a Sensitive Search for High-Velocity Gas in the Outer Galactic Halo -CYC3

Details a Sensitive Search for High-Velocity Gas in the Outer Galactic Halo -CYC3 a Sensitive Search for High-Velocity Gas in the Outer Galactic Halo -CYC3

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993hst..prop.4677m&link_type=abstract

HST Proposal ID #4677

Physics

Hst Proposal Id #4677

Scientific paper

We propose to obtain observations of the Mg II absorption toward a sample of eight bright Seyfert galaxies and BL Lac objects with the GHRS on HST in an effort to search for the metal-line counterparts of high-velocity H I clouds (HVCs) over pathlengths which cover the full extent of the Galactic halo. Based on a statistical extrapolation beyond the 10e18 cm-2 sensitivity of recent H I 21 cm emission surveys, Wakker (1991) has estimated that HVCs with N(H I)>2x10e17 cm-2 cover somewhere between 30% and 60% of the sky. Such a limiting column density of high-velocity gas would be detectable in Mg II absorption against bright extragalactic targets with GHRS provided that the gas is not too metal-poor (<0.1 solar). The lack of such high-velocity absorption toward Danly's (1989) IUE sample of bright halo stars has led Wakker to conclude that, in general, HVCs are located at large distances beyond the inner Galactic halo. With the exception of the Magellanic Cloud and 3C 273 sightlines, previous UV and optical absorption-line studies of halo gas toward extragalactic targets have not had the sensitivity to detect low N(HI) HVCs. Thus, the primary goal of our Mg II survey is to determine if such HVCs are a common constituent of the Galactic halo. Our results will also have an impact on the question of metallicity in HVCs. Furthermore, if we find ubiquitous high-velocity Mg II absorption, it would provide new support for the idea that at least some QSO absorbers are due to the halos of intervening galaxies.

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