DISCOVERY OF HIGHLY IONIZED HIGH-VELOCITY CLOUDS TOWARD MARKARIAN 509

摘要

We present Goddard High Resolution Spectrograph measurements of the Milky Way halo gas absorption toward Mrk 509 (l = 36.°0, b = -29.°9) in the 1233-1268 and 1521-1558 A spectral regions at a resolution of ≈15 km s~(-1). We detect high-velocity (V_(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 < v_(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 x 10~(17) cm~(-2), but mapping of the 21 cm emission in the area around Mrk 509 indicates that H I HVCs with < v_(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 ≥ 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 metal-licity 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~(-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.