dwarf systems. The widely adopted separation of early-type galaxies between core and power-law types is questioned based on the present study.

The unusual afterglow of the γ-ray burst of 26 March 1998 as evidence for a supernova connection

Cosmic γ-ray bursts have now been firmly established as one of the most powerful phenomena in the Universe, releasing almost the rest-mass energy of a neutron star within the space of a few seconds (ref. 1). The two most popular models to explain γ-ray bursts are the coalescence of two compact objects such as neutron stars or black holes, or the catastrophic collapse of a massive star in a very energetic supernova-like explosion. Here we show that, about three weeks after the γ-ray burst of 26 March 1998, the transient optical source associated with the burst brightened to about 60 times the expected flux, based upon an extrapolation of the initial light curve. Moreover, the spectrum changed dramatically, with the colour becoming extremely red. We argue that the new source is an underlying supernova. If our hypothesis is true then this provides evidence linking cosmologically located γ-ray bursts with deaths of massive stars.Palomar Observatory 105-24, Caltech, Pasadena, CA 91125, USA National Radio Astronomy Observatory, P. O. Box O, Socorro, NM 87801, USA Department of Astronomy, University of California, Berkeley, CA 94720-3411 USA National Optical Astronomy Observatories, 950 N. Cherry, Ave. Tucson, AZ 85719, USA Institute of Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, 7000 East Avenue, P. O. Box 808, L-413, Livermore, CA 94551-9900, USA Center for Particle Astrophysics, University of California, Berkeley, CA 94720 USA Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA European Southern Observatory, D-85748 Garching, Germany Istituto di Astrofisica Spaziale, CNR, via Fosso del Cavaliere, Roma I-00133, Italy

The Formation of Giant Elliptical Galaxies and Their Globular Cluster Systems

We examine the formation of giant elliptical galaxies using their globular cluster (GC) systems as probes of their evolutionary history. The bimodal distributions of GC metallicities in such galaxies are often cited as evidence for the formation of giant elliptical galaxies through mergers involving gas-rich spirals, with the metal-rich GCs forming during the merger process. We explore an alternative possibility: that these metal-rich clusters represent the galaxys intrinsic GC population and the metal-poor component of the observed GC metallicity distribution arises from the capture of GCs from other galaxies, either through mergers or through tidal stripping. Starting with plausible assumptions for the luminosity function of galaxies in the host cluster and for the dependence of GC metallicity on parent galaxy luminosity, we show using Monte Carlo simulations that the growth of a preexisting seed galaxy through mergers and tidal stripping is accompanied naturally by the capture of metal-poor GCs whose chemical abundances are similar to those that are observed to surround giant ellipticals. We also investigate the spatial distribution of GCs in isolated galaxies of low and intermediate luminosity and conclude that, at the epoch of formation, the GC systems of such galaxies are likely to have been more spatially extended than their constituent stars. Thus, the capture of GCs through tidal stripping, unlike mergers, does not necessarily conserve GC specific frequency. Comparisons of model GC metallicity distributions and specific frequencies to those observed for the well-studied galaxies NGC 4472 (=M49) and NGC 4486 (=M87), the two brightest cluster members of the nearby Virgo cluster, show that it is possible to explain their bimodal GC metallicity distributions and discordant specific frequencies without resorting to the formation of new GCs in mergers or by invoking multiple bursts of GC formation. Finally, we discuss the possibility of using the ratio of the numbers of metal-poor to metal-rich GCs in giant elliptical galaxies as a diagnostic of their merger histories. We use this method to derive upper limits on the number of galaxies and total luminosity accreted to date by NGC 4472.

The acs virgo cluster survey. IX. The color distributions of globular cluster systems in early-type galaxies

We present the color distributions of globular cluster (GC) systems for 100 early-type galaxies observed in the ACS Virgo Cluster Survey, the deepest and most homogeneous survey of this kind to date. 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% and increasing with luminosity. The colors of both subpopulations correlate with host galaxy luminosity and color, with the red GCs having a steeper slope. To convert color to metallicity, we 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 ∝ L0.16±0.04 ∝ M and [Fe/H]MR ∝ L0.26±0.03 ∝ M, respectively. Although these relations are shallower than the mass-metallicity relation predicted by wind models and observed for dwarf galaxies, they are very similar to the relation observed for star-forming galaxies in the same mass range. The offset between the two GC populations is approximately 1 dex across 3 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 implies that the conditions of GC formation for metal-poor and metal-rich GCs could not have been too different. Like the color-magnitude relation, these relations derived from globular clusters present stringent constraints on the formation and evolution of early-type galaxies.

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

We have measured half-light radii, rh, for thousands of globular clusters (GCs) belonging to the 100 early-type galaxies observed in the ACS Virgo Cluster Survey and the elliptical galaxy NGC 4697. 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 by Jordan. 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 = 2.7 ± 0.35 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 μz = 21 mag arcsec-2. Using this technique, we place an upper limit of 3.4 Mpc on the 1 σ 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 that successfully describes this distribution.

A Fundamental Relation between Compact Stellar Nuclei, Supermassive Black Holes, and Their Host Galaxies

Imaging surveys with the Hubble Space Telescope (HST) have shown that ?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 NGC 205 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 an SBH or a compact stellar nucleus?that contains a mean fraction, ?0.2%, of the total galactic mass. In galaxies with masses greater than gal ~ a few × 1010 ?, SBHs appear to be the dominant mode of CMO formation.

The ACS Virgo Cluster Survey XV. The Formation Efficiencies of Globular Clusters in Early-Type Galaxies: The Effects of Mass and Environment

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 galaxys 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, the largest homogeneous catalog of its kind. We find the following: (1) GC mass fractions can be high in both giants and dwarfs but are universally low in galaxies with intermediate luminosities. (2) The behavior of specific frequency across galaxy mass is dominated by the blue GCs. (3) GC fractions of low-mass galaxies exhibit a dependence on environment. Nearly all dwarf galaxies with high GC fractions are within 1 Mpc of the cD galaxy M87, presenting the first strong evidence that GC formation in dwarfs is biased toward dense environments. (4) GC formation in central dwarfs is biased because their stars form earliest and most intensely. Comparisons to the Millennium Simulation show that central dwarfs have older stellar populations and form more stars at higher star formation rates (SFRs) and SFR surface densities. The SFR surface density in simulated dwarfs peaks before the total SFR, naturally producing GC populations that are older and more metal-poor than the field stars. (5) Dwarfs within ~40 kpc of the giant ellipticals M87 and M49 are red 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.

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: