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 MgII, 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 MgI and MgII absorption obtained from Keck/LRIS and Lya, SiII and SiIII 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. The transverse bi-conical outflow model, assuming constant-velocity flows perpendicular to the disk, requires wind velocities $V_{outflow}=40-80$ km/s to reproduce the transverse MgII absorption kinematics, which is consistent with the range of $V_{dtb}$. The galaxy has a metallicity, derived from H$\alpha$ and NII, of $[{\rm O/H}]=-0.21\pm0.08$, whereas the transverse absorption has $[{\rm X/H}]=-1.12\pm0.02$. The galaxy star-formation rate is constrained between $4.6-15$ M$_{\odot}$/yr while the estimated outflow rate ranges between $1.6-4.2$ M$_{\odot}$/yr 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 $\sim$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.