Physical properties of galactic winds using background quasars

Abstract

Background quasars are potentially sensitive probes of galactic outflows\nprovided that one can determine the origin of the absorbing material since both\ngaseous disks and strong bipolar outflows can contribute to the absorption\ncross-section. Using a dozen quasars passing near spectroscopically identified\ngalaxies at $z\\sim0.1$, we find that the azimuthal orientation of the quasar\nsight-lines with strong MgII absorption (with EW>0.3 \\AA) is bi-modal: about\nhalf the MgII sight-lines are aligned with the major axis and the other half\nare within 30deg. of the minor axis, showing that bipolar outflows contribute\nsignificantly to the MgII cross-section. This bi-modality is also present in\nthe instantaneous star-formation rates (SFRs) of the hosts. For the sight-lines\naligned along the minor axis, a simple bi-conical wind model is able to\nreproduce the observed MgII kinematics and the MgII dependence with impact\nparameter b, (EW $\\propto b^{-1}$). Using our wind model, we can directly\nextract key wind properties such as the de-projected outflow speed $V_{out}$ of\nthe cool material traced by MgII and the outflow rates. The outflow speeds are\nfound to be 150-300 \\kms, i.e. of the order of the circular velocity, and\nsmaller than the escape velocity by a factor of ~2. The outflow rates are\ntypically two to three times the instantaneous SFRs. Our results demonstrates\nhow background quasars can be used to measure wind properties with high\nprecision.\n