Astrophys. J., 727, 46 (2011/January-3)
Fundamental properties of the highly ionized plasmas in the Milky Way.
LEHNER N., ZECH W.F., HOWK J.C. and SAVAGE B.D.
Abstract (from CDS):
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 x104 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 resolution spectra of Si IV, C IV, and N V absorption from the Space Telescope Imaging Spectrograph, and 15 km/s 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 < 7x104 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 = 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.
Galaxy: disk - ISM: clouds - ISM: structure - ultraviolet: ISM
VizieR on-line data:
<Available at CDS (J/ApJ/727/46): table1.dat table2.dat table3.dat table4.dat table5.dat table6.dat>
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