Physics
Scientific paper
Jan 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993egte.conf..159h&link_type=abstract
In NASA. Ames Research Center, The Evolution of Galaxies and Their Environment p 159-160 (SEE N93-26706 10-90)
Physics
Carbon Monoxide, Massive Stars, Molecular Clouds, Molecular Gases, Rotational Spectra, Star Formation, Starburst Galaxies, Submillimeter Waves, Excitation, Thermometers, Trapping
Scientific paper
Over the past several years, short-submillimeter observations of carbon monoxide's (CO) mid-J rotational levels have revealed the presence of a large amount of excited molecular gas in luminous giant molecular clouds in our Galaxy. Submillimeter lines are specific probes of excited material: collisional excitation of the level energy of 116 K above ground, and 6-5 transition's critical density is approximately 10(exp 6) cm(exp -3) in optically thin gas. Radiative trapping effects reduce the excitation requirements to some extent, but detection of the CO J=6-5 line is nearly indisputable proof of the existence of gas that is both warm and dense. The excitation conditions also imply that cool (T less than 20 K) molecular clouds within the beam neither emit nor absorb in the short-submillimeter lines; in our Galaxy, clouds with active massive star formation emit the strongest short-submillimeter CO rotational lines. We used these properties to explore the distribution of excited molecular material and physical conditions within the star formation regions of several classical starburst nuclei: NGC253, M82, and IC342. We have used the 6-5 transition as a thermometer of warm molecular gas in starburst nuclei, unambiguously finding that the nuclear molecular gas in starburst galaxies is substantially warmer than in typical disk clouds.
Genzel Reinhard
Graf Urs U.
Harris Andrew I.
Hills Richard E.
Russell Adrian P. G.
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