17 January 2012

Circumgalactic Media and Their Hosts

Title: The Large, Oxygen-Rich Halos of Star-Forming Galaxies Are a Major Reservoir of Galactic Metals
Authors: J. Tumlinson et al.

Galaxies grow and evolve by accreting gas from the intergalactic medium (IGM), forming stars with this material, and ejecting the often-enriched remnants through galactic-scale outflows.  At the convergence of these processes lies the circumgalactic medium (CGM), gas surrounding galaxies out to 100 to 300 kpc.  This paper investigates the relationship between properties of host galaxies and their CGM using the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope with absorption-line spectroscopy.

The method of measuring absorption lines through CGM is as follows.  The study focuses on 42 sample galaxies that are close to distant QSOs on the plane of the sky.  As light from the QSOs passes through the CGM of the galaxies, absorption lines can be measured, specifically the ultraviolet O VI doublet 1032 and 1038.  The data is (are?) used to measure O VI column densities, line profiles, and radial velocities of the CGM with respect to the host galaxies.  Furthermore, the Keck Observatory Low-Resolution Imaging Spectrograph (LRIS) and the Las Campanas Observatory Magellan Echellette (MagE) spectrograph were used to measure redshift, star formation rate, and metallicity for each galaxy.

Not surprisingly, the study found that each CGM was close in radial velocity to its host galaxy, suggesting a close physical and gravitational relationship.  Furthermore, there is a correlation between O VI column density and specific star formation rate (sSFR): active, star-forming galaxies have much higher column densities than passive galaxies.  This reflects the bimodality of galaxies and suggests that the CGM either directly affects or is affected directly by the galaxy's star formation.

The CGM also contains a substantial fraction of the metals in the galaxy, and the ratio of CGM metals to ISM metals increases with decreasing galaxy mass.  These metals were most likely created in the galaxies and then transported into the CGM by outflows.  Taking into account the amount of oxygen returned to the ISM during star formation and the typical star formation rate, the authors estimate that the oxygen in the CGM could have been deposited there over several billion years of star formation and outflow.  Furthermore, the observed O VI outflows do not exceed the galaxies' escape velocities, suggesting that this enrichment could eventually fall back onto the galaxy to fuel further star formation.