TRACING OUTFLOWS AND ACCRETION: A BIMODAL AZIMUTHAL DEPENDENCE OF Mg II ABSORPTION

Abstract

We report a bimodality in the azimuthal angle distribution of gas around galaxies as traced by Mg II 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 Mg II absorption-selected galaxies [W r (2796) ≥ 0.1 Å] and 35 spectroscopically confirmed nonabsorbing galaxies [W r (2796) < 0.1 Å] 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) < 0.1 Å. We find that blue star-forming galaxies clearly drive the bimodality while red passive galaxies may exhibit an excess along their major axis. These results are consistent with galaxy evolution scenarios where star-forming galaxies accrete new gas, forming new stars and producing winds, while red galaxies exist passively due to reduced gas reservoirs. We further compute an azimuthal angle dependent Mg II absorption covering fraction, which is enhanced by as much as 20−30% along the major and minor axes. The W r (2796) distribution for gas along the major axis is likely skewed toward weaker Mg II 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. Adopting these assumptions, we find that the opening angle of outflows and inflows to be 100 • and 40 • , respectively. We find the probability of detecting outflows is ∼60%, implying that winds are more commonly observed.

EVIDENCE FOR UBIQUITOUS COLLIMATED GALACTIC-SCALE OUTFLOWS ALONG THE STAR-FORMING SEQUENCE ATz∼ 0.5

“…All together, this analysis suggests that 50% of the star-forming galaxy population at z ∼ 0.5 has θ 0 greater than ∼42 • -60 • . These opening angles are consistent with the wind opening angle measured in the molecular gas outflow from M82 (θ 0 = 55 • ; Walter et al 2002), the opening angle implied by the detection of Na i-absorbing outflows from SDSS starforming galaxies (θ 0 ∼ 60 • ; Chen et al 2010), and the opening angles inferred from analysis of the azimuthal angle dependence of Mg ii absorption toward background QSOs (θ 0 ≈ 50 • ; Kacprzak et al 2012). Rupke et al (2005b) inferred somewhat smaller opening angles for Na i-absorbing winds from infraredselected starbursts and ULIRGs out to z ∼ 0.5; however, they also noted a possible dependence of opening angle on the IR luminosity of their sample: galaxies with 10 10 < L IR /L < 10 12 exhibited θ 0 ∼ 32 • , while galaxies with L IR /L > 10 12 exhibited θ 0 ∼ 46 • .…”

“…In more distant galaxies, for which emission from outflows is typically prohibitively faint, outflow morphology is not yet well understood. However, the enhanced Mg ii absorption detected toward background QSOs located along the minor axis of absorber host galaxies at z 1 is suggestive of wind collimation (Bordoloi et al 2011;Bouché et al 2012;Kacprzak et al 2012). Further evidence for anisotropic outflows is provided by Kornei et al (2012), who measured higher outflow velocities toward z ∼ 1 galaxies having i < 45 • .…”

“…Studies searching for the galaxies associated with very strong Mg ii absorbers having EW 2796 > 2 Å have identified potential counterparts at impact parameters ρ ∼ 10-60 kpc (e.g., Bouché et al 2007;Nestor et al 2011); however, these surveys may miss the true galaxy counterparts if their emission is blended with the bright QSO emission or below the detection limit. To constrain the frequency with which high-EW absorbers occur beyond 5-10 kpc, larger samples of halo absorption strength measurements for galaxies whose redshifts are known a priori at impact parameters <50 kpc are needed, preferably in combination with analysis of the galaxy disk orientation (e.g., Bouché et al 2012;Kacprzak et al 2012).…”

Star formation sustained by gas accretion

“…A particularly telling deviation from axisymmetry has been found recently by Kacprzak et al (2012a). They report a bimodality in the azimuthal angle distribution of low ionization gas around galaxies as traced by MgII absorption along QSO lines-of-sight.…”

Gas Accretion via Lyman Limit Systems

“…The z < 1.5 epoch is also a time where galaxy transformation is occurring with blue galaxies turning into red galaxies. The morphology, inclination, and geometry of z < 1 galaxies can also be robustly determined (e.g., Bordoloi et al 2011;Kacprzak et al 2012a;Bouché et al 2012), which will help us providing additional insights on the actual gas origin (see §9).…”