Physical properties of galactic winds using background quasars

Abstract Background quasars are potentially sensitive probes of galactic outflows provided that one can determine the origin of the absorbing material since both gaseous discs and strong bipolar outflows can contribute to the absorption cross-section. Using a dozen quasars passing near spectroscopically identified galaxies at z ∼ 0.1, we find that the azimuthal orientation of the quasar sightlines with strong Mg ii absorption (with ) is bi-modal: about half the Mg ii sightlines are aligned with the major axis and the other half are within α = 30° of the minor axis, suggesting that bipolar outflows can contribute to the Mg ii cross-section. This bi-modality is also present in the instantaneous star formation rates (SFRs) of the hosts. For the sightlines aligned along the minor axis, a simple bi-conical wind model is indeed able to reproduce the observed Mg ii kinematics and the Mg ii dependence with impact parameter b, (). Using our wind model, we can directly extract key wind properties such as the de-projected outflow speed V out of the cool material traced by Mg ii and the outflow rates . The outflow speeds V out are found to be 150-300 km s−1, i.e. of the order of the circular velocity, and smaller than the escape velocity by a factor of ∼2. The outflow rates are typically two to three times the instantaneous SFRs. Our results demonstrate how background quasars can be used to measure wind properties with high precision.