We can say with reasonable certainty that GWs driven by energy from stellar processes are a common feature of galaxies with moderate-to-high star formation rates and/or surface densities out to z ∼ 2−3. Stacking analyses of large rest-frame UV and optical spectroscopic surveys have established that the average star-forming galaxy has an ionized and/or neutral wind whose velocity scales with star formation rate, stellar mass, and possibly ΣSFR. At low redshift, these winds are most prominent in starburst galaxies that lie above the galaxy main sequence; at higher redshift, where galaxies on the main sequence have higher SFR, the situation may be different. Hints exist that GWs are common but simply hard to detect even in galaxies with low ΣSFR (for instance, the low-surface-brightness Milky Way GW).
The correlations between outflow and galaxy properties found in some of the first large surveys of stellar GWs have been verified and refined by larger and more diverse samples and different gas probes. Besides serving merely as input to parameterizations of outflows in numerical simulations, measurements of GWs can now be compared to the predicted properties of GWs from simulations that better implement the physics of stellar feedback.
Detailed, multiwavelength studies of star-forming galaxies continue to reveal new layers of GWs. Most notable is that stellar GWs entrain large quantities of molecular gas, including dense clumps, and loft dust and soot (PAHs) far above the galactic disk. The promising technique of combining resonant-line absorption and emission with non-resonant re-emission channels has been successfully used to detect winds at high z and may prove a powerful probe of GW structure and extent when widely deployed. Observations of a wider range of galaxies besides the usual suspects (e.g., M82) with 3D imaging spectroscopy shows that a complex, multiphase structure of filaments of dusty ionized and neutral gas collimated along the minor axis is a common feature of GWs. Transverse-sightline spectroscopy and correlations with galaxy inclination at a variety of redshifts bolster this picture. Finally, increasingly in-depth studies of local galaxies with extended Lyα and LyC may eventually put meaningful constraints on how outflows contribute to reionization and help interpret high-z observations of Lyα.
Future progress will occur on a variety of fronts. At low z, a new generation of ongoing multi-object IFS surveys (SAMI Galaxy Survey, MaNGA) will soon produce results on thousands of nearby galaxies. Future, much larger IFS surveys are being planned (using, e.g., Hector; [193]). Sensitive, wide-field IFS instruments on large telescopes (such as MUSE and KCWI, the Keck Cosmic Web Imager) will probe the full extent of GWs in nearby, well-resolved targets and enable efficient, spatially resolved characterization of many galaxies at once in high-z deep fields. Next generation multi-object spectroscopy surveys (e.g., the Dark Energy Spectroscopic Instrument, or DESI, Survey and 4MOST, the 4-metre Multi-Object Spectroscopic Telescope) will increase the fidelity of stacking analyses over a wider range of redshift and galaxy properties.
Continued measurements with ALMA, particularly at high spatial resolution, will provide more detailed understanding of the structure and chemistry of molecular gas in outflows. ALMA will also certainly expand on its currently short list of detections of high-z stellar GWs. The high resolution and sensitivity of the James Webb Space Telescope (JWST) in the MIR will undoubtedly produce useful measurements of molecular gas and dust in stellar GWs, as well. However, the spatial resolution and sensitivity of JWST is likely to provide the most dramatic advances in measuring the properties of outflows in high-z galaxies by characterizing them in individual main-sequence galaxies at z ∼ 2−3 and detecting them at very high z, where their impact could be especially significant but where measurements currently do not exist.
Finally, we note two areas of study that have seen little recent progress, but whose prospects should eventually rise. Measurements of the radio-emitting plasma in GWs are very rare except for a few recent detections [194, 195, 196]. The next generation of wide-field radio arrays may make this a growth area. The field of X-rays studies of GWs has also lain fallow, with a few exceptions (e.g., [197, 198]). The hottest gas phase of GWs, which may drive the outflows in starburst galaxies, has proven extremely difficult to detect except in the nearest cases [27]. More sensitive X-ray telescopes in the coming two decades will eventually lead to a better characterization of this pivotal component.
Acknowledgments : The author thanks the referees for their feedback.
Funding : This research received no external funding. The author is supported by the J. Lester Crain chair at Rhodes College.
Conflicts of Interest : The author declares no conflict of interest.
Abbreviations : The following abbreviations are used in this manuscript:
| ALMA | Atacama Large Millimeter/submillimeter Array |
| COS | Cosmic Origins Spectrograph |
| FIR | far-infrared |
| GW | galactic wind |
| IFS | integral field spectrograph |
| LBG | Lyman-break galaxy |
| LIRG | luminous infrared galaxy |
| MIR | mid-infrared |
| NIR | near-infrared |
| SFR | star formation rate |
| sSFR | specific star formation rate |
| ULIRG | ultraluminous infrared galaxy |