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

Abstract

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_⊙) ∼ 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 (Vmax), and normalized maximum velocity (V_(norm) = V_(max)/V_(circ,halo), where V_(circ,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 (>3σ) 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 (>3σ) 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 ∼ 2 and z ∼ 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.

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