Calcium H & K Induced by Galaxy Halos

Abstract

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.