Keck HIRES Spectroscopy: Low-Redshift Mg II and Lyman α Systems

Abstract

Motivated by the proposed connection between galaxies and the Ly α forest at low redshifts, we report on a sensitive search for Mg II absorption corresponding to Ly α lines detected by the HST Key Project at z ≤ 1.3. Spectra of the bright QSO, PG 1634+706 were obtained using the High Resolution Echelle Spectrometer on the Keck Telescope. We do not detect Mg II absorption corresponding to the majority of Ly α lines although many have similar strength to those which do have associated Mg II lines. The non-detections provide useful limits on the metal abundance in both individual lines and averaged over all Ly α redshifts. We detect an excess of weak Mg II systems which also show corresponding C IV and Lyman-limit absorption when such data are available In several cases, the extremely narrow line widths are consistent with thermal broadening at temperatures characteristic of Ly α forest lines. From the observed number density, we discuss implications for the size and luminosity of weak-line absorbers.