The Absorption and Emission Kinematics in the z = 0.7450 Mg ii Absorber toward Q1331+17

Abstract

We present a comparative analysis of the galaxy emission and quasar(QSO) absorption kinematics of a zabs~0.7450 Mg II system and itscandidate absorbing galaxy (G5) located 3.86" (28.3 h-170 kpc) fromthe QSO. We have obtained a spectrum of the galaxy candidate, previouslyidentified as a luminous edge-on disk, and detect the [O II] ?3727doublet at a systemic redshift of zsys=0.7450. From slit spectroscopyof this galaxy, we find vrot>~210 km s-1, possibly as large as 350km s-1. Plotted on the same velocity scale, the systemic redshiftof the galaxy coincides with the center of the absorption system,although the absorption components span more than 100 km s-1 in eitherdirection. However, once the sense of the rotation is taken intoaccount, there is no absorbing gas at the projected velocity of thedisk rotation curve. This implies incompatibility with a simple diskscenario. Moreover, a reanalysis of archival Hubble Space Telescope(HST) data reveals that the galaxy is only 0.3L*, considerably lessluminous than previously reported in the literature. This is incompatiblewith the established Tully-Fisher relation at this redshift, unlessapproximately 2 mag of total extinction is invoked. Careful inspectionof the archival HST data reveals that G5 may well be composed oftwo galaxies, although the quality of the data does not permit adetailed investigation of this. This possibility is further supportedby the identification of a second faint emission line at ?obs=5674�, whose distinct spatial and velocity profiles indicate that itarises in a different galaxy at a different redshift. Analysis ofthe absorption lines shows evidence for superbubbles in the interstellarmedium of the absorbing galaxy, based on the striking symmetry betweencomponents and large Mg I/Mg II and Mg I/Fe II ratios, indicativeof large densities. The large velocity separations between line pairings,?v~150 km s-1, indicate that these bubbles may be powered by OB associationscomparable to the largest observed at z=0 and that the gas is probablyenriched to at least 1/10 solar metallicity. This is consistent withobservations at low redshift that extended Mg II halos are oftenseen in galaxies that contain disturbed gas. Superbubbles may alsoexplain why the absorber has a relatively large Mg II equivalentwidth relative to the luminosity of the associated galaxy (or galaxies).