Physical properties of galactic winds using background quasars

TL;DR

This study uses background quasars to investigate galactic winds, revealing a bi-modal distribution of outflows aligned with galaxy axes, and measuring wind speeds and outflow rates directly from absorption features.

Contribution

The paper introduces a method to determine wind properties from quasar absorption lines, demonstrating the significance of bipolar outflows and providing quantitative wind measurements.

Findings

Bipolar outflows significantly contribute to MgII absorption.

Outflow speeds are 150-300 km/s, comparable to galaxy circular velocities.

Outflow rates are two to three times the star formation rates.

Abstract

Background quasars are potentially sensitive probes of galactic outflows provided that one can determine the origin of the absorbing material since both gaseous disks and strong bipolar outflows can contribute to the absorption cross-section. Using a dozen quasars passing near spectroscopically identified galaxies at $z\sim0.1$, we find that the azimuthal orientation of the quasar sight-lines with strong MgII absorption (with EW>0.3 \AA) is bi-modal: about half the MgII sight-lines are aligned with the major axis and the other half are within 30deg. of the minor axis, showing that bipolar outflows contribute significantly to the MgII cross-section. This bi-modality is also present in the instantaneous star-formation rates (SFRs) of the hosts. For the sight-lines aligned along the minor axis, a simple bi-conical wind model is able to reproduce the observed MgII kinematics and the MgII dependence with impact parameter b, (EW $\propto b^{-1}$). 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 MgII and the outflow rates. The outflow speeds are found to be 150-300 \kms, 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 demonstrates how background quasars can be used to measure wind properties with high precision.