Revisiting Attenuation Curves: the Case of NGC 3351
  Multi-wavelength images from the farUV (~0.15 micron) to the sub-millimeter
of the central region of the galaxy NGC 3351 are analyzed to constrain its
stellar populations and dust attenuation. Despite hosting a ~1 kpc
circumnuclear starburst ring, NGC 3351 deviates from the IRX-beta relation, the
relation between the infrared-to-UV luminosity ratio and the UV continuum slope
(beta) that other starburst galaxies follow. To understand the reason for the
deviation, we leverage the high angular resolution of archival nearUV-to-nearIR
HST images to divide the ring into ~60-180 pc size regions and model each
individually. We find that the UV slope of the combined intrinsic (dust-free)
stellar populations in the central region is redder than what is expected for a
young model population. This is due to the region's complex star formation
history, which boosts the nearUV emission relative to the farUV. The resulting
net attenuation curve has a UV slope that lies between those of the starburst
attenuation curve (Calzetti et al. 2000) and the Small Magellanic Cloud
extinction curve; the total-to-selective attenuation value, R'(V)=4.93, is
larger than both. As found for other star-forming galaxies, the stellar
continuum of NGC 3351 is less attenuated than the ionized gas, with
E(B-V)_{star}=0.40 E(B-V)_{gas}. The combination of the `red' intrinsic stellar
population and the new attenuation curve fully accounts for the location of the
central region of NGC 3351 on the IRX-beta diagram. Thus, the observed
characteristics result from the complex mixture of stellar populations and dust
column densities in the circumnuclear region. Despite being a sample of one,
these findings highlight the difficulty of defining attenuation curves of
general applicability outside the regime of centrally-concentrated starbursts.
