Molecular gas conditions in NGC 4945 and the Circinus galaxy
Molecular gas conditions in NGC 4945 and the Circinus galaxy
复制标题
NGC 4945 和圆规座星系中的分子气体条件
DOI：
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发表时间：
2001
期刊：
影响因子：
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通讯作者：
S. Aalto
中科院分区：
文献类型：
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作者：
S. Curran;L. Johansson;P. Bergman;A. Heikkilä;S. Aalto
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 $n_{{
m H}_2}=3 10^3{-}10^4$ cm -3 and $T_{
m kin}approx100$ K and in Circinus $n_{{
m H}_2}=2 10^3{-}10^5$ cm -3 and $T_{
m kin}approx50{-}80$ K for the molecular hydrogen density and kinetic temperature, respectively. As well as density/temperature tracing molecules, we have observed C 17 O and C 18 O in each galaxy and the value of C 18 O/C$^{17}{
m O}approx6$ 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.
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 $n_{{
m H}_2}=3 10^3{-}10^4$ cm -3 and $T_{
m kin}approx100$ K and in Circinus $n_{{
m H}_2}=2 10^3{-}10^5$ cm -3 and $T_{
m kin}approx50{-}80$ K for the molecular hydrogen density and kinetic temperature, respectively. As well as density/temperature tracing molecules, we have observed C 17 O and C 18 O in each galaxy and the value of C 18 O/C$^{17}{
m O}approx6$ 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.