[en] CO emission has been mapped along four strips in M51 using the 14 m Five College Radio Astronomy Observatory telescope (HPBW = 50'') in order to sample the radial distribution of molecular gas. The derived abundance of H₂ shows that three-fourths of the interstellar medium (ISM) nucleons are in H₂ rather than H I in the disk at R<10 kpc. For galactic radii 1--10 kpc, the radial distributions of CO emission, optical light from the disk, far-infrared flux, and nonthermal radio emission can be described by similar exponentials in R between 1 and 10 kpc with scale lengths approx.4 kpc. This contrasts sharply with the distribution of H I emission, which exhibits a hole in the center and a much flatter disk

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No abstract available

[en] High-resolution (2.4-arcsec) CO observations of the interacting system Arp 299 (Mrk 171: IC 694 and NGC 3690) reveal major gas condensations at the nuclei of both galaxies and in the disk overlap region. The 0.9 x 10 to the 9th solar masses of H2 at the nucleus of NGC 3690 extends to radii of 310 pc and probably sustains a starburst. A compact source (radius not greater than 250 pc) of mass 3.9 x 10 to the 9th solar masses is found at the nucleus of IC 694; this galaxy may harbor an AGN. At least three discrete subcondensations, probably active star-forming knots, are detected in the 1800 x 640 pc overlap region. 24 refs

[en] In recent years it has become clear, mainly from observations made by radio telescopes that most stars must form in molecular clouds. This conclusion follows from studies of nearby star-forming regions such as the Orion Nebula; it is also implied by the strong correlation found in the external galaxies between the molecular gas and the radiation from young stars. The other major gaseous component of these galaxies, atomic hydrogen, does not show this correlation. The authors review both observational and theoretical work done in an attempt to define the role of giant molecular clouds in galactic evolution and to establish the links between star formation and the large-scale structure of galaxies

[en] The predicted values of the gas temperature, T, and the OH abundance, n/subO//subH/, depend upon the rate of mass loss from the central star, Phi. The results quoted above are based on a calculation with Phi=3 x 10^-5 M/sub sun/ yr^-1. In general, T varies inversely and n/sub OH/ varies directly with Phi

[en] A total of 19 fields in the nearby spiral galaxy M33 have been observed by millimeter-wave interferometer at about 30 pc resolution. A total of 38 individual giant molecular clouds (GMCs) were detected. The velocity widths, diameters, peak brightness temperatures, and masses are very similar to those of Galactic GMCs. The virial and molecular masses for the clouds agree to within 10 percent, confirming for the first time that the Galactic value of the conversion factor alpha from CO flux to H2 column density is valid for an external galaxy. The mass distribution of the M33 clouds is in good agreement with the Galactic mass distribution, but shows a cutoff at masses greater than 400,000 solar. This can be explained in terms of a simple model where the rate of accretion of material by a cloud is balanced by the rate of dissociation of molecular gas by stars within the cloud. 37 refs

[en] A model is presented for the 1612 MHz OH masers associated with infrared stars. Its principal conclusions are as follows. The central stars are losing approx.3 x 10^-5 M/sub sun/yr^-1, and the masers operate in the outer regions (r>10¹⁶ cm) of the circumstellar envelopes. The maser radiation is narrowly beamed in the radial direction, both inward and outward. Thus the two maser emission features originate in the near and the far sides of the expanding circumstellar gas. The 1612 MHz maser is powered by the absorption of 35 μ photons which excite the OH molecules from the ²Pi₃/₂,J=3/2 ground state to the ²Pi₁/₂,J=5/2 state. The excited OH molecules return to the ground state by a series of radiative decays. In most cases, the radiative cascade proceeds directly down the ²Pi₁/₂ ladder. The final and the most important step in the pump cycle is the radiative decay from the ²Pi₁/₂,J=1/2 state to the ²Pi₃/₂,J= =3/2 state. If the transitions which link these two states are optically thick, a strong inversion of the F=1→F=2 1612 MHz transition is produced

[en] The CO emission in the two Scd galaxies, IC 342 and NGC 6946, has been mapped using the 14 m FCRAO telescope (HPBW = 50''). In each galaxy, the radial distribution of CO out to Rroughly-equal10 kpc exhibits a falloff which follows the exponential luminosity profile from the stellar disk. Neither galaxy shows a breach in the molecular gas like that found in our Galaxy between the nucleus and the molecular cloud annulus. The occurrence of this gap in our own Galaxy, but in neither of the external galaxies, suggests a possible link between the deficiency of gas and the presence of either a nuclear bulge or inner Lindblad resonance (ILR). From its rotation curve, the Milky Way has a large nuclear bulge and also has an ILR expected in the vicinity of the hole in the CO distribution. The CO rotation curves for IC 342 and NGC 6946 agree well with previous HI observations; at the present resolution these galaxies appear to undergo nearly rigid rotation inside R = 5 kpc and therefore have no ILRs and have small nuclear bulges. Further support for the association of the gap with ILR or bulge is provided by brief observations reported here of NGC 4321, a more distant Sc galaxy with a rotation curve similar to the Milky Way; the CO distribution appears like that in our own Galaxy with coarser resolution

[en] For a sample of 38 resolved molecular clouds in M33, new H-alpha data and previously published high-resolution H I maps are analyzed to study the star formation properties of the clouds and the cycle of the gas from one phase of the interstellar medium to another. Roughly 2/3 of the molecular clouds of more than 50,000 solar masses show recent massive star formation as traced by the H-alpha emission. The high-mass star formation rates for the individual clouds range from 0.000005 to 0.0005 solar masses/yr. The relative offset of the H I and the CO peaks in the southern spiral arm is not consistent with the picture in which the atomic gas condenses in the arms to form the molecular clouds, since the H I emission appears most strongly on the downstream side of the molecular gas. 23 refs