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The Nature of Ionized Gas in the Milky Way Galactic Fountain

Werk, Jessica K. and Rubin, K. H. R. and Bish, H. V. and Prochaska, J. X. and Zheng, Y. and O'Meara, J. M. and Lenz, D. and Hummels, C. and Deason, A. J. (2019) The Nature of Ionized Gas in the Milky Way Galactic Fountain. Astrophysical Journal, 887 (1). Art. No. 89. ISSN 1538-4357.

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We address the spatial scale, ionization structure, mass, and metal content of gas at the Milky Way disk–halo interface detected as absorption in the foreground of seven closely spaced, high-latitude halo blue horizontal branch stars with heights z = 3–14 kpc. We detect transitions that trace multiple ionization states (e.g., Ca ii, Fe ii, Si iv, C iv) with column densities that remain constant with height from the disk, indicating that the gas most likely lies within z < 3.4 kpc. The intermediate ionization state gas traced by C iv and Si iv is strongly correlated over the full range of transverse separations probed by our sight lines, indicating large, coherent structures greater than 1 kpc in size. The low ionization state material traced by Ca ii and Fe ii does not exhibit a correlation with either N H i or transverse separation, implying cloudlets or clumpiness on scales less than 10 pc. We find that the observed ratio log(N_(Si iv)/N_(C iv)), with a median value of −0.69 ± 0.04, is sensitive to the total carbon content of the ionized gas under the assumption of either photoionization or collisional ionization. The only self-consistent solution for photoionized gas requires that Si be depleted onto dust by 0.35 dex relative to the solar Si/C ratio, similar to the level of Si depletion in DLAs and in the Milky Way interstellar medium. The allowed range of values for the areal mass infall rate of warm, ionized gas at the disk−halo interface is 0.0003 < dM_(gas)/dtdA [M_⊙ kpc⁻² yr⁻¹] <0.006. Our data support a physical scenario in which the Milky Way is fed by complex, multiphase processes at its disk−halo interface that involve kiloparsec-scale ionized envelopes or streams containing parsec-scale, cool clumps.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Werk, Jessica K.0000-0002-0355-0134
Rubin, K. H. R.0000-0001-6248-1864
Bish, H. V.0000-0002-7483-8688
Prochaska, J. X.0000-0002-7738-6875
Zheng, Y.0000-0003-4158-5116
O'Meara, J. M.0000-0002-7893-1054
Lenz, D.0000-0001-5820-475X
Hummels, C.0000-0002-3817-8133
Deason, A. J.0000-0001-6146-2645
Additional Information:© 2019. The American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2019 April 23; revised 2019 October 29; accepted 2019 November 4; published 2019 December 13. Support for this work was provided by NASA through program GO-14140. J.K.W. and H.V.B. acknowledge additional support from a 2018 Sloan Foundation Fellowship and from the Research Royalty Fund grant 65-5743 at the University of Washington. A.D. is supported by a Royal Society University Research Fellowship. We thank B. Winkel for help with the processing of the EBHIS data and Chris Howk, Mary Putman, Josh Peek, Karin Sandstrom, Dusan Keres, Drummond Fielding, Evan Schneider, Chang-Goo Kim, Max Gronke, and Todd Tripp for helpful input and discussions regarding the results from this study. Y.Z. acknowledges support from the Miller Institute for Basic Research in Science. Finally, A.D. and J.K.W. thank Risa Wechsler and Charlie Conroy for organizing a speed collaboration session at the Mayacamas Ranch Conference in 2015, where the idea for this project originally took shape. Finally, we thank the anonymous referee, who provided valuable, in-depth comments that greatly improved the manuscript. The optical data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Facility: HST: COS Keck: HIRES - . Software: CALCOS (; MPFIT1 (; CLOUDY (
Funding AgencyGrant Number
Alfred P. Sloan FoundationUNSPECIFIED
University of Washington65-5743
Miller Institute for Basic Research in ScienceUNSPECIFIED
W. M. Keck FoundationUNSPECIFIED
Subject Keywords:Ultraviolet astronomy ; Photoionization ; Warm ionized medium ; Intergalactic dust clouds ; Circumgalactic medium ; Galaxy evolution ; Halo stars ; High resolution spectroscopy
Issue or Number:1
Classification Code:Unified Astronomy Thesaurus concepts: Ultraviolet astronomy (1736); Photoionization (2060); Warm ionized medium (1788); Intergalactic dust clouds (810); Circumgalactic medium (1879); Galaxy evolution (594); Halo stars (699); High resolution spectroscopy
Record Number:CaltechAUTHORS:20191213-102417575
Persistent URL:
Official Citation:Jessica K. Werk et al 2019 ApJ 887 89
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:100284
Deposited By: George Porter
Deposited On:16 Dec 2019 15:31
Last Modified:16 Dec 2019 15:31

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