Kavan U. Ratnatunga

The Morphological Mix of Field Galaxies to mI = 24.25 Magnitudes (bJ ~ 26 Magnitudes) from a Deep Hubble Space Telescope WFPC2 Image

We determine the morphological mix of field galaxies down to mI 24.25 mag (mB ~ 26.0 mag) from a single ultradeep Hubble Space Telescope wide field planetary camera (WFPC2) image in both the V606 and the I814 filters. In total, we find 227 objects with mI ? 24.5 mag and classify these into three types: ellipticals (16%), early-type spirals (37%), and late-type spirals/irregulars (47%). The differential number counts for each type are compared with simple models in a standard flat cosmology. We find that both the elliptical and the early-type spiral number counts are well described by little-or-no-evolution models, but only when normalized at bJ = 18.0 mag. Given the uncertainties in the luminosity function (LF) normalization, both populations are consistent with a mild evolutionary scenario based on a normal/low rate of star formation. This constrains the end of the last major star formation epoch in the giant galaxy populations to z ? 0.8. Conversely, the density of the observed late-type/irregular population is found to be a factor of 10 in excess of the conventional no-evolution model. This large population might be explained by a modified local dwarf-rich LF and/or strong evolution acting on the local LF. For the dwarf-rich case, a steep faint-end Schechter slope (? -1.8) is required, plus a fivefold increase in the dwarf normalization. For a purely evolving model based on a flat Loveday et al. LF (? -1.0), a ubiquitous starburst of ?I ~ 2.0 mag is needed at z 0.5 for the entire late-type population. We argue for a combination of these possibilities, and show that for a steep Marzke et al. LF (? -1.5) a starburst of ~1.3 mag is required at z 0.5 in the entire late-type population, or ~2.0 mag in ~20% of the population.

A Study of Nine High-Redshift Clusters of Galaxies. III. Hubble Space Telescope Morphology of Clusters 0023+0423 and 1604+4304

We present a detailed morphological analysis of the galaxy populations in the first two clusters to be completed in an extensive observational study of nine high-redshift clusters of galaxies. These two clusters, Cl 0023+0423 and Cl 1604+4304, are at redshifts of z = 0.84 and z = 0.90, respectively. The morphological studies are based on high angular resolution imagery taken with Wide Field Planetary Camera 2 aboard the Hubble Space Telescope. These data are combined with deep, ground-based BVRI photometry and spectra taken with the Keck 10 m telescopes. The morphological classifications presented in this paper consist of two parts. First, we provide a quantitative description of the structural properties of ~600 galaxies per cluster field using the Medium Deep Survey automated data reduction and object classification software. This analysis includes the galaxy position, photometry, and best-fit bulge+disk model. Second, for the brightest subsample of ~200 galaxies per cluster field, we provide a more detailed morphological description through a visual classification based on the revised Hubble classification scheme. Based on these classifications, we have examined the general relation between galaxy morphology and other photometric and spectral properties. We find that, as expected, the elliptical and S0 galaxies are redder, on average, than are the spiral and irregular galaxies. In addition, there is a strong correlation between morphology and spectral type. Of the galaxies that are visually classified as ellipticals, the majority show K star absorption spectra that are typical of nearby, red early-type galaxies; however, a few are actually blue compact galaxies with spectra characterized by fairly strong, narrow emission lines. Normal late-type galaxies typically have spectra with blue colors and [O II] emission, while the presence of strong star formation features, such as extremely high equivalent width [O II], Hβ, and/or [O III] emission, is always accompanied by peculiar morphologies that suggest recent mergers or interactions. We have used the statistical distributions of cluster galaxy morphologies to probe the overall morphological composition of these two systems. This analysis reveals that the two clusters contain very different galaxy populations. Cl 0023+0423 has a galaxy population that is more similar to groups of galaxies and the field. This system is almost completely dominated by spiral galaxies. Cl 1604+4304, however, has a morphological composition that is more typical of a normal, present-day cluster; early-type galaxies make up ~76% of all galaxies brighter than MV = -19.0 + 5 log h in the central ~0.5 h-1 Mpc. The ratio of S0 galaxies to elliptical galaxies in this cluster is 1.7 ± 0.9, consistent with local cluster populations. The morphological results support the conclusions of the dynamical analysis presented in the second paper of this series. Cl 0023+0423 consists of two galaxy groups that are separated by ~2900 km s-1 in radial velocity. Cl 1604+4304, on the other hand, has a velocity distribution indicating that it is already well formed and relaxed. The morphological composition, velocity dispersion, and implied mass of the Cl 1604+4304 system are consistent with an Abell richness class 2 or 3 cluster.

