Outflow vs. Infall in Spiral Galaxies: Metal Absorption in the Halo of NGC 891

TL;DR

This study measures metal absorption lines in the halo of NGC 891 to determine whether the gas is accreting from intergalactic space or expelled from the galaxy disk, finding evidence for a galactic fountain origin.

Contribution

First direct measurement of halo gas metallicity in NGC 891, distinguishing between accretion and fountain models using absorption line analysis.

Findings

Halo gas metallicity is near-solar, indicating a galactic fountain origin.

Metal absorption lines show differential depletion, affecting abundance estimates.

Results support gas being expelled from the galaxy disk rather than accreted.

Abstract

Gas accreting onto a galaxy will be of low metallicity while halo gas due to a galactic fountain will be of near-solar metallicity. We test these predictions by measuring the metal absorption line properties of halo gas 5 kpc above the plane of the edge-on galaxy NGC 891, using observations taken with HST/STIS toward a bright background quasar. Metal absorption lines of Fe II, Mg II, and Mg I in the halo of NGC 891 are clearly seen, and when combined with recent deep H I observations, we are able to place constraints on the metallicity of the halo gas for the first time. The H I line width defines the line broadening, from which we model opacity effects in these metal lines, assuming the absorbing gas is continuously distributed in the halo. The gas-phase metallicities are [Fe/H] = -1.18 +/- 0.07 and [Mg/H] = -0.23 +0.36/-0.27 (statistical errors) and this difference is probably due to differential depletion onto grains. When corrected for such depletion using Galactic gas as a guide, both elements have approximately solar or even supersolar abundances. This suggests that the gas is from the galaxy disk, probably expelled into the halo by a galactic fountain, rather than from accretion of intergalactic gas, which would have a low metallicity. The abundances would be raised by significant amounts if the absorbing gas lies in a few clouds with thermal widths smaller than the rotational velocity of the halo. If this is the case, both the abundances and [Mg/Fe] would be supersolar.