First measurements of extragalactic CO(4-3)

We report the first measurements of extragalactic CO(4-3) toward a sample of prominent nearby galactic nuclei (NGC 253, IC 342, M82, and NGC 6946). With the J = 4 level 55 K above ground, and critical densities for excitation of order ∼10⁴-10⁵ cm^-3, detection of the CO(4-3) line provides direct evidence for large amounts of warm, dense molecular gas. A multitransition CO excitation analysis, based on all presently observed ¹²CO and ¹³CO transitions up to J = 6-5, has been performed to confine the physical state of the gas. Single-component homogeneous models fail to fit the observations. As a next approximation, simple two-component models provide good matches to the observations, with a warm, dense, compact phase superposed on extended, less excited gas. The decomposition suggests that M82 (and likely NGC 253) harbors comparable masses of the warm and dense (∼50-70 K, ∼10^4-5 cm^-3), and low-excitation (T · n^1/2 ∼ 8 × 10²) phases, ∼10⁸ M⊙ each. Toward IC 342, our excitation study determines the average properties of the massive molecular clouds and the more extensive "intercloud" medium seen in recent interferometric maps. The massive clouds (∼10⁶ M⊙ each) are dense (∼2 × 10⁴ cm^-3) and warm (50-70 K), resembling those in the center of our Galaxy. While the highly excited gas phase in M82 (and NGC 253) is likely related to nuclear star formation activity and concomitant high UV flux, for the more moderate excitation conditions in IC 342 (and NGC 6946), large-scale heating processes, such as the dissipation of turbulence, may be viable.

Duke Scholars

Author Ken H Young Pathology