EVOLUTION OF IR-SELECTED GALAXIES IN Z~0.4 CLUSTERS

Wide-field optical and near--IR ($JHK$) imaging is presented for two rich galaxy clusters: Abell~370 at $z=0.374$ and Abell~851 (Cl0939+47) at $z=0.407$. Galaxy catalogs selected from the near--IR images are 90\% complete to approximately 1.5 mag below $K^\ast$ resulting in samples with $\sim$100 probable member galaxies per cluster in the central $\sim$2 Mpc. Comparison with $HST$ WFPC images yields subsamples of $\sim$70 galaxies in each cluster with morphological types. Analysis of the complete samples and the $HST$ subsamples shows that the $z\sim 0.4$ E/S0s are bluer than those in the Bower et al.\ (1992) Coma sample in the optical$-K$ color by $0.13$~mag for Abell~370 and by $0.18$~mag for Abell~851. If real, the bluing of the E/S0 populations at moderate redshift is consistent with that calculated from the Bruzual and Charlot (1993) models of passive elliptical galaxy evolution. In both clusters the intrinsic scatter of the known E/S0s about their optical$-K$ color--mag relation is small ($\sim 0.06$ mag) and not significantly different from that of Coma E/S0s as given by Bower et al.\ (1992), indicating that the galaxies within each cluster formed at the same time at an early epoch.Comment: uuencoded gzipped tar file containing latex files of manuscript (42 pages) plus tables (9 pages); figures available by anonymous ftp at ftp://ipac.caltech.edu//pub/pickup/sed ; accepted for publication in the Ap

The K-selected Butcher-Oemler Effect

[abridged] We investigate the Butcher-Oemler effect in a sample of K-selected galaxies in 33 clusters at 0.15 < z < 0.92. We attempt to duplicate the original Butcher-Oemler analysis as closely as possible given the characteristics of our data. We find that the infrared selected blue fractions are lower than those measured in the optical and that the trend with redshift is much weaker. Comparison with optical data in clusters in common with Butcher & Oemler (1984) shows that infrared selection is the primary difference between our study and optically selected samples. We suggest that the Butcher-Oemler effect is in large part due to a population of star-forming low mass galaxies which will evolve into dwarf galaxies. These early results point to the need for larger and deeper infrared samples of cluster galaxies to address this issueComment: 37 pages, 19 figures, ApJ accepted (vol 598 n1

Near-IR imaging of moderate redshift galaxy clusters

We have obtained near-IR imaging of 3 moderate-z clusters on the 1.3 m at KPNO with SQIID, a new camera offering wide-field (5.5 arcmin) simultaneous JHK band imaging. Our photometry on a sample of approximately 100 likely member galaxies in one of the clusters, Abell 370 at z = 0.37, shows that we can obtain magnitudes good to 20 percent down to K = 18, considerably below the estimated K* = 16.5 at this redshift. These data indicate that there are no systematic problems in obtaining photometry at faint levels with SQIID. With the development of larger arrays, the field is open to progress. The resulting J, H, and K data for three clusters are combined with previously obtained multiband optical photometry. We present an investigation of the spectral properties and evolution of the dominant cold stellar populations by comparing optical-to-IR colors and color-magnitude diagrams to predictions from population synthesis models and galaxy spectral evolution codes

An IR-Selected Galaxy Cluster at z=1.27

We report the discovery of a galaxy cluster at z=1.27. ClG J0848+4453 was found in a near-IR field survey as a high density region of objects with very red J-K colors. Optical spectroscopy of a limited number of 24 < R < 25 objects in the area shows that 6 galaxies within a 90 arcsec (0.49/h Mpc, q_O = 0.1) diameter region lie at z=1.273 +/- 0.002. Most of these 6 member galaxies have broad-band colors consistent with the expected spectral energy distribution of a passively-evolving elliptical galaxy formed at high redshift. An additional 2 galaxies located ~2 arcmin from the cluster center are also at z=1.27. Using all 8 of these spectroscopic members, we estimate the velocity dispersion is 700 +/- 180 km/s, similar to that of Abell R=1 clusters in the present epoch. A deep Rosat PSPC observation detects X-ray emission at the 5 sigma level coincident with the nominal cluster center. Assuming that the X-ray flux is emitted by hot gas trapped in the potential well of a collapsed system (no AGN is known to exist in the area), the resulting X-ray luminosity in the rest frame 0.1-2.4 keV band of L_x = 1.5 x 10^44 ergs/s suggests the presence of a moderately large mass. ClG J0848+4453 is the highest redshift cluster found without targetting a central active galaxy.Comment: Accepted for publication in The Astronomical Journal; 22 pages, 6 figures; corrected titl

Discovery of a Color-Selected Quasar at z=5.50

We present observations of RD J030117+002025, a quasar at z=5.50 discovered from deep, multi-color, ground-based observations covering 74 square arcmin. This is the most distant quasar or AGN currently known. The object was targeted as an R-band dropout, with R(AB)>26.3 (3-sigma limit in a 3 arcsec diameter region), I(AB)=23.8, and z(AB)=23.4. The Keck/LRIS spectrum shows broad Lyman-alpha/NV emission and sharp absorption decrements from the highly-redshifted hydrogen forests. The fractional continuum depression due to the Lyman-alpha forest is D(A)=0.90. RD J030117+002025 is the least luminous, high-redshift quasar known (M(B)~-22.7).Comment: 9 pages, 2 figures; to appear in the The Astrophysical Journal Letter

IDCS J1426.5+3508: The Most Massive Galaxy Cluster at z>1.5z > 1.5

We present a deep (100 ks) Chandra observation of IDCS J1426.5+3508, a spectroscopically confirmed, infrared-selected galaxy cluster at $z = 1.75$. This cluster is the most massive galaxy cluster currently known at $z > 1.5$, based on existing Sunyaev-Zel'dovich (SZ) and gravitational lensing detections. We confirm this high mass via a variety of X-ray scaling relations, including $T_X$-M, $f_g$-M, $Y_X$-M and $L_X$-M, finding a tight distribution of masses from these different methods, spanning M$_{500}$ = 2.3-3.3 $\times 10^{14}$ M$_{\odot}$, with the low-scatter $Y_X$-based mass $M_{500,Y_X} = 2.6^{+1.5}_{-0.5} \times 10^{14}$ M$_\odot$. IDCS J1426.5+3508 is currently the only cluster at $z > 1.5$ for which X-ray, SZ and gravitational lensing mass estimates exist, and these are in remarkably good agreement. We find a relatively tight distribution of the gas-to-total mass ratio, employing total masses from all of the aforementioned indicators, with values ranging from $f_{gas,500}$ = 0.087-0.12. We do not detect metals in the intracluster medium (ICM) of this system, placing a 2$\sigma$ upper limit of $Z(r < R_{500}) < 0.18 Z_{\odot}$. This upper limit on the metallicity suggests that this system may still be in the process of enriching its ICM. The cluster has a dense, low-entropy core, offset by $\sim$30 kpc from the X-ray centroid, which makes it one of the few "cool core" clusters discovered at $z > 1$, and the first known cool core cluster at $z > 1.2$. The offset of this core from the large-scale centroid suggests that this cluster has had a relatively recent ($\lesssim$500 Myr) merger/interaction with another massive system.Comment: Minor changes to match accepted version, results unchanged; ApJ in pres