Calcium H & K Induced by Galaxy Halos
  We present a measurement of the mean density profile of Ca II gas around
galaxies out to ~ 200 kpc, traced by Fraunhofer's H & K absorption lines. The
measurement is based on cross-correlating the positions of about one million
foreground galaxies at z ~ 0.1 and the flux decrements induced in the spectra
of about 10^5 background quasars from the Sloan Digital Sky Survey. This
technique allows us to trace the total amount of Ca II absorption induced by
the circumgalactic medium, including absorbers too weak to be detected in
individual spectra. We can statistically measure Ca II rest equivalent widths
down to several mA, corresponding to column densities of about 5x10^10 cm^{-2}.
We find that the Ca II column density distribution follows N ~ rp^{-1.4} and
the mean Ca II mass in the halo within 200 kpc is ~ 5x10^3 Msolar, averaged
over the foreground galaxy sample with median mass ~ 10^10.3 Msolar. This is
about an order-of-magnitude larger than the Ca II mass in the interstellar
medium of the Milky Way, suggesting more than 90% of Ca II in the Universe is
in the circum- and inter-galactic environments. Our measurements indicate that
the amount of Ca II in halos is larger for galaxies with higher stellar mass
and higher star formation rate. For edge-on galaxies we find Ca II to be more
concentrated along the minor axis, i.e. in the polar direction. This suggests
that bipolar outflows induced by star formation must have played a significant
role in producing Ca II in galaxy halos.
