F. D. A. Hartwick

The Average Mass Profile of Galaxy Clusters

The average mass density profile measured in the Canadian Network for Observational Cosmology cluster survey is well described with the analytic form ρ(r) = Ar-1(r + aρ)-2, as advocated on the basis of n-body simulations by Navarro, Frenk, & White. The predicted core radii are aρ = 0.20 (in units of the radius where the mean interior density is 200 times the critical density) for an Ω = 0.2 open cold dark matter model and aρ = 0.26 for a flat Ω = 0.2 model, with little dependence on other cosmological parameters for simulations normalized to the observed cluster abundance. The dynamically derived local mass-to-light ratio, which has little radial variation, converts the observed light profile to a mass profile. We find that the scale radius of the mass distribution, 0.20 ≤ aρ ≤ 0.30 (depending on modeling details, with a 95% confidence range of 0.12-0.50), is completely consistent with the predicted values. Moreover, the profiles and total masses of the clusters as individuals can be acceptably predicted from the cluster rms line-of-sight velocity dispersion alone. This is strong support for the hierarchical clustering theory for the formation of galaxy clusters in a cool, collisionless, dark-matter-dominated universe.

The Dynamical Equilibrium of Galaxy Clusters

If a galaxy cluster is effectively in dynamical equilibrium, then all galaxy populations within the cluster must have distributions in velocity and position that individually reflect the same underlying mass distribution, although the derived virial masses can be quite different. Specifically, within the Canadian Network for Observational Cosmology cluster sample, the virial radius of the red galaxy population is, on the average, a factor of 2.05 ± 0.34 smaller than that of the blue population. The red galaxies also have a smaller rms velocity dispersion, a factor of 1.31 ± 0.13 within our sample. Consequently, the virial mass calculated from the blue galaxies is 3.5 ± 1.3 times larger than from the red galaxies. However, applying the Jeans equation of stellar hydrodynamic equilibrium to the red and blue subsamples separately gives statistically identical cluster mass profiles. This is strong evidence that these clusters are effectively equilibrium systems and therefore demonstrates empirically that the masses in the virialized region are reliably estimated using dynamical techniques.

The Redshift Distribution and Luminosity Functions of Galaxies in the Hubble Deep Field

Photometric redshifts have been determined for the galaxies in the Hubble Deep Field. The resulting redshift distribution shows two peaks: one at z ~ 0.6 and one at z ~ 2.2. Luminosity functions derived from the redshifts show strong luminosity evolution as a function of redshift. This evolution is consistent with the Babul & Rees scenario wherein massive galaxies form stars at high redshift while star formation in dwarf galaxies is delayed until after z = 1.

The Structure of the Outer Halo of the Galaxy and its Relationship to Nearby Large-Scale Structure

We present evidence to support an earlier indication that the Galaxy is embedded in an extended, highly inclined, triaxial halo outlined by the spatial distribution of companion galaxies to the Milky Way. Signatures of this spatial distribution are seen in (1) the angular variation of the radial velocity dispersion of the companion galaxies, (2) the spatial distribution of the M31 subgroup of galaxies, (3) the spatial distribution of the isolated (mainly dwarf irregular) galaxies of the Local Group, (4) the velocity anisotropy quadrupole of a subgroup of high-velocity clouds, and (5) the spatial distribution of galaxies in the Coma-Sculptor cloud. Tidal effects of M31 and surrounding galaxies on the Galaxy are not strong enough to have affected the observed structure. We conclude that this distribution is a reflection of initial conditions. A simple galaxy formation scenario is proposed that relates the results found here to those of Holmberg and Zaritsky et al. regarding the peculiar distribution of satellites around a large sample of spiral galaxies.

The Evolution of Stellar Mass in Galaxies in the NICMOS Ultra Deep Field

We measure the buildup of the stellar mass of galaxies from z = 6 down to z = 1. Using 15 band multicolor imaging data in the NICMOS Ultra Deep Field we derive photometric redshifts and masses for 796 galaxies down to HAB = 26.5?mag. The derived evolution of the global stellar mass density of galaxies is consistent with previous star formation rate density measurements over the observed range of redshifts. Beyond the observed range, maintaining consistency between the global stellar mass and the observed star formation rate suggests the epoch of galaxy formation was z = 16.We measure the build-up of the stellar mass of galaxies from z = 6 to z = 1. Using 15 band multicolor imaging data in the NICMOS Ultra Deep Field we derive photometric redshifts and masses for 796 galaxies down to H AB = 26.5. The derived evolution of the global stellar mass density of galaxies is consistent with previous star formation rate density measurements over the observed range of redshifts. Beyond the observed range, maintaining consistency between the global stellar mass and the observed star formation rate suggests the epoch of galaxy formation was z = 16.

