High-velocity outflows persist up to 1 Gyr after a starburst in recently quenched galaxies at z > 1

ABSTRACT High-velocity outflows are ubiquitous in star-forming galaxies at cosmic noon, but are not as common in passive galaxies at the same epoch. Using optical spectra of galaxies selected from the UKIDSS Ultra Deep Survey at $z\gt 1$, we perform a stacking analysis to investigate the transition in outflow properties along a quenching time sequence. To do this, we use Mg ii ($\lambda$2800 Å) absorption profiles to investigate outflow properties as a function of time since the last major burst of star formation (t$_{\textrm {burst}}$). We find evidence for high-velocity outflows in the star-forming progenitor population (v$_{\textrm {out}}$$\sim$ 1400 $\pm$ 210 km s$^{-1}$), for recently quenched galaxies with t$_{\textrm {burst}}$ < 0.6 Gyr (v$_{\textrm {out}}$$\sim$ 990 $\pm$ 250 km s$^{-1}$), and for older quenched galaxies with 0.6 < t$_{\textrm {burst}}$ < 1 Gyr (v$_{\textrm {out}}$$\sim$ 1400 $\pm$ 220 km s$^{-1}$). The oldest galaxies (t$_{\textrm {burst}}$ > 1 Gyr) show no evidence for significant outflows. Our samples show no signs of active galactic nucleus (AGN) in optical observations, suggesting that any AGN in these galaxies have very short duty cycles, and were ‘off’ when observed. The presence of significant outflows in the older quenched galaxies (t$_{\textrm {burst}}$ > 0.6 Gyr) is difficult to explain with starburst activity, however, and may indicate energy input from episodic AGN activity as the starburst fades.