Discovery of a New Type of High Redshift Galaxy?

Abstract

Most currently known high redshift (z>2) galaxies are either powerful radio sources, or compact blue star-forming galaxies (eg. Steidel et al. 1996, ApJ 462, L17). We have discovered a group of three galaxies at redshift 2.38 that fall into neither category: radio-quiet galaxies with spectacular line emission and extremely red continuum colours. All three galaxies were discovered through narrow-band Lyalpha imaging and have confirmed multi-line redshift identifications. These three galaxies have the continuum colors of old (> 1 Gyr) massive elliptical galaxies. If true, this implies that at least some ellipticals had formed the vast bulk of their stars at redshifts above 5, and had already settled into comfortable middle age by redshift 2.38. Our galaxies appear to harbour the long sought high redshift analogues of Seyfert II nuclei: luminous radio-quiet QSOs obscured from our line of sight, perhaps by a dusty molecular torus. The three galaxies are surrounded by a compact group (radius ~ 300 projected kpc) of other galaxies with similar continuum colours: we hypothesize that these are other, less active galaxies in the same physical group. If true, the total group must have formed above redshift 5 with a total baryonic mass of > 10(12) solar: quite a challenge for cosmological models. This compact group of proto-ellipticals is surrounded by an extensive halo (radius > 3 Mpc) of blobs of neutral hydrogen, seen in absorption against the spectra of three background QSOs. An admittedly tenuous chain of reasoning leads us to propose that this halo contains at least 1000 such blobs, each with neutral hydrogen masses of ~ 10(8) solar: proto-dwarf galaxies? A paper on this work is in preparation; preliminary results were published as Francis et al. 1996, ApJ 457, 490. Show more