A,12CO Survey of the large magellanic cloud

Abstract

Frank Kerr, in collaboration with J.V. Hindman and B.J. Robinson (1954), undertook the first 21-cm survey of the LMC, so I thought this meeting to commemorate Frank's retirement was appropriate for a brief presentation of the results of the first CO survey of molecular clouds in the LMC. A few CO observations toward some of the more prominent Population I objects in the LMC had been done prior to our work or were underway (Israel 1984; Israel et al. 1986; and references therein), but no systematic survey existed. It was clear to us from the previous work that CO is weak in the LMC relative to the Milky Way, owing presumably to the lower metallicity, and that a sensitive receiver and long integration times would be required to accomplish a useful inventory of the molecular clouds which the flamboyant star formation in the LMC suggested must surely exist. Our survey was done with an improved copy of the 1.2 m millimeter-wave telescope with which we have been studying molecular clouds with CO in the northern sky since 1975 (Cohen et al. 1986). The southern telescope was installed in Chile on Cerro Tololo late in 1982, and the LMC survey was assigned a high priority: roughly 6 hours/day for the next two years were dedicated to observing its inner 6 ° x 6 ° on a 7!5 grid, with a typical integration of about 40 rain per position. The line surveyed was the standard 1 ---* 0 CO transition at 115 GHz used for nearly all large-scale molecular cloud work in both the Milky Way and external galaxies. The beamwidth of the telescope at that frequency is 8{8, so the sampling interval is somewhat less than one beamwidth (85%). Our spectrometer was a standard filter bank with 256 channels, each 0.5 MHz wide, for a resolution in radial velocity of 1.3 km s -1 and a range of 333 km s -1. Integrations were generally continued until the rms noise per channel was reduced to 0.06 K in radiation temperature (antenna temperature corrected for the beam efficiency of 0.82), but a few positions of particular interest were observed for as long as 10 hr. Like most of the projects undertaken with the Chile Telescope the LMC survey was the result of a collaboration between our laboratory and scientists from the University of Chile. Richard Cohen was responsible for building the telescope and installing it in Chile; he provided a superb instrument--one that was extremely reliable and rugged with a sensitive liquid nitrogen cooled