Hot gas in galaxy halos traced by coronal broad Lyman alpha absorbers
  We explore the possibility to systematically study the extended, hot gaseous
halos of low-redshift galaxies with Coronal Broad Ly alpha Absorbers (CBLAs).
These are weak, thermally broadenend HI absorption lines arising from the tiny
fraction of neutral hydrogen that resides in the collisionally ionized,
million-degree halo gas in these galaxies. Using a semi-analytic approach, we
model the spatial density and temperature distribution of hot coronal gas to
predict strength, spectral shape, and cross section of CBLAs as a function of
galaxy-halo mass and line-of-sight impact parameter. For virial halo masses in
the range log (M/M_sun)=10.6-12.6, the characteristic logarithmic CBLA HI
column densities and Doppler parameters are log N(HI)=12.4-13.4 and
b(HI)=70-200 km/s, indicating that CBLAs represent weak, shallow spectral
features that are difficult to detect. Yet, the expected number density of
CBLAs per unit redshift in the above given mass range is dN/dz(CBLA)~3,
implying that CBLAs have a substantial absorption cross-section. We compare the
model predictions with a combined set of ultraviolet (UV) absorption-line
spectra from HST/COS and HST/STIS that trace the halos of four low-redshift
galaxies. We demonstrate that CBLAs already might have been detected in these
spectra, but the complex multi-component structure and the limited
signal-to-noise ratio (S/N) complicate the interpretation of these CBLA
candidate systems. Our study suggests that CBLAs represent a very interesting
absorber class that potentially will allow us to further explore the hot
coronae of galaxies with UV spectral data.
