Molecular gas excitation in NGC 253
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Abstract
We have observed the J = 3 - 2 and J = 2 - 1 lines of ^13^CO and ^12^CO with 22"-24" resolution in the central region of the spiral galaxy NGC 253. The ^13^CO J = 3 - 2 emission is pointlike (<~15"), whereas the emission in the other CO lines is consistent with emission from a ~40" x 10" FWHM bar. The ratio of ^13^CO J = 3 - 2 to ^13^CO J = 2 - 1 line intensities toward the nucleus requires warm (T_K_~> 100 K), dense [n(H_2_ )~> 10^4^-10^5^ cm^-3^] molecular gas. Spatial coincidence of 3 mm continuum and [Ne II] 12.8 micron emission from H II regions and mid-IR emission from hot dust with the ^13^CO J = 3 - 2 emission implies a common source of excitation, possibly related to the intense star formation that has occurred in the nucleus. The decline in the ^13^CO J = 3 - 2 intensity with distance from the nucleus reflects a decline in both density and temperature. The declining density, for the bulk of the CO line-emitting gas, is inferred from the drop in the ^13^CO J = 3 - 2/^13^CO j = 2 - 1 intensity ratio, while the small ^13^CO J = 3 - 2 source size compared to the size of the emission region for lines of high dipole moment molecules (e.g., CS J = 2 - 1) implies a decline in gas temperature. A drop in the ^12^CO J = 3 - 2/J = 2 - 1 intensity ratio indicates that the molecular gas density is low 30" from the nucleus. The molecular gas mass in the central 50", estimated from the optically thin lines of ^13^CO, is ~4 X 10^7^ M_sun_.
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