arXiv subjects

Mark Dickinson

Publications and source records attributed to Mark Dickinson.

311 records · Page 18Linked to original sources

arXiv1995

What is 3C 324?

We report ground based and HST observations of the z=1.206 radio galaxy 3C 324, a prototypical example of the radio-optical ``alignment effect.'' While infrared images shows a simple, round object reminiscent of a giant elliptical galaxy, the HST images reveal a spectacular, linear chain of UV-bright subcomponents closely aligned with the radio axis. In light of the available data, we consider various scenarios to explain the properties of 3C 324, as well as evidence for the presence of dust which may obscure the central active nucleus and scatter its light to produce the polarized, aligned continuum seen in the rest-frame UV.

astro-ph↗

arXiv1995

Lensing by Distant Clusters: HST Observations of Weak Shear in the Field of 3C324

We present the detection of weak gravitational lensing in the field of the radio galaxy 3C324 (z=1.206) using deep HST imaging. ~From an analysis of the shapes of faint R=24.5-27.5 galaxies in the field we measure a weak, coherent distortion centered close to the radio source. This shear field most likely arises from gravitational lensing of distant field galaxies by a foreground mass concentration. In the light of previous observations of this region, which indicate the presence of a rich cluster around the radio source, we suggest that the most likely candidate for the lens is the cluster associated with the radio galaxy at z=1.2. If so, this is the most distant cluster to have been detected by weak shear observations. Such a statement has two important consequences. Firstly, it shows that massive, collapsed structures exist in the high redshift Universe, and secondly that a significant fraction of the R=24.5-27.5 field galaxy population lies beyond z=1.2.

astro-ph↗

arXiv1995

Cluster Ellipticals at High Redshift: The View from the Ground and with HST

New ground-based and HST observations of distant clusters make it possible to trace the history of E/S0 galaxies to lookback times of ~10 $h_{50}^{-1}$ Gyr. The data strongly favor a scenario in which cluster ellipticals formed very early with a narrow spread in ages. By z = 1.2 there are changes in color and luminosity consistent with simple passive evolution, but even at that redshift the galaxies appear mature, with red colors suggesting ages of several Gyr. Apparently we are still falling well short of seeing the true epoch of formation.

astro-ph↗

arXiv1995

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.

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arXiv1994

Discovery of a z=2.76 Dusty Radio Galaxy?

We report the discovery of a z=2.76 radio galaxy, MG1019+0535, with an unusual spectrum. Ly$α$, which usually dominates the spectra of high redshift radio galaxies, is very weak and the strongest lines in the spectrum are CIV$λ$1549 and HeII$λ$1640. We speculate that dust is responsible for attenuating the Ly$α$ line. This object provides more evidence for dust formation at early epochs. The optical counterpart of the compact radio source has a double morphology which is orthogonal to the radio source axis and most likely due to the superposition of two distinct objects which may or may not be physically related. There is only one detected line from the brighter component (B) which is close in observed wavelength to the redshifted HeII$λ$1640 line from the z=2.76 galaxy (A). We discuss whether B is at the same redshift as A or whether it is a foreground galaxy at z=0.66. We also report the serendipitous discovery of a very red ($R-K>7$) field object.

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