NASA/IPAC EXTRAGALACTIC DATABASE
Date and Time of the Query: 2019-06-15 T22:14:47 PDT
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For refcode 1995PASJ...47..801T:
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Copyright by Astronomical Society of Japan. 1995PASJ...47..801T Detection of the SO J_N_ = 1_0_ - 0_1_ Rotational Transition in NGC 253: Comparison of the Molecular Abundances among Extragalaxies Shuro TAKANO Institute for Molecular Science, Myodaiji, Okazaki, Aichi 444 E-mail(ST) takano @ims. ac.jp and Naomasa NAKAI and Kentarou KAWAGUCHI Nobeyama Radio Observatory Nobeyama, Minamimaki-mura, Minamisaku-gun, Nagano 384-13 (Received 1995 February 22; accepted 1995 August 9) Abstract 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) x 10^14^ cm^- 2^ and (1-2) x 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. Key words: Atomic and Molecular Processes Galaxies: Abundances Galaxies: individual (NGC 253, M82, and NGC 6946)
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