A Sensitive Search for High-Velocity Galactic {\sc Hi}

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Session 12 -- Interstellar medium
Display presentation, Monday, 9, 1995, 9:20am - 6:30pm

[12.10] A Sensitive Search for High-Velocity Galactic {\sc Hi}

E.M. Murphy (U. Virginia and NRAO), F.J. Lockman (NRAO), B.D. Savage (U. Wisconsin)

We have conducted a sensitive 21--cm search for Galactic {\sc Hi} high--velocity ($|{\rm V_{LSR}}|\: > \: 100 \;{\rm km\;s}^{-1}$) clouds in the directions of quasars which have appeared on planning lists for the Quasar Absorption Line Key Project of the Hubble Space Telescope. Our aim was to understand the distribution and properties of the high--velocity galactic clouds that will be seen in UV absorption lines against the quasars. We have found that 37\% of the sky is covered by high--velocity clouds, excluding the Galactic warp, to a $5\:\sigma$ completeness limit of $7 \times 10^{17} \:{\rm cm}^{-2}$. This is a significant increase over the 18\% found by previous surveys. The majority of this new high--velocity emission can be associated with previously known HVC complexes. From this, we conclude that the HVC complexes are surrounded by extensive low column density, neutral envelopes. This result may have important implications for the hypothesis that the narrow metal line systems seen in the spectra of quasars arise in the intervening gaseous halos of galaxies. If there is a population of HVCs not associated with the complexes, they must have a column density less than $7 \times 10^{17} \:{\rm cm}^{-2}$, cover a small fraction of the sky, or they must be ionized.

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