The CGM-GRB Study II: Outflow-Galaxy Connection at z ~ 2-6

Abstract

We use a sample of 27 GRBs at redshift $z=2-6$ to probe the outflows in their respective host galaxies ($\log ({\mathrm{M}}_{\ast }/{\mathrm{M}}_{\odot })\sim 9-11$) and search for possible relations between the outflow properties and those of the host galaxies such as ${\mathrm{M}}_{\ast }$, SFR, and specific SFR. First, we consider three outflow properties $-$ outflow column density (${\mathrm{N}}_{\mathrm{out}}$), maximum outflow velocity (${\mathrm{V}}_{\max }$), and normalized maximum velocity (${\mathrm{V}}_{\mathrm{norm}}$ = ${\mathrm{V}}_{\max }/{\mathrm{V}}_{\mathrm{circ},\mathrm{halo}}$, where ${\mathrm{V}}_{\mathrm{circ},\mathrm{halo}}$ is the halo circular velocity). We observe clear trends of ${\mathrm{N}}_{\mathrm{out}}$ and ${\mathrm{V}}_{\max }$ with increasing SFR in high-ion-traced outflows, with a stronger ($>3\ddot{I}ƒ$) ${\mathrm{V}}_{\max }-$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-ions. This, along with the lower detection fraction in low-ions, indicates that the outflow is primarily high-ion dominated. We also observe a strong ($>3\ddot{I}ƒ$) trend of normalized velocity (${\mathrm{V}}_{\mathrm{norm}}$) 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 CGM and IGM with metals. By comparing the CGM-GRB stacks with those of starbursts at $z\sim 2$ and $z\sim 0.1$, 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.

Published in Astrophysical Journal, 28 pages, 20 figures. Comments are welcome