Detection of the SO {JN =} 1_0 -> 0_1 Rotational Transition in NGC 253: Comparison of the Molecular Abundances among Extragalaxies

Details Detection of the SO {JN =} 1_0 -> 0_1 Rotational Transition in NGC 253: Comparison of the Molecular Abundances among Extragalaxies Detection of the SO {JN =} 1_0 -> 0_1 Rotational Transition in NGC 253: Comparison of the Molecular Abundances among Extragalaxies

Dec 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995pasj...47..801t&link_type=abstract

Publications of the Astronomical Society of Japan, v.47, p.801-813.

Astronomy and Astrophysics

Astronomy

15

Atomic And Molecular Processes, Galaxies: Abundances, Galaxies: Individual (Ngc 253, M82, And Ngc 6946)

Scientific paper

The J_N = 1_0 -> 0_1 (30.001630 GHz) and the J_N = 3_2 -> 2_1 (99.299875 GHz) rotational transitions of the SO molecule (X(3) Sigma (-) ) were searched in several extragalaxies NGC 253, M82, and NGC 6946 to study the relation between the molecular abundances and physical conditions. Only the J_N = 1_0 -> 0_1 transition of the SO molecule was detected in NGC 253, and no spectral line was detected toward M82 and NGC 6946. The column density and the relative abundance of SO with respect to H_2 in NGC 253 have been determined to be (2 -- 5)times 10(14) cm(-2) and (1 -- 2)times 10(-9) , respectively. The observed line profile is asymmetric, and the velocity component near 280 km s(-1) is prominent (V_LSR=230 km s(-1) ). Based on the present study and on the difference in the molecular abundances so far studied between NGC 253 and M82, we discuss the molecular formation mechanism in NGC 253 and M82. Two factors are considered to mainly contribute to the molecular formation: (1) ion-molecule reactions and neutral-neutral reactions (with small or no activation energy), which are effective in both NGC 253 and M82, and (2) neutral-neutral reactions with significant activation energy and grain-related reactions, which are effective in NGC 253, but not in M82. The probable reason for the effectiveness of the second factor in NGC 253 is that the temperature is higher and/or the size of high-temperature region is larger than that in M82. The SO molecule was detected in NGC 253 presumably due to this second factor, because a significant activation energy exists in at least one of the formation reactions of SO. Molecules effectively produced by the second factor have been also found in other nearby galaxies (IC 342, NGC 4945 and so on) in addition to NGC 253. Consequently, the ineffectiveness of the second factor in M82 may be striking among nearby galaxies.

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