R. S. Cohen

A composite CO survey of the entire milky way

Large-scale CO surveys of the entire Galactic plane and specific nearby clouds have been combined to produce a panorama of the entire Milky Way in molecular clouds at an angular resolution of 1/2°. Covering 10°–20° in latitude at all longitudes and all large, nearby clouds at higher latitude, the composite survey is the only molecular line survey to date with sky coverage and resolution comparable to that of the early 21 cm surveys.

Largest molecular cloud complexes in the first galactic quadrant

The CO emission within a few kiloparsecs of the Sun is dominated by a small number of very large molecular complexes, including those associated with the Orion Nebula (Thaddeus 1982), M16 and M17 (Elmegreen, Lada, and Dickinson 1979), and NGC7538 (Cohen et al. 1980). These complexes have masses from several 105 to 106 M⊙ and are generally very well-defined objects. They are also well endowed with HII regions, stellar clusters and associations, masers, and other Population-I objects whose distances can be measured. The complexes are thus valuable probes of the large-scale structure of the Galaxy.

Molecular clouds in the Carina arm - The largest objects, associated regions of star formation, and the Carina arm in the Galaxy

The Columbia CO survey of the southern Galactic plane is used to identify giant molecular clouds and cloud complexes in the Vela-Carina-Centaurus section of the Galaxy. Twenty-seven giant molecular clouds between l = 270 and 300 deg are catalogued and their heliocentric distances given. In addition, 16 clouds at l greater than 300 deg beyond the solar circle extend the catalog to include the very distant portion of the Carina arm. The most massive clouds in the catalog trace the Carina arm over 23 kpc in the plane of the Galaxy. The average mass of these objects is 1.4 x 10 to the 6th solar, and their average spacing along the arm is 700 pc. The composite distribution projected onto the Galactic plane of the largest molecular clouds in the Carina arm and of similarly massive clouds in the first and second quadrants strongly suggests that the Carina and Sagittarius arms form a single spiral arm about 40 kpc in length wrapping two-thirds of the way around the Galaxy. Descriptions of each cloud, including identification of associated star-forming regions, are presented in an appendix. 76 references.

Molecular clouds in the Carina arm - Large-scale properties of molecular gas and comparison with H I

Results from the first large-scale survey in the CO (J = 1 - 0) line of the Vela-Carina-Centaurus region of the southern Milky Way are reported. The results demonstrate that molecular clouds in the Galaxy are largely confined to the spiral arms and that CO is therefore an extremely good tracer of the large-scale structure of the system. The Carina arm is the dominant feature in the data. Its abrupt tangent at l of roughly 280 deg and characteristic loop in the (l, v) diagram are unmistakable evidence for CO spiral structure. When the emission is integrated over velocity and latitude, the height of the step seen in the tangent direction indicate that the arm-interarm contrast is at least 13:1. 44 references.

A deep CO survey of molecular clouds in the southern Milky Way

The complete set of observations from the first out-of-plane CO survey of the southern Milky Way is presented. A spatial map, obtained by integrating the CO emission over the entire observed spectral range, 333 km/s, summarizes the survey concisely and can be compared with gamma-ray, IR, and other continuum data. The kinematic information is summarized in two longitude-velocity diagrams, which represent in one case the CO emission integrated over the whole latitude coverage of the survey, and in the other, to improve the contrast for the distant molecular clouds, over a narrower latitude region within 0.75 deg of the Galactic equator. The information is unfolded in a complete set of longitude-velocity contour diagrams, one for each latitude observed. 18 refs.

Molecular clouds and galactic spiral structure

Two large-scale 2.6 mm CO surveys of the galactic plane, one in the first quadrant (l=12/sup 0/ to 60/sup 0/, b=-1/sup 0/ to +1/sup 0/), the other in the second (l=105/sup 0/ to 139/sup 0/, b=-3/sup 0/ to +3/sup 0/), have provided evidence that, contrary to previous findings, molecular clouds constitute a highly specific tracer of spiral structure. Molecular counterparts of five of the classical 21 cm spiral arms have been identified: the Perseus arm, the local arm (including Lindblads local expanding ring), the Sagittarius arm, the Scutum arm, and the 4-kpc arm. The region between the local arm and the Perseus arm is apparently devoid of molecular clouds, and the interarm regions of the inner Galaxy appear largely so. CO spiral structure implies that the mean lifetime of molecular clouds cannot be greater than 1 x 10/sup 8/ years, the time required for interstellar matter to cross a spiral arm. Conservation of mass then sets a limit on the fraction of the interstellar medium in the form of molecular clouds: it cannot exceed one-half at any distance from the galactic center in the range 4--12 kpc.

An out-of-plane galactic carbon monoxide survey

Galactic CO line emission at 115 GHz has been surveyed in the region from 15 to 60 deg galactic longitude and from about -1.5 to 1.5 deg latitude. In addition to confirming the findings of previous in-plane surveys that galactic CO emission is concentrated in a ring 6 kpc in radius, a fit of a cylindrically symmetric galactic model to the observational data has provided a determination of the thickness of this molecular ring and its displacement from the conventional galactic plane, both as functions of galactocentric distance. The average half-thickness at half-maximum of the molecular ring is 59 pc, and the average displacement of the ring with respect to the galactic equatorial plane is -40 pc.

The Columbia CO survey of molecular clouds in the first galactic quadrant

A systematic, unbiased survey of CO emission from much of the first Galactic quadrant has been completed using the Columbia-Goddard millimeter wave telescope. The velocity resolution is 1.3 km/s and the coverage is from -13 to 153 km/s at latitude less than 55 deg and from -55 to 111 km/s at latitudes of 55 deg and higher. The noise is 0.45 K rms per 1/3 km/s resolution element. The instrument and observing technique are described and the instruments calibration is compared with that of other telescopes. When the different beams are taken into account, the calibration agrees well with measurements of individual molecular clouds and the Galactic plane done with both the Bell Laboratories 7 m and the NRAO 11 m telescopes. The entire collection of over 3000 spectra are presented along with spatial and longitude-velocity maps.

A large molecular cloud in Lupus far from the galactic plane

The detection of a large molecular cloud at a distance of more than 200 pc from the Galactic plane, situated above a prominent hole in the CO distribution in the plane, is reported. The cloud has a radial velocity of -41 km/s, much larger than is characteristic of most local, high-latitude gas. The clouds displacement above the plane is the largest for any cloud yet detected. The mass of the cloud is about 100,000 solar masses, and its gravitational potential energy is about 7 x 10 to the 50th ergs. A single event may have created both the cloud and the nearby hole. 27 references.

Sandqvist 187 - A dense molecular cloud in Norma

Observations of Sandqvist 187, an elongated dust cloud in the southern constellation Norma are presented and discussed. The cloud contains two Herbig-Haro objects, HH 56 and HH 57. HH 57 currently displays on its NE edge a 17th mag variable star of the FU Ori type. Using the Columbia University 1.2 m millimeter-wave telescope at Cerro Tololo, the region is mapped and an extended CO cloud which envelops and is elongated along the optical dust cloud is found. The position of maximum CO emission coincides with HH 56 and HH 57. Assuming a distance of 0.7 kpc, the total mass of the cloud is found to be close to 500 solar masses. The CO spectra show evidence of a molecular flow. Photographs and CCD images obtained mostly with the CTIO 4 m telescope show the detailed optical structure of the dark clouds core region. The Herbig-Haro object HH 56 appears to be related to an emission-line star embedded in the small nebula Reipurth 13, not to the FU Ori star in HH 57.