High altitude molecular clouds

A population of molecular clouds with a significantly greater scale height than that of giant molecular clouds (GMCs) has been identified by examining maps of the latitude distribution of the (12)CO(1 - 0) emission in the first quadrant of the Galaxy. These clouds are found by identifying emission more than 2.6 times the scale-height away from the Galactic midplane (centroid of CO emission) at the tangent points. Since the distance to the tangent points is known, we know the heights and the sizes of these clouds. They are smaller and fainter than the GMCs and do not seem to be gravitationally bound. These clouds have properties similar to the high-latitude clouds in the solar neighborhood. Although they lie outside the molecular cloud layer, the high-altitude clouds are well within the H I layer in the Galaxy and coincide with distinct peaks in the H I distribution. These clouds represent a Galaxy-wide population of small molecular clouds having a larger scale height. They may be clouds in transition between molecular and atomic phases.