J. B. Oke

A systematic survey for distant galaxy clusters

A photographic survey for faint clusters of galaxies has been carried out with fine-grained photographic emulsions using the 1.2 m Schmidt and 5 m Hale telescopes, as well as the 4 m Mayall telescope. A total of 418 clusters have been found with redshifts mostly in the range from 0.15 to 0.92. The survey was planned to minimize distance-dependent selection effects in the resulting catalog. In areas of sky where the deepest search was made, the sample is complete to about z = 0.50; there are 11 clusters per square degree at this limit. At a redshift of 1.0 there should be 63 or 45 clusters per square degree depending on whether q0 is 0.0 or 0.5, provided there is no evolution.

A Study of Nine High-Redshift Clusters of Galaxies. II. Photometry, Spectra, and Ages of Clusters 0023+0423 and 1604+4304

We present an extensive photometric and spectroscopic study of two high-redshift clusters of galaxies based on data obtained from the Keck 10 m telescopes and the Hubble Space Telescope. The clusters Cl 0023+0423 (z = 0.84) and Cl 1604+4304 (z = 0.90) are part of a multiwavelength program of Oke, Postman & Lubin to study nine candidate clusters at z 0.6. Based on these observations, we study in detail both the field and cluster populations. From the confirmed cluster members, we find that Cl 0023+0423 actually consists of two components separated by ~2900 km s-1. A kinematic analysis indicates that the two components are a poor cluster with ~3 ? 1014 M? and a less massive group with ~1013 M?. Cl 1604+4304 is a centrally concentrated, rich cluster at z = 0.8967 with a velocity dispersion of 1226 km s-1 and a mass of ~3 ? 1015 M?. A large percentage of the cluster members show high levels of star formation activity. Approximately 57% and 50% of the galaxies are active in Cl 0023+0423 and Cl 1604+4304, respectively. These numbers are significantly larger than those found in intermediate-redshift clusters. We also observe many old, red galaxies. Found mainly in Cl 1604+4304, they have spectra consistent with passive stellar evolution, typical of the populations of early-type galaxies in low- and intermediate-redshift clusters. We have calculated their ages by comparing their spectral energy distributions to standard Bruzual & Charlot evolutionary models. We find that their colors are consistent with models having an exponentially decreasing star formation rate with a time constant of 0.6 Gyr. We also observe a significant luminosity brightening in our brightest cluster galaxies. Compared with brightest cluster galaxies at z ~ 0.1, we find a luminosity increase of ~1 mag in the rest MB and ~0.8 mag in the rest MV. In the field, we find that ~76% of the galaxies with z > 0.4 show emission-line activity. These numbers are consistent with previous studies. We find that an exponentially decaying star formation rate is required to produce the observed amount of star formation for the majority of the galaxies in our sample. A time constant of ? = 0.6 Gyr appears to be optimal. We also detect several interesting galaxies at z > 1. Two of these galaxies are extremely luminous, with strong Mg II ?2800 absorption and Fe II resonance-line absorption. These lines are so strong that we conclude that they must be generated within the atmospheres of a large population of young, hot stars.

The double quasar Q0957 + 561 A, B - A gravitational lens image formed by a galaxy at z = 0.39

We believed that we have observed the gravitational lens that is responsible for producing the double quasar. Extremely deep CCD pictures of the region show that the QSOs are behind a rich cluster of galaxies. The CCD data and spectrophotometry of the QSOs indicate that the southern QSO image is seen through the brightest cluster galaxy, whose redshift is 0.39.

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.

Diffraction-limited imaging with ground-based optical telescopes

The construction of diffraction-limited images with ground-based optical telescopes by means of closure-phase observations is explored. The effect of redundant baselines on closure-phase observations is analyzed, and it is shown that for bright objects a nonredundant-mask approach is superior to the use of the full aperture. Observations of closure phase with a nonredundant mask on the 200 in. Hale Telescope are presented, and compared with the predictions of atmospheric seeing theory. It is demonstrated that fairly complex images can be constructed from closure-phase observations alone.

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.

A Study of Nine High-Redshift Clusters of Galaxies. I. The Survey

We present a description of the observations and data reduction procedures for an extensive spectroscopic and multiband photometric study of nine high-redshift, optically selected cluster candidates. The primary goal of the survey is to establish new constraints on cluster and galaxy evolution, with specific emphasis on the evolution of galaxy morphology and on the star formation history of the galaxies within and around distant clusters. We have measured 892 new redshifts for galaxies with R ≤ 23.3. The data will also serve as deep probes of the foreground and background large-scale structures. The observations include broadband optical imaging and spectroscopy with the Low Resolution Imaging Spectrograph at the 10 m W. M. Keck Observatory telescope; K-band imaging with the IRIM camera at the 4 m Kitt Peak National Observatory telescope; and deep, high angular resolution imaging with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. We also describe the procedures used to obtain morphological information. We have established that six of the nine cluster candidates are indeed real space density enhancements and are representative of those typically associated with clusters of galaxies. The remaining three candidates appear to be projections of several smaller groups at widely separated distances. This success rate is consistent with estimates of the false-positive rate in two-dimensional optical high-z cluster searches.

Evidence for Cluster Evolution from an Improved Measurement of the Velocity Dispersion and Morphological Fraction of Cluster 1324+3011 at z = 0.76

We have carried out additional spectroscopic observations in the field of cluster Cl 1324+3011 at z = 0.76. Combined with the spectroscopy recently presented by Postman, Lubin, & Oke, we now have spectroscopically confirmed 47 cluster members. With this significant number of redshifts, we measure accurately the cluster velocity dispersion to be 1016 km s-1. The distribution of velocity offsets is consistent with a Gaussian, indicating no substantial velocity substructure. As previously noted for other optically selected clusters at redshifts of z 0.5, a comparison between the X-ray luminosity (LX) and the velocity dispersion (σ) of Cl 1324+3011 implies that this cluster is underluminous in X-rays by a factor of ~3–40 when compared with the LX-σ relation for local and moderate-redshift clusters. We also examine the morphologies of those cluster members that have available high angular resolution imaging with the Hubble Space Telescope (HST). There are 22 spectroscopically confirmed cluster members within the HST field of view. Twelve of these are visually classified as early-type (elliptical or S0) galaxies, implying an early-type fraction of 0.55 in this cluster. This fraction is a factor of ~1.5 lower than that observed in nearby rich clusters. Confirming previous cluster studies, the results for cluster Cl 1324+3011, combined with morphological studies of other massive clusters at redshifts of 0 ≤ z 1, suggest that the galaxy population in massive clusters is strongly evolving with redshift. This evolution implies that early-type galaxies are forming out of the excess of late-type (spiral, irregular, and peculiar) galaxies over the ~7 Gyr timescale.

Supernova 1972e in NGC 5253

New absolute energy distributions of the Type I supernova 1972e in NGC 5253 extending to about 700 days after maximum light have been obtained. A physical model of the expanding envelope, based on the identification of the feature at 6550 A with H

Diffraction-Limited Imaging II. Optical Aperture-Synthesis Imaging of Two Binary Stars

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