Abstract
(Abridged) This paper presents an absorption-line study of the multiphase
circumgalactic medium (CGM) based on observations of a suite of absorption
features including Lya, CII, CIV, SiII, SiIII, and SiIV transitions.
Cross-matching between public galaxy and QSO survey data has yielded 111
independent galaxy and QSO pairs for which high-quality archival UV spectra of
the QSOs and multi-wavelength observations of the galaxies are available. The
galaxy sample is characterized by a median redshift of <z> = 0.0232, a median
projected distance of <d> = 342 kpc, and a median stellar mass of log
(Mstar/Msun) = 9.040.93. It is therefore dominated by low-mass dwarf
galaxies. Comparing the absorber features identified in the QSO spectra with
galaxy properties has led to strong constraints for the CGM at z<~0.06. While
abundant hydrogen gas is found beyond the dark matter halo radius Rh and all
through d~500 kpc with a mean covering fraction of ~50%, no heavy elements are
detected at d>~0.5 Rh. The lack of heavy elements at large distances is
unlikely due to ionization effects, since it persists through all ionization
states included in the study. Considering all galaxies at d>Rh leads to a
strict upper limit for the covering fraction of heavy elements of ~4% (at a 95%
confidence level) over d=(1-10) Rh. At d<0.5 Rh, differential covering fraction
between low- and high-ionization gas is observed, suggesting that the CGM
becomes progressively more ionized from d<0.2 Rh to larger distances. Comparing
absorption-line observations of the CGM at low and high redshifts shows that
massive starburst galaxies at z=2.2 exhibit significantly stronger mean
absorption than dwarf galaxies at z~0 and the distinction is most pronounced in
low-ionization species traced by CII and SiII absorption lines, suggesting
distinct ionization conditions between the CGM at low and high redshifts.
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