New Perspective on Galaxy Outflows From the First Detection of Both Intrinsic and Traverse Metal-Line Absorption

Abstract

We present the first observation of a galaxy (z = 0.2) that exhibits metal-line absorption back-illuminated by the galaxy ("down-the-barrel") and transversely by a background quasar at a projected distance of 58 kpc. Both absorption systems, traced by Mg II, are blueshifted relative to the galaxy systemic velocity. The quasar sight-line, which resides almost directly along the projected minor axis of the galaxy, probes Mg I and Mg II absorption obtained from Keck/LRIS and Ly α, Si II and Si III absorption obtained from HST/COS. For the first time, we combine two independent models used to quantify the outflow properties for down-the-barrel and transverse absorption. We find that the modeled down-the-barrel deprojected outflow velocities range between V dtb = 45 − 255 km s −1 . The transverse bi-conical outflow model, assuming constant-velocity flows perpendicular to the disk, requires wind velocities V out f low = 40 − 80 km s −1 to reproduce the transverse Mg II absorption kinematics, which is consistent with the range of V dtb . The galaxy has a metallicity, derived from Hα and N II, of [O/H]=−0.21±0.08, whereas the transverse absorption has [X/H] = −1.12 ± 0.02. The galaxy star-formation rate is constrained between 4.6-15 M ⊙ yr −1 while the estimated outflow rate ranges between 1.6-4.2 M ⊙ yr −1 and yields a wind loading factor ranging between 0.1 − 0.9. The galaxy and gas metallicities, the galaxy-quasar sight-line geometry, and the down-the-barrel and transverse modeled outflow velocities collectively suggest that the transverse gas originates from ongoing outflowing material from the galaxy. The ∼1 dex decrease in metallicity from the base of the outflow to the outer halo suggests metal dilution of the gas by the time it reached 58 kpc.

POSSIBLE SIGNATURES OF A COLD-FLOW DISK FROM MUSE USING A z ∼ 1 GALAXY–QUASAR PAIR TOWARD SDSS J1422−0001*

“…However, the Mg II absorption harbors a significant component from the ISM at zero velocity. Hence, performing a double-Gaussian fit to the absorption representing the ISM 18 and the wind components (as in Martin et al 2012;Kacprzak et al 2014) shown in Figure 4, we find that the wind speed (at peak optical depth) is about » - V 80 15 out km s −1 . This is the bulk velocity where most of the optical depth is, but the Mg II profile (and the Fe IIprofile) shows absorption up to ∼−150 km s −1 .…”

“…As discussed in the next subsection, we can gain insights into the nature of the absorbing gas by comparing the line-ofsight kinematics (with respect to the host galaxy kinematics) to simple models. Such analyses are powerful, but have only been possible in very few cases, such as in Steidel et al (2002), Bouché et al (2012Bouché et al ( , 2013, Kacprzak et al (2010Kacprzak et al ( , 2014,and Schroetter et al (2015) with background quasars and in Rubin et al (2010) and Diamond-Stanic et al (2015) with a bright background galaxy. This analysis requires good constraints on the galaxy systemic redshift and onthe galaxyʼs relative orientation with respect to the quasar sightline.…”

“…This possibility is not supported by the statistical results in the literature and by our data, as discussed inSection 4.6. Furthermore, a simple bi-conical flow model-which has been successful in reproducing absorption profiles in Bouché et al (2012), Kacprzak et al (2014), andSchroetter et al (2015)-would produce absorption at a single speed because the line of sight is almost entirely radial, and thus it would not account for the velocity range observed in the low-ionization profile shown in Figure 10(a). Hence, we rule out the wind scenario "A.…”

ALMA suggests outflows in z ∼ 5.5 galaxies

“…However, thanks mostly to absorption line spectroscopy, the presence of powerful outflows in galaxies close to the peak of the cosmic star formation (z ≈ 2 − 3), has been firmly assessed (Shapley et al 2003;Steidel et al 2010;Kacprzak et al 2014;Rubin et al 2014;Heckman et al 2015;Schroetter et al 2015). Pushing observations into the Epoch of Reionization (EoR, z > 6) is clearly the next frontier.…”

Gas Accretion via Lyman Limit Systems

“…One can also undertake a combination of both methods just described above whereby absorption is observed toward a galaxy (down-the-barrel) and toward a QSO transversely projected at some distance (e.g., Kacprzak et al 2014;Bouché et al 2016). I suspect the number of galaxy-QSO pairs will increase dramatically in the future with IFU observations and larger aperture telescope.…”