A Study of Nine High-Redshift Clusters of Galaxies : III. HST Morphology of Clusters 0023+0423 and 1604+4304

We present a detailed morphological analysis of the galaxy populations in the first two clusters to be completed in an extensive observational study of nine high-redshift clusters of galaxies. These two clusters, Cl 0023+0423 and Cl 1604+4304, are at redshifts of z = 0.84 and z = 0.90, respectively. The morphological studies are based on high angular resolution imagery taken with Wide Field Planetary Camera 2 aboard the Hubble Space Telescope. These data are combined with deep, ground-based BVRI photometry and spectra taken with the Keck 10 m telescopes. The morphological classifications presented in this paper consist of two parts. First, we provide a quantitative description of the structural properties of ~600 galaxies per cluster field using the Medium Deep Survey automated data reduction and object classification software. This analysis includes the galaxy position, photometry, and best-fit bulge+disk model. Second, for the brightest subsample of ~200 galaxies per cluster field, we provide a more detailed morphological description through a visual classification based on the revised Hubble classification scheme. Based on these classifications, we have examined the general relation between galaxy morphology and other photometric and spectral properties. We find that, as expected, the elliptical and S0 galaxies are redder, on average, than are the spiral and irregular galaxies. In addition, there is a strong correlation between morphology and spectral type. Of the galaxies that are visually classified as ellipticals, the majority show K star absorption spectra that are typical of nearby, red early-type galaxies; however, a few are actually blue compact galaxies with spectra characterized by fairly strong, narrow emission lines. Normal late-type galaxies typically have spectra with blue colors and [O II] emission, while the presence of strong star formation features, such as extremely high equivalent width [O II], Hβ, and/or [O III] emission, is always accompanied by peculiar morphologies that suggest recent mergers or interactions. We have used the statistical distributions of cluster galaxy morphologies to probe the overall morphological composition of these two systems. This analysis reveals that the two clusters contain very different galaxy populations. Cl 0023+0423 has a galaxy population that is more similar to groups of galaxies and the field. This system is almost completely dominated by spiral galaxies. Cl 1604+4304, however, has a morphological composition that is more typical of a normal, present-day cluster; early-type galaxies make up ~76% of all galaxies brighter than MV = -19.0 + 5 log h in the central ~0.5 h-1 Mpc. The ratio of S0 galaxies to elliptical galaxies in this cluster is 1.7 ± 0.9, consistent with local cluster populations. The morphological results support the conclusions of the dynamical analysis presented in the second paper of this series. Cl 0023+0423 consists of two galaxy groups that are separated by ~2900 km s-1 in radial velocity. Cl 1604+4304, on the other hand, has a velocity distribution indicating that it is already well formed and relaxed. The morphological composition, velocity dispersion, and implied mass of the Cl 1604+4304 system are consistent with an Abell richness class 2 or 3 cluster.

Disk and Bulge Morphology of WFPC2 Galaxies: The HUBBLE SPACE TELESCOPE Medium Deep Survey Database

Quantitative morphological and structural parameters are estimated for galaxies detected in Hubble Space Telescope observations of WFPC2 survey fields. A modeling approach based on maximum likelihood has been developed for two-dimensional decomposition of faint undersampled galaxy images into components of disk and bulge morphology. Decomposition can be achieved for images down to F814W (I) ≈ 23.0, F606W (V) ≈ 23.8, and F450W (B) ≈ 23.3 mag in WFPC2 exposures of 1 hr. We discuss details of the fitting procedure and present the observed distributions of magnitude, color, effective half-light radius, disk and bulge axis ratios, bulge-to-(disk+bulge) flux ratio, bulge-to-disk half-light radius ratio, and surface brightness. We also discuss the various selection limits on the measured parameters. The Medium Deep Survey catalogs and images of random pure parallel fields and other similar archival primary WFPC2 fields have been made available via the Internet with a searchable browser interface to the database.

A Measurement of the Cosmological Constant Using Elliptical Galaxies as Strong Gravitational Lenses

We have identified seven (field) elliptical galaxies acting as strong gravitational lenses and have used them to measure cosmological parameters. To find the most likely value for Ωm (=Ωmatter) and Λ, we have used the combined probabilities of these lens systems having the observed critical radii (or image deflection) for the measured or estimated values of lens redshifts, source redshifts, and lens magnitudes. Our measurement gives Λ=0.64+ 0.15−0.26 if Ωm + Λ = 1, and the Ωm = 1 model is excluded at the 97% confidence level. We also find, at the 68% (Ω = 0)-82% (Ω = 0.3) confidence level, that an open universe is less likely than a flat universe with nonzero Λ. Except for the possibility of strong perturbations due to cluster potentials and the systematic overestimate of the lens magnitudes, other possible systematic errors do not seem to influence our results strongly: correction of possible systematic errors seems to increase the significance of the result in favor of a nonzero Λ model.

