Аннотация
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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