HiGas in Higher Density Regions of the Intergalactic Medium

Abstract

Using H I absorption alone, we attempt to separate H I absorption lines in quasar spectra into two categories: higher density lines (HDLs) and lower density lines (LDLs), and we discuss the difference in their physical properties. We deblend and fit all H I lines with Voigt profiles and make an unbiased sample of H I lines covering a wide column density range (12 < log NH I [cm-2] < 19). To reduce the influence of line blending, we simultaneously fit several Lyman series lines. As a result of a two-point correlation analysis, we found that higher column density H I lines are clustering at Δv < 200 km s-1, whereas lower ones cluster at Δv < 100 km s-1. We define HDLs as H I lines with 15 < log NH I < 19 and all H I lines within 200 km s-1 of a line with log NH I > 15 and LDLs as all others with 12 < log NH I < 15. We found that the HDLs have smaller minimum b-values for a given column density than the LDLs. This difference is successfully reproduced by our hydrodynamic simulation. The LDLs seem to be cool or shock-heated diffuse intergalactic medium gas, whereas the HDLs are likely to be cooler dense gas near to galaxies.