Ca II Absorbers in the Sloan Digital Sky Survey: Statistics
  We present the results of a survey for CaII 3934,3969 absorption-line systems
culled from ~ 95,000 Sloan Digital Sky Survey (SDSS) Data Release 7 and Data
Release 9 quasar spectra. With 435 doublets identified in the catalog, this
list is the largest CaII catalog compiled to date, spanning redshifts z < 1.34,
which corresponds to the most recent ~ 8.9 Gyrs of the history of the Universe.
We derive statistics on the CaII rest equivalent width distribution (REW) and
incidence (number density per unit redshift). We find that the lambda3934 REW
distribution cannot be described by a single exponential function. A double
exponential function is required to produce a satisfactory description. The
function can be written as a sum of weak and strong components: dn/dW =
(N_wk*/W_wk*) exp(-W/W_wk*) + (N_str*/W_str*) exp(-W/W_str*). A maximum
likelihood fit to the unbinned data indicates: N_wk*=0.140 +/- 0.029,
W_wk*=0.165 +/- 0.020 A, N_str*=0.024 +/- 0.020, and W_str*=0.427 +/- 0.101 A.
This suggests that the CaII absorbers are composed of at least two distinct
populations. The incidence (product of integrated absorber cross section and
their co-moving number density) of the overall CaII absorber population does
not show evidence for evolution in the standard cosmology. The normalization of
the no-evolution curve, i.e., the value of the CaII incidence extrapolated to
redshift z=0, for lambda 3934 >= 0.3 A, is n_0=0.017 +/- 0.001. In comparison
to MgII surveys, we found that only 3% of MgII systems in the SDSS have CaII,
confirming that it is rare to identify CaII in quasar absorption-line surveys.
We also report on some preliminary investigations of the nature of the two
populations of CaII absorbers, and show that they can likely be distinguished
using their MgII properties.
