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Title:
Molecular gas conditions in NGC 4945 and the Circinus galaxy
Authors:
Curran, S. J.; Johansson, L. E. B.; Bergman, P.; Heikkilä, A.; Aalto, S.
Affiliation:
AA(Onsala Space Observatory, Chalmers University of Technology, 439 92 Onsala, Sweden; European Southern Observatory, Casilla 19001, Santiago 19, Chile), AB(Onsala Space Observatory, Chalmers University of Technology, 439 92 Onsala, Sweden), AC(Onsala Space Observatory, Chalmers University of Technology, 439 92 Onsala, Sweden), AD(Onsala Space Observatory, Chalmers University of Technology, 439 92 Onsala, Sweden; Observatory, PO Box 14, 00014 University of Helsinki, Finland), AE(Onsala Space Observatory, Chalmers University of Technology, 439 92 Onsala, Sweden)
Publication:
Astronomy and Astrophysics, v.367, p.457-469 (2001) (A&A Homepage)
Publication Date:
02/2001
Origin:
A&A
A&A Keywords:
GALAXIES: ABUNDANCES, GALAXIES: INDIVIDUAL: NGC 4945, CIRCINUS, GALAXIES: NUCLEI, GALAXIES: SEYFERT, GALAXIES: STAR-BURST
DOI:
10.1051/0004-6361:20000462
Bibliographic Code:
2001A&A...367..457C

Abstract

We present results of a multi-transition study of the dense molecular gas in the central part of the hybrid star-burst/Seyfert galaxies NGC 4945 and the Circinus galaxy. From the results of radiative transfer calculations, we estimate in NGC 4945 nH_2=3 103-104 cm-3 and T_kin~100 K and in Circinus nH_2=2 103-105 cm-3 and T_kin~50-80 K for the molecular hydrogen density and kinetic temperature, respectively. As well as density/temperature tracing molecules, we have observed C17O and C18O in each galaxy and the value of C18O/C17O~6 for the isotopic column density ratio suggests that both have relatively high populations of massive stars. Finally, although star formation is present, the radiative transfer results combined with the high HCN/CO and (possibly) HCN/FIR, radio/FIR ratios may suggest that, in comparison with Circinus, a higher proportion of the dense gas emission in NGC 4945 may be located in the hypothesised central nuclear disk as opposed to dense star forming cloud cores. Contrary to the literature, which assumes that all of the far-infrared emission arises from star formation, our results suggest that in NGC 4945 some of this emission could arise from an additional source, and so we believe that a revision of the star formation rate estimates may be required for these two galaxies. Based on results collected at the European Southern Observatory, La Silla, Chile.
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