The CNOC2 field galaxy redshift survey

The second Canadian Network for Observational Cosmology ( CNOC) galaxy redshift survey, CNOC2, is designed to investigate the relations between the dramatic evolution of field galaxies and their clustering over the redshift range 0 to 0.7. The sample of about 6000 galaxies with accurate velocities is spread over four sky patches with a total area of about 1.5deg2. Here we report preliminary results based on two of the sky patches and within the redshift range of 0.12 to 0.55. After classifying the galaxy spectral energy distributions relative to non–evolving references, we find that the early and intermediate–type populations can be described with nearly pure luminosity evolution, whereas the late–type population requires nearly pure density evolution. The spatial two–point correlation functions have a strong colour dependence with scale, and a weaker, apparently scale–free, luminosity dependence. The population most likely to be conserved with redshift is the high–luminosity galaxies. In particular, we choose galaxies with MRke ⩽−20 mag as our tracer population. We find that the evolution of the clustered density in proper co–ordinates at r ≲ 10h−1 Mpc, ρgg ∝ r0γ(1+z)3, is best described as a ‘de–clustering’, proportional to (1+z)0.6±0.4); or equivalently, there is a weak growth of clustering in co–moving co–ordinates, x0 ∝(1+z)(−0.3±0.2). This conclusion is supported by the pairwise peculiar velocities, which show no significant change with redshift. The cosmic virial theorem applied to the CNOC2 data gives Q3ΩM/b = 0.11 ± 0.04, where Q3 is the three–point correlation parameter and b the bias.

Cosmic Star Formation History from Local Observations and an Outline for Galaxy Formation and Evolution

The goal of this investigation is to reconstruct the cosmic star formation rate density history from local observations and in doing so to gain insight into how galaxies might have formed and evolved. A new chemical evolution model is described that accounts for the formation of globular clusters, as well as the accompanying field stars. When this model is used in conjunction with the observed age-metallicity relations for the clusters and with input that allows for the formation of the nearly universally observed bimodal distribution of globular clusters, star formation rates are obtained. By confining attention to a representative volume of the local universe, these rates allow a successful reconstruction of the Madau plot, while complementary results simultaneously satisfy many local cosmological constraints. A physical framework for galaxy formation is presented that incorporates the results from this chemical evolution model and assumes an anisotropic collapse. In addition to providing the classical halo, bulge, and disk components, the model also predicts a new stellar halo component with peak ~ -0.8 and disklike angular momentum and allows for the formation of a thick disk, as outlined by the group of metal-rich globular clusters. Milky Way counterparts of the latter two components are identified.

Merger-Induced Globular Cluster Formation and Galaxy Evolution

Based on the earlier work of Gunn and McCrea, we model the formation of globular clusters in merging galaxies. Neutral hydrogen observations of dwarf irregular galaxies as well as more luminous systems are used to provide the key parameters of the model. The observations indicate that clusters with the mass of globular clusters should still be forming today. The model is incorporated into a phenomenological picture of galaxy evolution making use of a simple chemical evolution model. These results are compared to recent observations of the metallicity distributions of F and G stars from a recent large SDSS survey. The comparisons are consistent with an anisotropic collapse and merging of a large number of dwarf irregular galaxies for the formation of the Galaxy.

MODELING OBSERVATIONAL CONSTRAINTS FOR DARK MATTER HALOS

Observations show that the underlying rotation curves at intermediate radii in spiral and low-surface-brightness galaxies are nearly universal. Further, in these same galaxies, the product of the central density and the core radius (ρ0 r 0) is constant. An empirically motivated model for dark matter halos that incorporates these observational constraints is presented and shown to be in accord with the observations. A model fit to the observations of the galaxy cluster A611 shows that ρ0 r 0 for the dark matter halo in this more massive structure is larger by a factor of ~20 over that assumed for the galaxies. The model maintains the successful Navarro-Frenk-White form in the outer regions, although the well-defined differences in the inner regions suggest that modifications to the standard cold dark matter picture are required.

Early Cosmic Chemical Evolution: Relating the Origin of a Diffuse Intergalactic Medium and the First Long-Lived Stars

Nucleosynthetic signatures in common are found between the gas responsible for the high-redshift Lyα forest and a subsample of extremely metal-poor stars. A simple mass-loss model of chemical evolution with physically motivated parameters provides a consistent picture in which the gas is identified with that lost by supernova-driven winds during the first generation of star formation. Substantial mass loss occurs, which can account for a diffuse IGM with up to 80% of the total baryon content and a peak [C-O/H] abundance of ~-2.9. This mass-loss component differs from one produced later, during galaxy formation and evolution, which contributes to a circumgalactic medium (CGM). The CGM has earlier been shown to have a mass of ~10% of all baryons and peak [Fe/H] ~ -1.