2000ApJ...537..221U

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 0°.125 in latitude and longitude, covers 55 deg² between l=107° and 116°. A smaller area of ~15 deg² between l=108° and 113° was similarly surveyed in ¹³CO. In the region covered in both isotopic species we find at least seven GMCs with masses on the order of 10⁵ M_☉. An intensity-weighted radius gives a more meaningful measure of cloud size than the simple geometrical area and is best used to estimate the virial mass. The ratio of total cloud luminosities in CO and ¹³CO, S₁₂/S₁₃, ranges from 6 to 10, with a mean of 8.5. The distribution of molecular gas is very 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.