Probing Halos of Galaxies at Very Large Radii Using Background QSOs.

Abstract

Gaseous halos of nine nearby galaxies (with redshifts cz < 6000 km s-1) were probed at large galactocentric radii using background quasars observed with the Hubble Space Telescope Goddard High Resolution Spectrograph and the Space Telescope Imaging Spectrograph. The projected quasar-galaxy separations range from 55 to 387 h75-1 kpc. Lyα absorption lines were successfully detected in the spectra of five quasars, at impact parameters of up to ∼170 h75-1 kpc from the center of the nearby galaxy, and in each case at wavelengths consistent with the galaxy's redshift. Our observations include the lowest redshift Lyα lines detected to date. H I velocity fields were obtained at the Very Large Array for three of the galaxies in our sample (in one case the velocity field was available from the literature) to derive their rotation curves. When comparing the inner rotation curves of the galaxies with the velocity at large radius provided by the Lyα line, it is apparent that it is very difficult to explain the observed Lyα velocity as due to gas in an extended rotating disk. In most cases, one would need to invoke large warps in the outer gas disks and also thick gas disks to reconcile the observed velocities with the predicted ones. Indeed, in one case, the Lyα line velocity indicates, in fact, counterrotation with respect to the inner disk rotation. In light of these results, we conclude that in a typical galaxy there is no longer detectable atomic gas corotating in an extended disk at radii greater than 35α-1, where α-1 is the stellar disk exponential scale length. The cosmic web is the most likely origin for the detected Lyα lines. Our observations confirm the recent Bowen et al. correlation of equivalent widths with the local volume density of galaxies around the sight line, and the observed equivalent widths of the lines are consistent with expectations of the cosmic web.