Excitation and Kinematics if the Central Molecular Clouds in NGC 1808

Susanne Aalto, Roy S. Booth (Onsala Space Observatory), J.H. Black (Steward Observatory), Bärbel Koribalski (MPIfR Bonn; now ATNF), Richard Wielebinski (MPIfR Bonn)

(1994) A&A 286, 365

Abstract. We have mapped the central region of the spiral galaxy NGC 1808 in the 12CO J=1-0, J=2-1, J=3-2 lines and measured 13CO J=1-0, J=2-1, C18O 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 12CO 2-1/1-0 and 12CO/13CO J=1-0 intensity ratios at two positions outside the disk. NGC 1808 has highly concentrated 12CO emission and an asymetrical velocity field in 12CO that shows distinct peaks in both sides of the 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-phase'' model that can explain the observations with a warm Tk = 100 K, diffuse low-mass medium of moderate density, n(H2) = 2000 cm^-3, surrounding colder Tk = 15 K, dense cores, n(H2) = 10e5 cm^-3. A simple estimate of the pressure felt by these clouds leads to further constrains on their properties.
The statistically enhanced 12CO 2-1 map supports the notion of a central molecular ring, but the 12CO emission line profile shows three peaks, two peaks at the high velocity (NW) side of the nominal central velocity and one on the low velocity (SE) side, indicating a somewhat asymmetric distribution of molecular gas. The 12CO 2-1/1-0 and 12CO/13CO J=1-0 intensity ratios suggest a gradient in the CO excitation in which the excitation decreases with increasing distance from the nucleus.

Keywords. galaxies: ISM -- galaxies: individual: NGC 1808 -- galaxies: kinematics and dynamics -- radio lines: galaxies

--> Halpha in NGC 1808
--> HI in NGC 1808
--> HI in the nuclear region of NGC 1808
--> PhD Thesis: NGC 1808
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Bärbel Koribalski * Baerbel.Koribalski @ csiro.au