The excitation of near-infrared H2 emission in NGC 253⋆
Sterrewacht Leiden, Universiteit Leiden,
PO Box 9513, 2300 RA Leiden, The Netherland
Received: 17 August 2012
Accepted: 6 December 2012
Context. Because of its large angular size and proximity to the Milky Way, NGC 253, an archetypal starburst galaxy, provides an excellent laboratory to study the intricacies of this intense episode of star formation.
Aims. We aim to characterize the excitation mechanisms driving the emission in NGC 253. Specifically we aim to distinguish between shock excitation and ultraviolet (UV) excitation as the dominant driving mechanism, using Brγ, H2 and [FeII] as diagnostic emission line tracers.
Methods. Using SINFONI observations, we create linemaps of Brγ, [FeII]1.64, and all detected H2 transitions. By using symmetry arguments of the gas and stellar gas velocity field, we find a kinematic center in agreement with previous determinations. The ratio of the 2-1 S(1) to 1-0 S(1) H2 transitions can be used as a diagnostic to discriminate between shock and fluorescent excitation.
Results. Using the 1-0 S(1)/2-1 S(1) line ratio as well as several other H2 line ratios and the morphological comparison between H2 and Brγ and [FeII], we find that excitation from UV photons is the dominant excitation mechanisms throughout NGC 253. We employ a diagnostic energy level diagram to quantitatively differentiate between mechanisms. We compare the observed energy level diagrams to photon-dominated region (PDR) and shock models and find that in most regions and over the galaxy as a whole, fluorescent excitation is the dominant mechanism exciting the H2 gas. We also place an upper limit of the percentage of shock excited H2 at 29%.
Conclusions. We find that UV radiation is the dominant excitation mechanism for the H2 emission. The H2 emission does not correlate well with Brγ but closely traces the polycyclic aromatic hydrocarbon emission, showing that not only is H2 fluorescently excited, but it is predominately excited by slightly lower mass stars than O stars which excite Brγ, such as B stars.
Key words: ISM: molecules / ISM: lines and bands / galaxies: ISM / galaxies: starburst / infrared: ISM
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