Gemini Preprint #36


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

Andrew Harrison
Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh, UK EH9 3HJ

Phil Puxley
Gemini 8m Telescopes Project, 950 N. Cherry Ave., Tucson AZ 85719 USA

Adrian Russell
Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh UK EH9 3HJ

Peter Brand
Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh, UK EH9 3HJ

Abstract.

We present measurements of several near-IR emission lines from the nearby galaxy NGC 253. We have been able to measure 4 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 photo-dissociation 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 Brgamma 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 HII regions which are encompassed by PDRs. Away from the nuclear region, the geometry becomes that of PDRs bathed in a relatively diffuse UV radiation field.

The rotation curves of 1-0 S(1) and Brgamma suggest that the ionised gas is tracing a kinematic system different to that of the molecular gas in NGC 253, particularly away from the nucleus.

Appears in the Monthly Notices of the Royal Astronomical Society.

Download the PostScript file.


Ruth A. Kneale / web@gemini.edu / May 1, 1998