Physical properties of galactic winds using background quasars

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 similar to 0.1, we find that the azimuthal orientation of the quasar sightlines with strong Mg?ii absorption (with Wr?2796 > 0.3 angstrom) is bi-modal: about half the Mg?ii sightlines are aligned with the major axis and the other half are within a = 30 degrees 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, (Wr?2796 ? b-1). Using our wind model, we can directly extract key wind properties such as the de-projected outflow speed Vout of the cool material traced by Mg?ii and the outflow rates M? out . The outflow speeds Vout are found to be 150300?km?s-1, i.e. of the order of the circular velocity, and smaller than the escape velocity by a factor of similar to 2. The outflow rates M? out 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.