Discovery of Highly Ionized High-Velocity Clouds toward Markarian 509

Abstract

We present Goddard High Resolution Spectrograph measurements of the Milky Way halo gas absorption toward Mrk 509 (1 = 36°.0, b = -29°.9) in the 1233-1268 and 1521-1558 Å spectral regions at a resolution of ≈15 km s^-1. We detect high-velocity (υ_LSR -340 to -170 km s^-1 absorption in both members of the C IV λλ1548, 1550 doublet but find no corresponding Si II λ1526 or N V λ1238 absorption. The high-velocity C IV absorption has substructure that can be modeled with two Gaussian components centered at υ_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 σ 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 <υ_LSR> between -270 and - 300 km s^-1 are located within approximately 1°.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 ≥ 10⁵ 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 (≥ 1 kpc) of low-density gas ( ≤ 10^-2 cm^-3) photoionized by an extragalactic ionizing background with J_v(Lyman-limit) ∼ 10^-22 ergs cm^-2 s^-1 Hz^-1sr^-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.