Absorption‐Line Probes of Gas and Dust in Galactic Superwinds

Abstract

We have obtained moderate resolution (R \ few thousand) spectra of the Na I jj5890, 5896 (Na D) absorption line in a sample of 32 far-IRbright starburst galaxies.In 18 cases, the Na D line in the nucleus is produced primarily by interstellar gas, while cool stars contribute signicantly in the others.In 12 of the 18 "" interstellar-dominated cases the Na D line is blueshifted by over 100 km s~1 relative to the galaxy systemic velocity (the "" outow sources ), while no case shows a net redshift of more than 100 km s~1.The absorption-line proles in these outow sources span the range from near the galaxy systemic velocity to a maximum blueshift of D400600 km s~1.The outow sources are galaxies systematically viewed more nearly face-on than the others.We therefore argue that the absorbing material consists of ambient interstellar material that has been entrained and accelerated along the minor axis of the galaxy by a hot starburst-driven superwind.The Na D lines are optically thick, but indirect arguments imply total hydrogen column densities of cm~2.This implies that the superwind N H D few ] 1021 is expelling matter at a rate comparable to the star formation rate.This outowing material is evidently very dusty : we nd a strong correlation between the depth of the Na D prole and the line-of-sight reddening.Typical implied values are E(B[V ) \ 0.31 over regions several-to-10 kpc in size.We briey consider some of the potential implications of these observations.The estimated terminal velocities of superwinds inferred from the present data and extant X-ray data are typically 400800 km~1, are independent of the galaxy rotation speed, and are comparable to (substantially exceed) the escape velocities for (dwarf) galaxies.The resulting selective loss of metals from shallower potential wells can establish L * the mass-metallicity relation in spheroids, produce the observed metallicity in the intracluster medium, and enrich a general IGM to of order 10~1 solar metallicity.If the outowing dust grains can survive their journey into the IGM, their e ect on observations of cosmologically distant objects would be sig-nicant.