The CGM-GRB Study. II. Outflow-Galaxy Connection at z ∼ 2-6

We use a sample of 27 gamma-ray bursts (GRBs) at redshift z = 2-6 to probe the outflows in their respective host galaxies (log(M-*/M-circle dot) similar to 9-11) and search for possible relations between the outflow properties and those of the host galaxies, such as M (*), the star formation rate (SFR), and the specific SFR (sSFR). First, we consider three outflow properties: outflow column density (N-out), maximum outflow velocity (V-max), and normalized maximum velocity (V-norm = V-max/V-circ,V-halo, where V-circ,V-halo is the halo circular velocity). We observe clear trends of N-out and V-max with increasing SFR in high-ion-traced outflows, with a stronger (>3 sigma) V-max-SFR correlation. We find that the estimated mass outflow rate and momentum flux of the high-ion outflows scale with SFR and can be supported by the momentum imparted by star formation (supernovae and stellar winds). The kinematic correlations of high-ion-traced outflows with SFR are similar to those observed for star-forming galaxies at low redshifts. The correlations with SFR are weaker in low-ion outflows. This, along with the lower detection fraction in low-ion outflows, indicates that the outflow is primarily high-ion dominated. We also observe a strong (>3 sigma) trend of normalized velocity (V-norm) decreasing with halo mass and increasing with sSFR, suggesting that outflows from low-mass halos and high-sSFR galaxies are most likely to escape and enrich the outer circumgalactic medium (CGM) and intergalactic medium with metals. By comparing the CGM-GRB stacks with those of starbursts at z similar to 2 and z similar to 0.1, we find that over a broad redshift range, the outflow strength strongly depends on the main-sequence offset at the respective redshifts, rather than simply the SFR.

Automated summary

We investigate the outflows in host galaxies of gamma-ray bursts (GRBs) at redshift z = 2-6 and find clear correlations between high-ion-traced outflows and the star formation rate (SFR). The mass outflow rate and momentum flux of high-ion outflows are supported by the momentum imparted by star formation. However, low-ion outflows show weaker correlations with SFR. We also observe a trend of normalized velocity decreasing with halo mass and increasing with specific SFR.