The Population of Weak Mg II Absorbers. I. A Survey of 26 QSO HIRES/Keck Spectra
Vogt, Steven S.; Charlton, Jane C.; Churchill, Christopher W.; Rigby, Jane R.
United States
Abstract
We present a search for ``weak'' Mg II absorbers [those with Wr(2796)<0.3 Å] in the HIRES/Keck spectra of 26 QSOs. We found 30, of which 23 are newly discovered. The spectra are 80% complete to Wr(2796)=0.02 Å and have a cumulative redshift path of ~17.2 for the redshift range 0.4<=z<=1.4. The number of absorbers per unit redshift, dN/dz, is seen to increase as the equivalent width threshold is decreased; we obtained dN/dz=1.74+/-0.10 for our 0.02 Wr(2796)<0.3 Å sample. The equivalent width distribution follows a power law, N(W)~W-δ, with δ~1.0 there is no turnover down to Wr(2796)=0.02 Å at <z>=0.9. Weak absorbers comprise at least 65% of the total Mg II absorption population, which outnumbers Lyman limit systems (LLSs) by a factor of 3.8+/-1.1 the majority of weak Mg II absorbers must arise in sub-LLS environments. Tentatively, we predict that ~5% of the Lyα forest clouds with Wr(Lyα)>=0.1 Å will have detectable Mg II absorption to Wminr(2796)=0.02 Å and that this is primarily a high-metallicity selection effect ([Z/Zsolar]>=-1). This implies that Mg II absorbing structures figure prominently as tracers of sub-LLS environments where gas has been processed by stars. We compare the number density of Wr(2796)>=0.02 Å absorbers with that of both high and low surface brightness galaxies and find a fiducial absorber size of 35 h-1-63 h-1 kpc, depending upon the assumed galaxy population and their absorption properties. The individual absorbing ``clouds'' have Wr(2796)<=0.15 Å, and their narrow (often unresolved) line widths imply temperatures of ~25,000 K. We measured Wr(1548) from C IV in Faint Object Spectrograph/Hubble Space Telescope archival spectra and, based upon comparisons with Fe II, found a range of ionization conditions (low, high, and multiphase) in absorbers selected by weak Mg II.
Based in part on observations obtained at the W. M. Keck Observatory, which is jointly operated by the University of California and the California Institute of Technology. Based in part on observations obtained with the NASA/ESA Hubble Space Telescope, which is operated by the STScI for the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.