Absorption-line probes of gas and dust in galactic superwinds.
HECKMAN T.M., LEHNERT M.D., STRICKLAND D.K. and ARMUS L.
Abstract (from CDS):
We have obtained moderate resolution (R=few thousand) spectra of the Na I λλ5890, 5896 (Na D) absorption line in a sample of 32 far-IR-bright starburst galaxies. In 18 cases, the Na D line in the nucleus is produced primarily by interstellar gas, while cool stars contribute significantly 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 ``outflow sources''), while no case shows a net redshift of more than 100 km.s–1. The absorption-line profiles in these outflow sources span the range from near the galaxy systemic velocity to a maximum blueshift of ∼400-600 km.s–1. The outflow 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 NH∼fewx1021 cm–2. This implies that the superwind is expelling matter at a rate comparable to the star formation rate. This outflowing material is evidently very dusty: we find a strong correlation between the depth of the Na D profile and the line-of-sight reddening. Typical implied values are E(B-V)=0.3-1 over regions several-to-10 kpc in size. We briefly 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 400-800 km–1, are independent of the galaxy rotation speed, and are comparable to (substantially exceed) the escape velocities for L* (dwarf) galaxies. The resulting selective loss of metals from shallower potential wells can establish 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 outflowing dust grains can survive their journey into the IGM, their effect on observations of cosmologically distant objects would be significant.
Galaxies: Active - Galaxies: Halos - Galaxies: ISM - Galaxies: Kinematics and Dynamics - Galaxies: Nuclei - Galaxies: Starburst - Galaxies: Intergalactic Medium