Excitation and kinematics of the central molecular clouds of NGC 1808

Details Excitation and kinematics of the central molecular clouds of NGC 1808 Excitation and kinematics of the central molecular clouds of NGC 1808

Jun 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...286..365a&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 286, no. 2, p. 365-380

Astronomy and Astrophysics

Astronomy

32

Astronomical Models, Carbon Monoxide, Emission Spectra, Galactic Bulge, Interstellar Matter, Kinematics, Line Spectra, Molecular Clouds, Molecular Excitation, Radio Astronomy, Radio Spectra, Spiral Galaxies, Extremely High Frequencies, Hydrocyanic Acid, Millimeter Waves, Submillimeter Waves, Sulfides

Scientific paper

We have mapped the central region of the spiral galaxy NGC 1808 in the (12)CO J=1-0, J=2-1, J=3-2 lines and measured (13)CO J=1-0, J=2-1, C(18)O J=1-0, HCN J=1-0 and an upper limit on CS J=2-1 at the central position. We have also measured the 12(CO) 2-1/1-0 and (12)CO/(13)CO J=1-0 intensity ratios at two positions out in the disk. NGC 1808 has highly concentrated (12)CO emission and an asymmetrical velocity field in (12)CO that shows distinct peaks on both sides of centre. The line intensity ratios indicate that the central molecular gas of NGC 1808 is in a state of high excitation compared with giant molecular clouds in the disk of the Milky Way. We present a 'two-phrase' model that can explain the observations with a warm Tk = 100 K, diffuse low-mass medium of moderate density, n(H2) = 2000/cu cm, surrounding colder Tk = 15 K, dense cores, n(H2) = 105/cu cm. A simple estimate of the pressure felt by these clouds leads to further constraints on their properties. The statistically enhanced (12)CO 2-1 map supports the notion of a central molecular ring, but the (12)CO emission line profile shows three peaks, two peaks at the high velocity (NW) side of the nominal central velocity Vc and one on the low velocity (SE) side, indicating a somewhat asymmetric distribution of molecular gas. The (12)CO 2-1/1-0 and (12)CO/(13)CO J=1-0 intensity ratios suggest a gradient in the CO excitation in which the excitation decreases with increasing distance from the nucleus.

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