Monday, January 07, 2008

Winds, winds everywhere...

The current (January 1st 2008) edition of the Astrophysical Journal has two interesting papers that discuss galactic or AGN-driven winds.

Prochaska et al, 2008, ApJ, 672, 59 "On the Nature of Velocity Fields in High‐z Galaxies" discusses statistical analyzes of the velocity fields in damped Lyman alpha systems (extended gas in high redshift galaxies either seen spectroscopically in absorption along the line of sight to either a higher redshift AGN, or galaxies that are host to a Gamma Ray burst). They consider a variety of statistics drawn from various absorption lines, and conclude that:

The data support a scenario in which the statistic reflects dynamics in the interstellar medium (ISM) and W1526 traces motions outside the ISM (e.g., halo gas and galactic‐scale winds). The W1526 statistic and gas metallicity [M/H] are tightly correlated, especially for the QSO‐DLAs: with and . We argue that the W1526 statistic primarily tracks dynamical motions in the halos of high‐z galaxies and interpret this correlation as a mass‐metallicity relation with very similar slope to the trend observed in local, low‐metallicity galaxies. Finally, the GRB‐DLAs exhibit systematically larger W1526 values (>0.5 Å) than the QSO‐DLAs ( Å), which may suggest that galactic‐scale outflows contribute to the largest observed velocity fields.

Interesting stuff at first glance. It will be interesting to see how distinct a probe of galactic wind dynamics this method may prove, as compared to the blue-shifted nebular absorption lines seen directly in Lyman Break galaxy spectra.

The other paper, Ganguly & Brotherton, 2008, ApJ, 672, 102 "On the Fraction of Quasars with Outflows" is again based on absorption line studies, but this time on outflows intrinsic to the AGN. They conclude that about 60% of AGN display evidence of driving outflows, over a wide range of intrinsic AGN luminosity. Note that these outflows may be quite compact, perhaps as small as a few parsecs (compared to ~10+ kiloparsec-scale of starburst-driven galactic winds), even if they are similar or greater in absolute mechanical power.

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