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Session 110 - Imaging of Cluster Galaxies.
Display session, Thursday, January 18
North Banquet Hall, Convention Center

[110.01] Discovery of a Cluster of Young Galaxies at z\simeq2.40 in Deep HST WFPC2 Images: Formation from Sub-galactic Size Clumps?

S. M. Pascarelle, R. A. Windhorst, S. C. Odewahn (ASU), W. C. Keel (U of Al)

We present the discovery of a large number of significant, compact Lyman-alpha emitting candidates at z\simeq2.40 in Cycle 5 HST WFPC2 images. The F410M filter (Lyman-alpha at z\simeq2.40) was used, in conjunction with F450W for continuum subtraction, to find at least 16 potential z\simeq2.40 objects within a 0.5h_100^-1Mpc\times0.5h_100^-1Mpc region (q_o=0.5). Earlier ground-based narrow- and broad-band photometry on the same field yielded only four cluster candidates (Pascarelle, et al. 1995, ApJL). Four of the 16 have been spectroscopically confirmed to be at z\simeq2.40. All candidates are much smaller than the median scale-length of WFPC2 field galaxies at the same magnitude. They have half-light radii \simeq0.11'' or \simeq0.43h_100^-1kpc, and have luminosities in the range M_v=--18 to M_v=--23 (<0.1-1L at z\simeq2.40). It is most likely because of the very small scale-lengths of these sub-galactic clumps that previous ground-based Lyman-alpha primeval galaxy searches have been largely unsuccessful. Lyman-alpha searches for such compact objects carried out in typical ground-based seeing and sky brightness would take about 200 times longer than with WFPC2 and F410M, indicating that HST may hold the secret to success in proto-galaxy searches. We propose that these sub-galactic clumps could have grown into the luminous giant galaxies (E/S0 and early-type spirals) seen today, through the CDM process of repeated (hierarchical) merging.

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