Detection of Highly Ionized Galactic HVCs Toward Markarian 509

Abstract

We present Goddard High Resolution Spectrograph measurements of the Milky Way halo gas absorption toward Mrk 509 (1 = 36, b = −30) in the 1233–1268 A and 1521–1558 A spectral regions at a resolution of 15 km s−1. We detect high velocity C IV absorption between −340 and −170 km s−1(LSR), but find no corresponding Si II 1526 A or N V 1238 A absorption. The high velocity C IV absorption has sub-structure that can be modeled with two Gaussian components centered at −227 and −283 km s−1 with N(C IV)/N(Si II) > 1.3 and > 5.1 and N(C IV)/N(N V) > 3.0 and > 6.6, respectively. We obtained sensitive observations of the H I 21 cm emission toward Mrk 509 with the NRAO 140 foot telescope. We find no detectable 21 cm emission at the velocities of the C IV-HVCs to a 4cr limit of N(H I) 4.9×1017 cm−2, but mapping of the 21 cm emission in the area around Mrk 509 indicates that H I-HVCs with velocities between −270 and −300 km s−1 arelocated within about 1.5 degrees of the sight line. The C IV-HVC absorption appears to probe the ionized boundaries of the H I-HVCs. Ionization model calculations indicate that it is likely that these C IV-HVCs are photoionized by extragalactic background radiation rather than by collisional ionization within a recombining, hot gas. These C IV high velocity clouds have ionization properties more closely resembling those of the high ionization quasar metal line systems than those of previously studied sight lines through the Milky Way disk and halo.