Evidence for Galaxy Interactions/Mergers from Medium Deep Survey WFPC2 Data*

We examine the morphological and statistical properties of close galaxy pairs from two sets of 28 WFPC2 fields, acquired for the Medium Deep Survey (MDS) and the Groth-Westphal Survey (GWS) in F606W (V) and F814W (I) passbands. In the GWS sample all fields have uniform 95% completeness down to I 24.3 mag, whereas in the MDS sample, fields have varying 95% completeness limits in the range I 23.6-24.8 mag. In each field ~400 galaxies per 5 square arcmin field are detected. We exploit high-resolution WFPC2 images to systematically determine morphological classifications of galaxies as disk or bulge dominated down to I ≤ 23 mag (V 24 mag) and to differentiate galaxies from stars 1 magnitude fainter. Down to I ≤ 25 mag the number of galaxy pairs with separations θ ≤ 30 is consistent with a shortward extrapolation of the angular two-point correlation function ω(θ) θ-0.8 observed from the same data; the fraction of such pairs showing morphological evidence for physical association accounts for a third of the total numbers suggested by a shortward extrapolation of ω(θ). The latter result may not be too surprising given the low surface brightness of the tidal tails resulting from galaxy interactions; i.e., much of the evidence for interactions may fall below our detection limit. Moreover, we find no trend between apparent physical association (on the basis of morphology) and (V - I) color or I-magnitude difference between pair members of the θ 30 pair sample. We use recent galaxy redshift surveys to estimate the rate of galaxy merging occurring in the MDS and GWS galaxy pair samples. From this work we find that merging has a moderate dependence on redshift: we derive an estimate for the galaxy pair fraction Pf (1 + z)m, with m = 1.2 ± 0.4 for galaxies with I ≤ 25 mag (zmed 1-2). Two scenarios are consistent both with this low value of m and for the low correlation amplitude: (a) a low-density universe with strong clustering evolution parameterized by a clustering exponent e 1.0 such that galaxy cluster-scale structures shrink relative to the proper coordinate frame; and/or (b) a weakly clustered galaxy population, the majority of which fade or dissipate below zmed 0.5 (I 20 mag), thus mimicking the apparently strong evolution in the correlation amplitude, Aω. Although not directly observed using our data, the possible flattening of the slope of ω(θ) with increasing survey depth can explain the strong decline in Aω and allow for greater pair fraction evolution limited to m ≤ 1.6.

The Morphologically Divided Redshift Distribution of Faint Galaxies

We have constructed a morphologically divided redshift distribution of faint field galaxies using a statistically unbiased sample of 196 galaxies brighter than I=21.5 for which detailed morphological information (from the Hubble Space Telescope) as well as ground-based spectroscopic redshifts are available. Galaxies are classified into three rough morphological types according to their visual appearance (E/S0s, spirals, Sdm/dE/Irr/Pec galaxies), and redshift distributions are constructed for each type. The most striking feature is the abundance of low- to moderate-redshift Sdm/dE/Irr/Pec galaxies at I<19.5. This confirms that the faint-end slope of the luminosity function (LF) is steep (α<-1.4) for these objects. We also find that Sdm/dE/Irr/Pec galaxies are fairly abundant at moderate redshifts, and this can be explained by a strong luminosity evolution. However, the normalization factor (or the number density) of the LF of Sdm/dE/Irr/Pec galaxies is not much higher than that of the local LF of Sdm/dE/Irr/Pec galaxies. Furthermore, as we go to fainter magnitudes, the abundance of moderate- to high-redshift Irr/Pec galaxies increases considerably. This cannot be explained by strong luminosity evolution of the dwarf galaxy populations alone: these Irr/Pec galaxies are probably the progenitors of present-day ellipticals and spiral galaxies that are undergoing rapid star formation or merging with their neighbors. On the other hand, the redshift distributions of E/S0s and spirals are fairly consistent with those expected from passive luminosity evolution and are only in slight disagreement with the nonevolving model.

Luminosity Functions of Elliptical Galaxies at z < 1.2

The luminosity functions of E/S0 galaxies are constructed in three different redshift bins (0.2 1) for elliptical galaxies, together with weak evolution of the major merger rate at z < 1.

Kinematic modeling of the galaxy. II. Two samples of high proper motion stars

The paper analyzes two independent surveys, due to Fouts and Sandage and to Carney and Latham, of high proper motion stars with photoelectric photometry and line-of-sight velocity information. A purely kinematic study of both samples indicates that a two-component classical (disk + spheroid) model of the Galaxy is inadequate to represent the kinematics of stars in the solar neighborhood. A component with intermediate kinematic properties (old disk) is required. An asymmetric drift of 50 + or - 15 km/s and principal velocity dispersions of (66, 37, 38) + or - 10 km/s are determined, in good agreement with the results of the metallicity-based analysis of Sandage and Fouts and of Carney and collaborators. The density of this component is poorly constrained, and is 10 percent + or - 5 percent of the local disk density for the same color range; the scale height is less than 1 kpc. 36 refs.

Kinematic modeling of the Galaxy. I - The Yale Bright Star Catalogue

A code has been developed to investigate the kinematic properties of stellar populations of the Galaxy by modeling the distributions of proper motion and line-of-sight velocity. The coordinate frames of reference are discussed in detail and the general matrix transformations valid beyond the solar neighborhood are given. As a first application, the kinematic properties of the magnitude-selected sample of 3993 stars in the Bright Star Catalogue with V less than 6.0 and (B-V) greater than 0 are selected. The observed kinematic distributions are compared with a conventional model that has an exponential disk with scale heights, components of velocity dispersion ellipsoid, and asymmetric drift taken from the literature and defined as functions of spectral type. The comparisons suggest that a significant fraction of the Bright Star Catalogue belongs to a relatively young population that is not properly represented in the model. 55 refs.