SIMBAD references

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 out in the disk. NGC 1808 has highly concentrated ¹²CO emission and an asymmetrical velocity field in ¹²CO 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-phase" model that can explain the observations with a warm T_k=100 K, diffuse low-mass medium of moderate density, n(H₂)=2000 cm^–3, surrounding colder T_k=15 K, dense cores, n( H₂)=10⁵ cm^–3. A simple estimate of the pressure felt by these clouds leads to further constraints 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 V_c 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.