The excitation of near-infrared H2 emission in NGC 253

TL;DR

This study investigates the excitation mechanisms of near-infrared H_2 emission in NGC 253, concluding that UV photon excitation dominates over shocks, with implications for understanding star formation processes.

Contribution

It provides a detailed diagnostic analysis distinguishing between shock and UV excitation in NGC 253, demonstrating UV excitation as the primary mechanism across the galaxy.

Findings

UV excitation dominates H_2 emission in NGC 253

Shock excitation contributes less than 29%

H_2 emission correlates with PAH emission, not Br extgamma

Abstract

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. We aim to characterize the excitation mechanisms driving the emission in NGC 253. Specifically we aim to distinguish between shock excitation and UV excitation as the dominant driving mechanism, using Br\gamma, H_2 and [FeII] as diagnostic emission line tracers. Using SINFONI observations, we create linemaps of Br\gamma, [FeII]_{1.64}, and all detected H_2 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) H_2 transitions can be used as a diagnostic to discriminate between shock and fluorescent excitation. Using the 1-0 S(1)/2-1 S(1) line ratio as well as several other H_2 line ratios and the morphological comparison between H_2 and Br\gamma 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 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 H_2 gas. We also place an upper limit of the percentage of shock excited H_2 at 29%. We find that UV radiation is the dominant excitation mechanism for the H_2 emission. The H_2 emission does not correlate well with Br\gamma but closely traces the PAH emission, showing that not only is H_2 fluorescently excited, but it is predominately excited by slightly lower mass stars than O stars which excite Br\gamma, such as B stars.