Galaxy formation and evolution

Abstract:

We report the discovery of a group of galaxies at redshift 2.38. We imaged ∼ 10% of a claimed supercluster of QSO absorption lines at z = 2.38. In this small field (2′ radius), we detect two Lyα-emitting galaxies. The discovery of two such galaxies in our tiny field supports Francis & Hewett's interpretation of the absorption-line supercluster as a high-redshift Great Wall. One of the Lyα galaxies lies 22″ from a background QSO and may be associated with a multicomponent Lyα absorption complex seen in the QSO spectrum. This galaxy has an extended (∼50 kpc), lumpy Lyα morphology surrounding a compact, IR-bright nucleus. The nucleus shows a pronounced break in its optical-UV colors at ∼4000 Å (rest frame), consistent with a stellar population of mass ∼7 × 10 M , an age of greater than 500 Myr, and little ongoing star formation. C iv emission is detected, which suggests that a concealed active galactic nucleus is present. The Lyα emission is redshifted by ∼490 km s with respect to the C IV emission, probably because of absorption. Extended Hα emission is also detected; the ratio of Lyα flux to Hα is abnormally low (∼0.7), probable evidence for extended dust. This galaxy is surrounded by a number of very red (B -K > 5 mag) objects, some of which have colors that suggest that they too are at z = 2.38. We hypothesize that this galaxy, its neighbors, and a surrounding lumpy gas cloud may be a giant elliptical galaxy in the process of bottom-up formation. © 1996. The American Astronomical Society. All rights reserved.