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

Open Access
Publisher Website
Google Scholar
Abstract
We have observed the 158 μm2P3/2←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 nH2 5 × 105 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' × 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.