QUASAR ABSORBING GALAXIES AT z approx < 1: DEEP IMAGING AND SPECTROSCOPY IN THE FIELD OF 3C 336

摘要

We present very deep WFPC2 images and FOS spectroscopy from the Hubble Space Telescope (HST) together with numerous supporting ground-based observations of the field of the quasar 3C 336 (z_(em) = 0.927). The observations are designed to investigate the nature of galaxies producing metal-line absorption systems in the spectrum of the QSO. Along a single line of sight, we find at least six metal-line absorption systems (of which three are newly discovered) ranging in redshift from 0.317 to 0.892. Through an extensive program of optical and IR imaging, QSO spectroscopy, and faint galaxy spectroscopy, we have identified five of the six metal-line absorption systems with luminous (L_K ≥ 0.1L_K~*) galaxies. These have morphologies ranging from very late-type spiral to S0, and they exhibit a wide range of inclination and position angles with respect to the QSO sight line. The only unidentified absorber, despite our intensive search, is a damped Lyman-α system at z_(abs) = 0.656. Analysis of the absorption spectrum suggests that the metal abundances ([Fe/H] = -1.2) in this system are similar to those in damped systems at z ～ 2 and to the two other damped systems for which abundances have been determined at z < 1. The absorption line system must either be associated with an underluminous, late-type spiral galaxy, which we find at a projected disk impact parameter of ～ 120 h~(-1) kpc, or with an as yet unseen, extremely faint galaxy (L < 0.05L_K~*) near the QSO sight line that eludes detection despite our deep HST and high-resolution ground-based near-IR images. We have found no examples of intrinsically faint galaxies (L < 0.1L~*) at small impact parameters that might have been missed as absorber candidates in our previous ground-based imaging and spectroscopic programs on Mg II absorbing galaxies. We have, however, identified several intrinsically faint galaxies within ～ 50 h~(-1) kpc of the QSO sight line that do not produce detectable metal-line absorption. There are no bright galaxies (L > 0.1 L_K) within 50 h~(-1) kpc that do not produce detectable metal lines (of Mg II λλ2796, 2803 and/or C IV λλ1548, 1550) in the QSO spectrum. All of these results generally support the inferences we have previously reached from a larger survey for absorption-selected galaxies at z approx < 1. There are several other galaxies with redshifts near that of 3C 336, suggesting that the QSO is situated in an overdense region, perhaps a galaxy cluster. Previously published reports of a cluster around 3C 336 were largely misled by the presence of many foreground galaxies seen in projection near the QSO. It is possible that a reported measurement of weak shear gravitational lensing in this field may be produced by the QSO cluster itself, since there appear to be no other groups or clusters in the foreground. We find no evidence for a normal, bright QSO host galaxy, although there are several faint objects very close to the quasar and at similar redshift that might either be companions or part of a disorganized QSO host.