Professor Andrew Bunker

Abstract:

THE most distant astronomical objects observed are quasars at redshifts of z ≃ 4.9 (ref. 1), corresponding to a time when the Universe was less than a billion years old. This leaves little time during which the quasars and their host galaxies could form. In principle, the evolutionary state of the host galaxies can be probed by determining how many stars have formed, but this task is not straightforward because light from the quasar itself overwhelms any accompanying starlight. High-redshift radio galaxies-the likely progenitors of luminous elliptical galaxies-provide better targets for such studies, as optical emissions from their active nuclei are observed to be faint. Here we report the discovery of a radio galaxy (6C0140 + 326) at z = 4.41 which shows no evidence for either a stellar continuum or an unobscured quasar nucleus. We conclude that the galaxy associated with the radio source is neither fully formed nor obviously in the process of forming stars. This implies that at least some giant elliptical galaxies are still immature at z ≃ 4.5, and that if the intense bursts of star formation thought to produce the bulk of their stellar populations occur during the radio-bright phase, these star-forming regions are obscured by dust and gas.

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.