H I gas in higher density regions of the intergalactic medium
  Using H I absorption alone, we attempt to separate H I absorption lines in quasar spectra into two categories; HDLs (Higher Density Lines) and LDLs (Lower Density Lines), 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 N_HI < 19 cm^-2). 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 $\Delta$ v < 200 km/s, while lower ones at $\Delta$ v < 100 km/s. We define HDLs as H I lines with 15 < log N_HI < 19 cm^-2 and all H I lines within +/- 200 km/s of a line with log N_HI > 15 cm^-2, and LDLs as others with 12 < log N_HI < 15 cm^-2. 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 IGM gas, while the HDLs are likely to be cooler dense gas near to galaxies. 
T. Misawa, D. Tytler, M. Iye, P. Paschos, M. Norman, D. Kirkman, J.   O'Meara, N. Suzuki and N. Kashikawa