Far-Infrared Spectroscopy of C II and High-J CO Emission from Warm Molecular Gas in NGC 3576

Astronomy and Astrophysics – Astronomy

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Infrared: Ism: Lines And Bands, Ism: Molecules, Ism: Individual Ngc Number: Ngc 3576, Line: Profiles

Scientific paper

We have observed the 158 mu m 2P3/2<--2P1/2 fine-structure line of C II, the J = 9 --> 8, 12 --> 11, and 17 --> 16 lines of 12CO, and the J = 9 --> 8 rotational transition of 13CO in the molecular cloud and photodissociation region (PDR) associated with NGC 3576. The line profiles were fully resolved using a heterodyne spectrometer with better than 1 km s-1 resolution. The high-J CO emission arises from a spatially compact region near the infrared peak that is characterized by a kinetic temperature near 150 K and a density n_{{H}2}>~5{x10}5 cm-3. The 12CO J = 9 --> 8 line is found to be optically thick and arises from a more extended, cooler region with Tkin ~ 60 K. The increase in excitation temperature with the rotational level of CO suggests that the ionizing source is located behind much of the molecular cloud. Less than 1% of the molecular cloud material is found in the warm PDR and cloud interface region. The C II fine-structure line emission is intense over more than 4' x 4' and shows a central "self-absorption" feature at many locations. This absorption may represent a cooler or less dense component of ionized gas associated with the foreground molecular cloud.

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