DISCOVERY OF HIGHLY IONIZED HIGH-VELOCITY CLOUDS TOWARD MARKARIAN-509

We present Goddard High Resolution Spectrograph measurements of the Milky Way halo gas absorption toward Mrk 509 (l = 36 degrees.0, b = -29 degrees.9) in the 1233-1268 and 1521-1558 Angstrom spectral regions at a resolution of approximate to 15 km s(-1). We detect high-velocity (upsilon(LSR) approximate to -340 to -170 km s(-1)) absorption in both members of the C IV lambda lambda 1548, 1550 doublet but find no corresponding Si II lambda 1526 or N v lambda 1238 absorption. The high-velocity C IV absorption has substructure that can be modeled with two Gaussian components centered at [upsilon(LSR)] = -227.5 +/- 6.1 and -283.4 +/- 3.2 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. To understand the relationship between these highly ionized C IV high-velocity clouds (HVCs) and the more familiar H I HVCs, we obtained sensitive observations of the H I 21 cm emission toward Mrk 509 with the NRAO 140 foot (43 m) telescope. We find no detectable 21 cm emission at the velocities of the C IV HVCs to a 4 sigma limit of N(H I) < 4.9 x 10(17) cm(-2), but mapping of the 21 cm emission in the area around Mrk 509 indicates that H I HVCs with [upsilon(LSR)] between -270 and -300 km s(-1) are located within approximately 1 degrees.5 of the sight line. This suggests that the C IV HVC absorption probes the ionized boundaries of the H I HVCs. Although we cannot completely rule out collisional ionization at T greater than or equal to 10(5) K as a means for producing some of the high-velocity C IV, the observed ionic ratios are sufficiently rigorous for us to reject many collisional ionization models and favor a photoionization interpretation of the ionization structure of the C IV HVCs. A plane-parallel slab model investigated using the photoionization code CLOUDY suggests that the high-velocity C IV absorption can be explained with an almost fully ionized [N(H I)/N(H-tot) < 0.01] subsolar metallicity layer(greater than or equal to 1 kpc) of low-density gas (less than or equal to 10(-2) cm(-3)) photoionized by an extragalactic ionizing background with J(n)u(Lyman-limit)similar to 10(-22) ergs cm(-2) s(-1) Hz(-1) sr(-1) The general properties of the high-velocity C IV absorption more closely resemble those of the high-ionization quasar metal line absorption systems than those of other sight lines through the Milky Way disk and halo.