Open AccessThe ortho‐ to para‐ ratio of H 2 in the starburst of NGC 253
Author(s) -
Harrison Andrew,
Puxley Phil,
Russell Adrian,
Brand Peter
Publication year - 1998
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1046/j.1365-8711.1998.01525.x
Subject(s) - physics , galaxy , astrophysics , excited state , star formation , photodissociation , infrared , luminous infrared galaxy , emission spectrum , active galactic nucleus , measure (data warehouse) , astronomy , spectral line , atomic physics , chemistry , photochemistry , database , computer science
We present measurements of several near‐infrared emission lines from the nearby galaxy NGC 253. 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 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.