A CO Survey of Giant Molecular Clouds near Cassiopeia A and NGC 7538

Abstract

With the 1.2 m telescope at the Center for Astrophysics we have undertaken a survey of the J 1 ) 0 CO line toward the conspicuous complex of giant molecular clouds (GMCs) in the Perseus arm in the region of Cas A and NGC 7538. The survey, at a spectral resolution of 250 kHz (0.65 km s~1), on a uniform grid of points spaced every in latitude and longitude, covers 55 deg2 between l 107i and 0i.125 116i. A smaller area of B15 deg2 between l 108i and 113i was similarly surveyed in 13CO. In the region covered in both isotopic species we —nd at least seven GMCs with masses on the order of 105 An intensity-weighted radius gives a more meaningful measure of cloud size than the simple geo- M _ . metrical area and is best used to estimate the virial mass. The ratio of total cloud luminosities in CO and 13CO, ranges from 6 to 10, with a mean of 8.5. The distribution of molecular gas is very S 12 /S 13 , similar in CO and 13CO, and the velocity-integrated intensities at each point are closely correlated. In the (l, v)-diagram the Perseus arm is kinematically separated from the local arm by an interarm gap that is nearly free of CO; the contrast in molecular gas surface density between the Perseus arm and the interarm gap is apparently at least 20. Subject headings: Galaxy: structureISM: cloudsISM: individual (NGC 7538) ¨ ISM: molecules