Judith G. Cohen — Page 7

arXiv1996

Redshift clustering in the Hubble Deep Field

We present initial results from a redshift survey carried out with the Low Resolution Imaging Spectrograph on the 10~m W. M. Keck Telescope in the Hubble Deep Field. In the redshift distribution of the 140 extragalactic objects in this sample we find 6 strong peaks, with velocity dispersions of ${\sim}400${\kms}. The areal density of objects within a particular peak, while it may be non-uniform, does not show evidence for strong central concentration. These peaks have characteristics (velocity dispersions, density enhancements, spacing, and spatial extent) similar to those seen in a comparable redshift survey in a different high galactic latitude field (Cohen et al 1996), confirming that the structures are generic. They are probably the high redshift counterparts of huge galaxy structures (``walls'') observed locally.

astro-ph↗

arXiv1995

The Discovery of Two Giant Arcs in the Rich Cluster A2219 with the Keck Telescope

We report the discovery with the Keck telescope of two new multiply-imaged arcs in the luminous X-ray cluster A2219 ($z=0.225$). The brighter arc in the field is red and we use spectroscopic and photometric information to identify it as a $z \sim 1$ moderately star-forming system. The brightness of this arc implies that it is formed from two merging images of the background source, and we identify possible candidates for the third image of this source. The second giant arc in this cluster is blue, and while fainter than the red arc it has a similarly large angular extent (32 arcsec). This arc comprises three images of a single nucleated source -- the relative parities of the three images are discernible in our best resolution images. The presence of several bright multiply imaged arcs in a single cluster allows detailed modelling of the cluster mass distribution, especially when redshift information is available. We present a lensing model of the cluster which explains the properties of the various arcs, and we contrast this model with the optical and X-ray information available on the cluster. We uncover significant differences between the distributions of mass and X-ray gas in the cluster. We suggest that such discrepancies may indicate an on-going merger event in the cluster core, possibly associated with a group around the second brightest cluster member. The preponderance of similar merger signatures in a large fraction of the moderate redshift clusters would indicate their dynamical immaturity.

astro-ph↗