Exploring the Origin and Fate of the Magellanic Stream with Ultraviolet and Optical Absorption

Type Journal Article
Names Andrew J. Fox, Bart P. Wakker, Jonathan V. Smoker, Philipp Richter, Blair D. Savage, Kenneth R. Sembach
Publication The Astrophysical Journal
Volume 718
Issue 2
Pages 1046-1061
Date August 1, 2010
URL http://adsabs.harvard.edu/abs/2010ApJ...718.1046F
Library Catalog NASA ADS
Abstract We present an analysis of ionization and metal enrichment in the Magellanic Stream (MS), the nearest gaseous tidal stream, using Hubble Space Telescope/STIS and FUSE ultraviolet spectroscopy of two background active galactic nuclei. The targets are NGC 7469, lying directly behind the MS with log N(H I)MS = 18.63 ± 0.03(stat) ± 0.08(syst), and Mrk 335, lying 24fdg7 away with log N(H I)MS = 16.67 ± 0.05. For NGC 7469, we include optical spectroscopy from VLT/UVES. In both sight lines, the MS is detected in low-ion (O I, C II, C III, Si II, Si III, Al II, Ca II) and high-ion (O VI, C IV, Si IV) absorption. Toward NGC 7469, we measure an MS oxygen abundance [O/H]MS = [O I/H I] = -1.00 ± 0.05(stat) ± 0.08(syst), supporting the view that the Stream originates in the Small Magellanic Cloud rather than the Large Magellanic Cloud. We use CLOUDY to model the low-ion phase of the Stream as a photoionized plasma using the observed Si III/Si II and C III/C II ratios. Toward Mrk 335, this yields an ionization parameter between log U = -3.45 and -3.15, a gas density log (n H/cm-3) between -2.51 and -2.21, and a hydrogen ionization fraction of 98.9%-99.5%. Toward NGC 7469, we derive sub-solar abundance ratios for [Si/O], [Fe/O], and [Al/O], indicating the presence of dust in the MS. The high-ion column densities are too large to be explained by photoionization, but also cannot be explained by a single-temperature collisional ionization model (equilibrium or non-equilibrium). This suggests that the high-ion plasma is multi-phase, with an Si IV region, a hotter O VI region, and C IV potentially contributing to each. Summing over the low-ion and high-ion phases, we derive conservative lower limits on the ratio N(total H II)/N(H I) of gsim19 toward NGC 7469 and gsim330 toward Mrk 335, showing that along these two directions the vast majority of the Stream has been ionized. The presence of warm-hot plasma together with the small-scale structure observed at 21 cm provides evidence for an evaporative interaction with the hot Galactic corona. This scenario, predicted by hydrodynamical simulations, suggests that the fate of the MS will be to replenish the Galactic corona with new plasma, rather than to bring neutral fuel to the disk. Based on observations from the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer mission (program P101), operated by Johns Hopkins University, supported by NASA contract NAS 5-32985; from the NASA/ESA Hubble Space Telescope (program 9802), obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555; and from the Ultraviolet and Visual Echelle Spectrograph (UVES) on the Very Large Telescope (VLT) Unit 2 (Kueyen) at Paranal, Chile, operated by the European Southern Observatory, under program ID 081.D-0697(A).
Tags Galaxy: evolution, Galaxy: halo, ISM: clouds, MAGELLANIC CLOUDS, Ultraviolet: ISM
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