Significant amounts of gas in galaxies move in outward radial trajectories due to energy imparted by star formation. This energy originates from a combination of phenomena rooted in stellar processes: radiation, winds, explosive events, and cosmic rays. These gas outflows powered by star formation, or stellar galactic winds (GWs), continue to be a dynamic topic of observational research in the era of large galaxy surveys and multi-messenger astronomy. Outflows are challenging to characterize largely because of two factors: the large contrast between the outflow and an underlying galaxy disk; and the complex, multiphase structure of outflows. High quality data in many gas tracers are essential to adequately quantify the mass, momentum, and energy budget of the wind.
Advances in observations of GWs driven by stellar processes in this decade have come from the use of new observational techniques and the application of old techniques to much larger samples. Both have been aided by new-and-improved telescopes or instrumentation. In the first category fall molecular gas transitions newly applied to study outflows (notably the hydroxyl molecule), mid- and far-infrared (MIR/FIR) imaging of dust, and resonant-line emission in the rest-frame ultraviolet (UV). In the second category are surveys with integral field spectrographs (IFSs), with multi-object long-slit spectrographs, and with the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST).
Excellent and thorough reviews from previous decades of theory and observations of GWs provide in-depth discussions of the quantities of observational interest and the astrophysics of GWs (e.g., [1, 2]). They also show the trajectory and progress of the field. The scope of this review is narrower. I synthesize observational results from the current decade (approximately 2010 through the present). I focus on direct measures of outflows and do not discuss studies that infer the presence or properties of GWs by studying other phenomena.
As an example of an indirect measurement of the presence or properties of GWs, studies of the mass-metallicity relation constrain how efficiently GWs eject metals from galaxy disks compared to metal production and reaccretion (e.g., [3]). A second example is the significant reservoirs of highly ionized carbon and oxygen that preferentially arise in the circumgalactic media of actively star-forming galaxies [4, 5, 6, 7, 8]. A logical source of these metals is stellar GWs. Third, the hot halos that appear to be a common feature of star-forming galaxies [9] may be produced by stellar GWs. Finally, cosmological simulations typically use numerical prescriptions for the unresolved physics of stellar GWs and compare simulated galaxy properties with observed galaxy properties (like the galaxy mass function) to constrain the nature of GWs (e.g., [10, 11, 12]). Such indirect measurements are essential for a complete picture of the relationship of GWs to their surrounding environments. However, at present, interpretations of these measurements based on stellar GWs typically compete with other physical models and are rarely definitive.
Stellar processes are not the only possible drivers of GWs. Evidence continues to accumulate that actively accreting supermassive black holes (active galactic nuclei, or AGN) are also important in powering GWs in galaxies with above-average masses (see reviews in [13, 14]). However, it can be difficult to distinguish whether the AGN is energetically important for the GW if significant star formation is also present, except in the most powerful AGN. This difficulty is due to the possible power sources being cospatial at low resolution and to the uncertain duty cycle of AGN. Consequently, in this review I focus on studies of purely star-forming systems, or at least those where star formation clearly dominates the galaxy's luminosity. AGN with low-to-moderate Eddington ratios are almost certainly present in many galaxies classified as purely star-forming [15, 16], but their energetic importance for GWs remains unquantified.
I organized this review at the highest level by redshift. This is useful for two reasons. First, star formation and galaxy properties at redshifts above unity differ significantly from those in the local universe. The global star formation rate (SFR) peaked at z ∼ 1.9 (e.g., [17]) and gas mass fraction increases with increasing redshift (e.g., [18]). Galaxies may grow from the inside out, meaning that disks are more extended compared to stars at high z [19, 20]. Second, low-z winds are much better characterized because many more techniques can be brought to bear. Some techniques are in practice easier to apply to high-redshift galaxies, but on balance this is not the case.