Journal Article

The ortho- to para- ratio of H<sub>2</sub> in the starburst of NGC 253

Andrew Harrison, Phil Puxley, Adrian Russell and Peter Brand

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 297, issue 2, pages 624-632
Published in print June 1998 | ISSN: 0035-8711
Published online June 1998 | e-ISSN: 1365-2966 | DOI:
The ortho- to para- ratio of H2 in the starburst of NGC 253

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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.

Journal Article.  0 words. 

Subjects: Astronomy and Astrophysics

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