The Population of Weak Mg ii Absorbers. I. A Survey of 26 QSO HIRES/Keck Spectra

Abstract

We present a search for "" weak ÏÏ Mg II absorbers [those with in the HIRES/Keck W r (2796) \ 0.3 A ] spectra of 26 QSOs. We found 30, of which 23 are newly discovered. The spectra are 80% complete to and have a cumulative redshift path of D17.2 for the redshift range 0.4 ¹ z ¹ 1.4. The W r (2796) \ 0.02 A 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 sample. The equivalent width W r (2796) \ 0.3 A distribution follows a power law, N(W ) P W~d, with d D 1.0 ; there is no turnover down to W r (2796) \ 0.02 at SzT \ 0.9. Weak absorbers comprise at least 65% of the total Mg II absorption population, A 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 D5% of the Lya forest clouds with will have detectable Mg II absorption to and that this W r (Lya) º 0.1 A W r min(2796) \ 0.02 A is primarily a high-metallicity selection e †ect This implies that Mg II absorbing struc-. tures Ðgure prominently as tracers of sub-LLS environments where gas has been processed by stars. We compare the number density of absorbers with that of both high and low surface W r (2796) º 0.02 A brightness galaxies and Ðnd a Ðducial absorber size of 35 h~1È63 h~1 kpc, depending upon the assumed galaxy population and their absorption properties. The individual absorbing "" clouds ÏÏ have W r (2796) ¹ 0.15 and their narrow (often unresolved) line widths imply temperatures of D25,000 K. We measured A , from C IV in Faint Object Spectrograph/Hubble Space T elescope archival spectra and, based W r (1548) upon comparisons with Fe II, found a range of ionization conditions (low, high, and multiphase) in absorbers selected by weak Mg II.

The Kinematic Connection between absorbing Gas toward QSOs and Galaxies at Intermediate Redshift

“…An a posteriori search of the HIRES spectrum yielded a weak Mg ii doublet with W 0 ð 2796Þ ¼ 0:08 Å , so that it is well below the detection threshold of the SS92 survey and most earlier Mg ii absorption-line surveys. We later realized that this weak system was cataloged by Churchill et al (1999) at z abs ¼ 0:55020.…”

High‐Redshift Superwinds as the Source of the Strongest MgiiAbsorbers: A Feasibility Analysis

“…The unÑuxed continuum was normalized using the methods described by Sembach & Savage (1992). Further details of the data reduction can be found in Churchill (1997) ; Churchill et al (1999bChurchill et al ( , 2000aChurchill et al ( , 2000b, and Churchill & Vogt (2001).…”

“…DATA AND ANALYSIS 2.1. Sample Selection and Observations During the observing runs for a larger program to study the kinematics of Mg II absorbers at 0.5 \ z \ 1.5 (Churchill, Steidel, & Vogt 1996 ;Charlton & Churchill 1998 ;Churchill et al 1999b ;Churchill & Vogt 2001), several very large equivalent width systems (W r [ 1.8 A ) were observed at z [ 1. The goal was to obtain a small sample of kinematic data from systems belonging to the strongly evolving population of Mg II absorbers W r [ 1.8 A documented by Steidel & Sargent (1992).…”

CIV absorbers inz ${>4}$ quasars: Tracing early galactic halos evolution

“…MgII systems are known to be associated with the gaseous envelopes of bright galaxies which have been detected in emission at z ∼ 0.6 (Bergeron & Boissé 1991). The typical impact parameter from the galaxy varies with the equivalent width threshold of the surveys for absorbers: 40 h −1 kpc at W rest = 0.30 Å and 60 h −1 kpc at W rest = 0.02 Å (Churchill et al 1999). In addition the absorbers optically thick to hydrogen ionizing radiation, i.e.…”