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 ^ { -1 } , while lower ones at \Delta v < 100 km s ^ { -1 } .
We define HDLs as H i lines with 15 < \log N _ { HI } < 19 cm ^ { -2 } and all H i lines within \pm 200 km s ^ { -1 } 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 .