Hot gas in galaxy halos traced by coronal broad Lyman alpha absorbers

Abstract: 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.