A Sensitive Search for Molecular Gas in High‐Velocity Clouds

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Abstract
We report on observations of the J = 1-0 line of 12CO at 115 GHz toward several high-velocity H I clouds (HVCs). High angular resolution observations at 21 cm show the presence of dense cores with N(H I) = (2-4) × 1020 cm-2 and n(H )=30-80D cm-3. We observed the CO line toward six of the densest HVC cores. No12CO emission is detected down to a best 5 σ limit of 0.077 K km s-1. Using the median I(CO) to N(H2) conversion factor appropriate for high-latitude translucent clouds, this limit corresponds to a column density limit for N(H2) of 9 × 1018 cm-2. We calculate the expected value of N(H2) assuming a standard formation mechanism for the molecules and accounting for the run of the interstellar radiation field with the assumed distance to the HVC. The nondetections then indicate that the observed HVCs are more distant than ~2 kpc. This is consistent with the results of interstellar absorption line studies, which yield lower limits of 2-5 kpc for most of the observed HVCs. The nondetections also suggest that HVC cores are not likely to be current sites of star formation in the halo.

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