HI Observations of two Molecular Clouds with Extremely Large Velocity Dispersions

Abstract

We have mapped two molecular clouds at (l, b) = (3.2 degrees, + 0.3 degrees) and (l, b) = (5.4 degrees, -0.5 degrees) in 21-cm line and continuum emission. These clouds show unusually large velocity dispersions of more than 100 km/s (FWHM) which has also been seen in $^{12}$CO, $^{13}$CO, and CS emissions. This dispersion is roughly an order of magnitude larger compared to giant molecular clouds. From our HI observation we estimate that the atomic mass of the cloud at l = 3.2 degrees is about $1.5\times 10^5 M_\odot$ and the mass of the cloud at l = 5.4 degrees is $7\times 10^4 M_\odot$. The ratio of total molecular mass to atomic mass for these clouds appears to be normal for clouds near the galactic center. The main peculiar feature of these clouds is their abnormally large velocity dispersion; the extent in velocity is from about 0 km/s (LSR) to 200 km/s. These clouds are connected, in the l-v plane, to high velocity ridges that extend over several degrees of the longitude. These properties, we believe, provide important clues to the physical process responsible for their large velocity dispersion.