G. G. Kacprzak, C. W. Churchill, N. M. Nielsen
We report a bimodality in the azimuthal angle distribution of gas around galaxies as traced by MgII absorption: Halo gas prefers to exist near the projected galaxy major and minor axes. The bimodality is demonstrated by computing the mean azimuthal angle probability distribution function using 88 spectroscopically confirmed MgII absorption-selected galaxies [W_r(2796)> 0.1A] and 35 spectroscopically confirmed non-absorbing galaxies [W_r(2796)<0.1A] imaged with HST and SDSS. The azimuthal angle distribution for non-absorbers is flat, indicating no azimuthal preference for gas characterized by W_r(2796)<0.1A. We find that blue star-forming galaxies clearly drive the bimodality. We compute an azimuthal angle dependent MgII absorption covering fraction and find that it is enhanced by as much as 20-30% along the major and minor axes. The equivalent width distribution for gas along the major axis is likely skewed toward weaker MgII absorption than for gas along the projected minor axis. These combined results are highly suggestive that the bimodality is driven by gas accreted along the galaxy major axis and outflowing along the galaxy minor axis. The opening angle of outflows is 2.5 times larger than for accreting gas. We find the probably of detecting outflows is 60%, implying that winds are more commonly observed. This scenario is consistent with ideas of galaxy evolution were star-forming galaxies accrete new gas reservoirs, forming new stars and producing winds, while red early-type galaxies exist passively due to a lack of new gas reservoirs to form new stars.
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http://arxiv.org/abs/1205.0245
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