The ortho- to para- ratio of H<sub><nobr>2</nobr></sub> in the starburst of NGC 253期刊界 All Journals 搜尽天下杂志传播学术成果专业期刊搜索期刊信息化学术搜索

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The ortho- to para- ratio of H₂ in the starburst of NGC 253

Authors:	Andrew Harrison Phil Puxley Adrian Russell & Peter Brand

Institution:	Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ,;Gemini 8m Telescopes Project, 950 N. Cherry Ave., Tucson, Arizona 85719, USA,;Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ

Abstract:	We present measurements of several near-infrared emission lines from the nearby galaxy NGC 253. We have been able to measure four H₂ lines across the circumnuclear starburst, from which we estimate the ortho- to para- ratio of excited H₂ to be ～2. This indicates that the bulk of the H₂ emission arises from photodissociation regions (PDRs), rather than from shocks. This is the case across the entire region of active star formation. As the H₂ emission arises from PDRs, it is likely that the ratio of H₂ to Brγ (the bright hydrogen recombination line) is a measure of the relative geometry of O and B stars and PDRs. Towards the nucleus of NGC 253 the geometry is deduced to be tightly clustered O and B stars in a few giant H II regions that are encompassed by PDRs. Away from the nuclear region, the geometry becomes that of PDRs bathed in a relatively diffuse ultraviolet radiation field. The rotation curves of 1–0 S(1) and Brγ suggest that the ionized gas is tracing a kinetic system different from that of the molecular gas in NGC 253, particularly away from the nucleus.

Keywords:	galaxies: individual: NGC 253 galaxies: ISM galaxies: starburst infrared: galaxies

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