The ortho- to para- ratio of H_2 in the starburst of NGC253

Astronomy and Astrophysics – Astronomy

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Galaxies: Individual: Ngc253, Galaxies: Ism, Galaxies: Starburst, Infrared: Galaxies

Scientific paper

We present measurements of several near-infrared emission lines from the nearby galaxy NGC253. We have been able to measure four H_2 lines across the circumnuclear starburst, from which we estimate the ortho- to para- ratio of excited H_2 to be ~2. This indicates that the bulk of the H_2 emission arises from photodissociation regions (PDRs), rather than from shocks. This is the case across the entire region of active star formation. As the H_2 emission arises from PDRs, it is likely that the ratio of H_2 to Brgamma (the bright hydrogen recombination line) is a measure of the relative geometry of O and B stars and PDRs. Towards the nucleus of NGC253 the geometry is deduced to be tightly clustered O and B stars in a few giant HII 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 Brgamma suggest that the ionized gas is tracing a kinetic system different from that of the molecular gas in NGC253, particularly away from the nucleus.

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