The presence and ubiquity of stellar GWs outside of the local universe was first evident in blueshifted rest-frame UV absorption lines and complex Lyα profiles in Lyman-break galaxies (LBGs) (e.g., [145, 146, 147]). Information on stellar GWs at high z comes predominantly from single-aperture, down-the-barrel spectroscopy of rest-frame UV lines. (“Down-the-barrel” refers to sightlines toward the galaxy itself.) However, newer instruments and techniques have in the past decade opened other avenues to study high z winds. These include rest-frame optical measurements of emission lines with wide-field, NIR, multi-object spectrographs; multiplexed or wide-field IFS instruments such as the K-band Multi-Object Spectrograph (KMOS) and the Multi-Unit Spectroscopic Explorer (MUSE) that enable multi-object and/or spatially resolved measurements; NIR, adaptive optics IFS to achieve high spatial resolution; FIR and submillimeter observations that probe molecular and atomic gas transitions; and transverse-sightline spectroscopic surveys. These new techniques have established the ubiquity and properties of stellar GWs in new galaxy populations and constrained the redshift evolution of their bulk properties.
Deep spectroscopy of strong rest-frame optical emission lines (mainly Hα and [N II]) at z ∼ 2 reveals broad wings that arise primarily from bright star-forming regions [148, 149, 150, 151]. These broad wings, which appear to extend over several kpc and strengthen with increasing ΣSFR, have been interpreted as evidence of stellar feedback. Similar wings are found in massive, compact star-forming galaxies over a wider wavelength range [152].
Larger spectroscopic surveys (up to ∼500 galaxies) of star-forming galaxies at z = 0.3−2 use the low-ionization metal lines Mg I, Mg II, and Fe II to probe GWs [154, 155, 156, 157, 158, 159, 160, 161, 162, 163]. Resonant transitions from outflows in absorption (blueshifted) and emission (redshifted), as well as corresponding non-resonant transitions, constrain basic outflow properties such as velocity and ionization state and, potentially, more complex structural parameters (Figure 4; [164]). These high-z, low-ionization outflows are broadly consistent with those found at low z: they have modest velocities (up to a few hundred km s−1 on average); their properties (velocity and equivalent width) correlate with SFR, ΣSFR, and M*; their detection rates in absorption average a few tens of percent, indicative primarily of the wind geometry (a high frequency of occurrence of non-spherical winds); they have estimated mass-loss rates of order the star formation rate, though with considerable uncertainty; and they are preferentially found in face-on galaxies (or their properties are more extreme in face-on galaxies), consistent with minor-axis flows. The velocities of these winds may increase with increasing z for galaxies of given SFR, possibly due to increasing star formation rate surface density [158, 165, 166, 167], and with increasing ionization potential [167, 168, 169]. Extended, scattered emission (both resonant and non-resonant) from low-ionization species has been probed in detail in a handful of systems [153, 170, 171, 172] and is now recognized as a common feature of star-forming galaxies at these epochs [163, 173, 174]. Pure resonant emission is seen at low SFR, transitioning to P-Cygni or pure-absorption resonant profiles plus non-resonant emission at high SFR, indicative of an increasing signature of outflowing gas [163]. In a handful of gravitationally lensed systems, absorption-line analyses of more ions indicate α enhancement and more robustly constrain mass-loss rates [175, 176].
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Figure 4. Absorption and emission lines in the outflow of a z = 1.3 star-forming galaxy [153]. The resonant absorption lines (labeled in blue) are blueshifted and trace the approaching near side of the outflow. The non-resonant iron emission lines (labeled in red), which are spatially extended along the minor axis, trace the bulk of the outflow. These resonant and non-resonant lines are powerful probes of the presence and properties of high-redshift outflows. Reproduced with permission from Figure 1 of reference [153], ©ESO. |
At moderate redshifts (z ∼ 0.7−0.8), the presence of low-velocity GWs in poststarburst galaxies points to the possibility that these winds help to quench star formation [173]. Very high-velocity winds (1000 km s−1) found in bluer, rarer post-starbursts [177] appear to be driven by very compact starbursts rather than AGN activity [31, 178].
The down-the-barrel technique, which probes low-ionization outflows that emerge along the line-of-sight toward the galaxy disk, is complemented by transverse sightlines through galaxy halos toward background quasars or galaxies. LBGs at z = 2−3 host both low- and high-ionization gas out to radii ∼100 kpc; because this gas is outflowing over a large solid angle (as inferred from down-the-barrel observations), it is plausibly doing so at large radii [147]. In other samples, Mg II and O 6 absorbers show a preference for alignment with galaxy major or minor axes, and the minor-axis gas is preferentially found in blue galaxies. This absorber alignment is suggestive of major-axis inflow and minor-axis outflow, and the connection to blue galaxies points to star formation as the power source [179, 180]. Simple geometric outflow models of halo absorbers yield outflow properties that are consistent with those seen at low z [181, 182, 183].
The prominence of Lyα in the spectra of high z galaxies makes it a tempting target for parameterizing outflows (e.g., [184]). However, as mentioned above (Section 2.1), radiation transfer effects make it an ambiguous indicator. Redshifted Lyα does typically accompany blueshifted low-ionization lines (e.g., [147]). Star forming galaxies also show an increase in the velocity of Lyα as SFR and Lyα equivalent width increase [185, 186, 187]. However, whether this is due solely to changing outflow properties or instead to an increase in gas near the systemic redshift is unclear [187].
Finally, a handful of molecular gas detections of outflows at moderate-to-high z are emerging. CO has been imaged in two high-velocity, apparently stellar GWs in post-starbursts at z ∼ 0.7 [32, 188]. A long Herschel integration allowed detection of an OH outflow in absorption in a z = 2.3 ULIRG [189]. A serendipitous ALMA discovery of extremely broad CH+ in several z ∼ 2.5 ULIRGs points to turbulent outflows [190]. Tentative detections of broad, faint [C II] line wings at z = 5.5 hint at the possibility of stellar GWs in modestly star-forming galaxies at this epoch [191]. Finally, an OH outflow exists in a gravitationally lensed, dusty galaxy at z = 5.3 [192].