The excitation of near-infrared H₂ emission in NGC 253^⋆

Sterrewacht Leiden, Universiteit Leiden, PO Box 9513, 2300 RA Leiden, The Netherland
e-mail: rosenberg@strw.leidenuniv.nl

Received: 17 August 2012
Accepted: 6 December 2012

Abstract

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.