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



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


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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.



Autor: Joel N. Bregman; Eric D. Miller; Patrick Seitzer; C. R. Cowley; Matthew J. Miller

Fuente: https://archive.org/







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