Neutral atomic-carbon QSO absorption-line systems at z>1.5: Sample
  selection, HI content, reddening, and 2175 A extinction feature
  We present the results of a search for cold gas at high redshift along QSO
lines-of-sight carried out without any a priori assumption on the neutral
atomic-hydrogen (HI) content of the absorbers. To do this, we systematically
looked for neutral-carbon (CI) 1560,1656 transition lines in low-resolution QSO
spectra from the SDSS database. We built up a sample of 66 CI absorbers with
redshifts 1.5<z<3.1 and equivalent widths 0.1<W_r(1560)<1.7 A. The completeness
limit of our survey is W_r,lim(1560)~0.4 A. CI systems stronger than that are
more than one hundred-times rarer than DLAs at z_abs=2.5. The number of CI
systems per unit redshift increases significantly below z=2. We suggest that
the CI absorbers are closely related to the process of star formation and the
production of dust in galaxies. We derive the HI content of the CI systems and
find that a majority of them are sub-DLAs with N(HI)~10^20 atoms cm^-2. The
dust content of these absorbers is yet significant as seen from the redder
optical colours of the background QSOs and their reddened SEDs. The overall
N(HI) distribution of CI systems is relatively flat however. As a consequence,
among the CI systems classifying as DLAs there is a probable excess of strong
DLAs with log N(HI)>21 compared to systematic DLA surveys. We study empirical
relations between W_r(CI), N(HI), E(B-V) and the strength of the 2175 A
extinction feature, the latter being detected in about 30% of the CI absorbers.
We show that the 2175 A feature is weak compared to Galactic lines-of-sight
exhibiting the same amount of reddening. This is probably the consequence of
current or past star formation in the vicinity of the CI systems. We also find
that the strongest CI systems tend to have the largest amounts of dust and that
the metallicity of the gas and its molecular fraction is likely to be high in a
large number of cases.
