A Deep Search for High Velocity Clouds Toward Quasars

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Session 21 -- Diffuse Galactic Emission
Display presentation, Wednesday, January 12, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[21.07] A Deep Search for High Velocity Clouds Toward Quasars

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

We have conducted a deep search for Galactic high velocity clouds in the direction of quasars. The observations were carried out using the NRAO 140-foot telescope at 21--cm. The intent of the survey was to look for low column density clouds that may have been overlooked in previous surveys. The quasars constitute a random sample of directions at high galactic latitudes and have the additional significant advantage that many will be studied in UV absorption lines as part of a Key Project on the Hubble Space Telescope.

Altogether, the lines of sight toward 99 quasars have been investigated. Of these, approximately 20\% show high velocity neutral hydrogen ($|v_{LSR}| > 100$ km/s), excluding the Galactic warp. Many of these clouds would not have been detected in previous surveys. The column density detection limit (5 $\sigma$) for the majority of the quasars is $7\times10^{17} \;{\rm cm}^{-2}$. Only 10\% of the high velocity clouds are at positive velocities. We find no evidence for a large population of low column density, neutral, high velocity clouds surrounding the Milky Way. We do find, however, that HVC complexes may be surrounded by low column density, neutral gas, that the complexes are larger than has been shown by previous surveys, and that holes in the complexes may be filled with low column density gas.

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