Far-infrared emission from the largest molecular complexes in the fourth Galactic quadrant

摘要

The main scientific objective of this project was to study the far-infrared properties of the largest molecular cloud complexes in the fourth Galactic quadrant using the Infrared Astronomical Satellite (IRAS) data and our full CO survey of the Southern Milky Way (300 deg is less than l is less than 348 deg; absolute value of b is less than 2 deg). Most if not all of the major molecular cloud complexes identified from this survey have substantial infrared counterparts; some of them, with masses exceeding 5 x 10(exp 6) solar mass, are among the strongest extended IRAS sources in the Galactic plane. Our analysis was initially focused on two objects of this kind, the conspicuous giant molecular cloud complexes between l = 337 deg and l = 338 deg. These complexes are in the far side of the Norma spiral arm, 12 kpc from the Sun, and 5 kpc from the Galactic center. Each of them is very luminous in the infrared; the whole region taken together is the strongest extended IRAS source with the exception of the Galactic center. The radial velocities of these clouds differ by about 20 km/s. Our observations of the (13)CO J = 1 approaches 0 line along an arc of gas joining the two clouds, with a resolution of 40 min, show a continuous velocity gradient, favoring a dynamic relation; available observations of OH and H2CO in absorption are consistent with the geometry of the clouds' relative motion only if the clouds are receding from each other. These clouds could have experienced a close encounter that triggered a massive star formation burst, causing also a flow of material between the two.