Abstract
Doubly ionized silicon (SiIII) is a powerful tracer of diffuse ionized gas
inside and outside of galaxies. It can be observed in the local Universe in
ultraviolet (UV) absorption against bright extragalactic background sources. We
here present an extensive study of intervening SiIII- selected absorbers and
their relation to the circumgalactic medium (CGM) of galaxies at low redshift
(z<=0.1), based on the analysis of UV absorption spectra along 303
extragalactic lines of sight obtained with the Cosmic Origins Spectrograph
(COS) on board the Hubble Space Telescope (HST). Along a total redshift path of
Dz=24 we identify 69 intervening SiIII systems that all show associated
absorption from other low and high ions. We derive a number density of
dN/dz(SiIII)=2.9 for absorbers with column densities log N(SiIII)>12.2. We
develop a geometrical model for the absorption-cross section of the CGM around
the local galaxy population and find excellent agreement between the model
predictions and the observations. We further compare redshifts and positions of
the absorbers with that of ~64,000 galaxies using archival galaxy-survey data.
For the majority of the absorbers we identify possible L>0.5L* host galaxies
within 300 km/s of the absorbers and derive impact parameters rho<200 kpc,
demonstrating that the spatial distributions of SiIII absorbers and galaxies
are highly correlated. Our study indicates that the majority of SiIII-selected
absorbers in our sample trace the CGM of nearby galaxies within their virial
radii at a typical covering fraction of ~75 per cent. From a detailed
ionization model we estimate that diffuse gas in the CGM around galaxies, as
traced by SiIII, contains substantially more baryonic mass than their neutral
interstellar medium.
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