C IV High Velocity Clouds: Remnants of the Local Group?

Details C IV High Velocity Clouds: Remnants of the Local Group? C IV High Velocity Clouds: Remnants of the Local Group?

Jul 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999hst..prop.8097s&link_type=abstract

HST Proposal ID #8097

Computer Science

Hst Proposal Id #8097 Interstellar And Intergalactic Matter

Scientific paper

We have recently studied two high velocity {V_LSR <-100 km/sec} clouds {HVCs} in the direction of Mrk 509 that have unusual ionization properties. The clouds exhibit strong C IV absorption with little or no detectable low ion {C II, Si II} absorption or H I 21cm emission. Our Cycle 6 program to study the properties of the HVCs toward Mrk 509 in detail revealed that they are likely very low density, low pressure gas clouds in the Local Group {or distant Galactic halo} subject to the extragalactic ionizing background. Their ionization properties resemble those of the outer diffuse halos of damped LyAlpha absorbers and the low N{H I} metal line absorption systems seen in the spectra of high redshift quasars. These C IV-HVCs present important opportunities for relating the properties of nearby intergalactic gas {or an extended Galactic halo} to the properties of gas in higher-z systems. Here, we will obtain STIS spectra to study the ionization properties of C IV-HVCs along the PG 1116+215 sight line. We will compare their properties with those of the C IV-HVCs toward Mrk 509 to test the hypothesis that these clouds are remnants from the formation of the Local Group. The results have implications for the origin of HVCs, the processes that distribute and ionize diffuse gas in the Local Group, the extent of the Galactic halo, and the interpretation of high-z quasar absorption line systems. PKS 2155-304 was originally in this program but was taken away by the super-TAC, so no PKS 2155-304 observations will be specified in this Phase 2 submission.

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