The excitation of near-infrared H2 emission in NGC 253
  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.
