Fundamental Properties of the Highly Ionized Plasmas in the Milky Way

Type Journal Article
Names N. Lehner, W. F. Zech, J. C. Howk, B. D. Savage
Publication The Astrophysical Journal
Volume 727
Issue 1
Pages 46
Date January 1, 2011
URL http://adsabs.harvard.edu/abs/2011ApJ...727...46L
Library Catalog NASA ADS
Abstract The cooling transition temperature gas in the interstellar medium (ISM), traced by the high ions, Si IV, C IV, N V, and O VI, helps to constrain the flow of energy from the hot ISM with T>106 K to the warm ISM with T < 2 × 104 K. We investigate the properties of this gas along the lines of sight to 38 stars in the Milky Way disk using 1.5-2.7 km s-1 resolution spectra of Si IV, C IV, and N V absorption from the Space Telescope Imaging Spectrograph, and 15 km s-1 resolution spectra of O VI absorption from the Far Ultraviolet Spectroscopic Explorer. The absorption by Si IV and C IV exhibits broad and narrow components while only broad components are seen in N V and O VI. The narrow components imply gas with T < 7 × 104 K and trace two distinct types of gas. The strong, saturated, and narrow Si IV and C IV components trace the gas associated with the vicinities of O-type stars and their supershells. The weaker narrow Si IV and C IV components trace gas in the general ISM that is photoionized by the EUV radiation from cooling hot gas or has radiatively cooled in a non-equilibrium manner from the transition temperature phase, but rarely the warm-ionized medium probed by Al III. The broad Si IV, C IV, N V, and O VI components trace collisionally ionized gas that is very likely undergoing a cooling transition from the hot ISM to the warm ISM. The cooling process possibly provides the regulation mechanism that produces langN(C IV)/N(Si IV)rang = 3.9 ± 1.9. The cooling process also produces absorption lines where the median and mean values of the line widths increase with the energy required to create the ion. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract no. NAS5-26555. Based also on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer. FUSE is operated for NASA by the Johns Hopkins University under NASA contract NAS5-32985.
Tags Galaxy: Disk, ISM: clouds, ISM: structure, Ultraviolet: ISM
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