NICMOS Imaging of a Cluster of Extremely Red Galaxies at Redshift z=2.38

Abstract

We present NICMOS observations of the central regions of a rich cluster of galaxies at redshift z=2.38. The observations were scheduled for two weeks after the abstract submission deadline, so no results here, I'm afraid. The cluster was originally identified as an enormous overdensity of QSO Damped Lyman-alpha absorption-line systems seen in the spectra of three background QSOs (Francis and Hewett 1993, AJ 105, 1633). Ground-based imaging revealed enormous overdensities of Lyman-alpha emitting galaxies and other extremely red objects associated with the absorption-line cluster (Francis et al. 1996, ApJ 457, 490, Francis, Woodgate and Danks 1997, ApJ 482, L25). This cluster is probably the best studied cluster in the early universe and poses a number of challenges to our current understanding of galaxy and structure formation. Firstly, the cluster is extraordinarily overdense (a factor of more than 10 on 10 Mpc scales) which implies remarkably high biassing. Secondly, most of the galaxies currently detected in the cluster have very red optical/IR colors (B-K>6, J-H>2), in great contrast to most other radio-quiet galaxies at high redshift. Naively interpreted, these colors suggest that the galaxies have baryonic masses of >10(12) solar masses, and completed the bulk of their star formation above redshift five, which is impossible is most hierarchical structure formation models. Thirdly, three of the galaxies appear to be radio-quiet counterparts of radio galaxies; the long sought QSO IIs. We will present broad-band and narrow-band H-beta NICMOS NIC3 imaging of the central region of the cluster, including three extremely red objects, one QSO II and a background QSO. We will determine the morphology of the galaxies, probe the luminosity function, and measure the line fluxs and redshifted 400nm break sizes.