Discovery of Highly Ionized High-Velocity Clouds toward Markarian 509

Details Discovery of Highly Ionized High-Velocity Clouds toward Markarian 509 Discovery of Highly Ionized High-Velocity Clouds toward Markarian 509

Oct 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...451..616s&link_type=abstract

Astrophysical Journal v.451, p.616

Astronomy and Astrophysics

Astronomy

50

Galaxies: Individual Name: Markarian 509, Galaxy: Halo, Ism: Abundances, Ism: Clouds, Radio Lines: Ism, Ultraviolet: Ism

Scientific paper

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 (υLSR -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 υLSR = -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 <υLSR> 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(Htot) < 0.01] subsolar metallicity layer (≥ 1 kpc) of low-density gas ( ≤ 10-2 cm-3) photoionized by an extragalactic ionizing background with Jv(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.

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