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 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.
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