N-Band Observations of Henize 2-10: Unveiling the Dusty Engine of a Starburst Galaxy

As part of an ongoing program to better understand the early stages of massive star cluster evolution and the physical conditions for their formation, we have obtained J, H, K′, and N (10.8 μm) images of the nuclear region of the starburst galaxy He 2-10. The N-band images were obtained with the Gemini North Telescope. In only 10 minutes of on-source integration time with Gemini, we were able to detect four of the five enshrouded clusters, or "ultradense H II regions" (UD H II regions) recently discovered in radio maps. None of these sources appears in either the optical Hubble Space Telescope images or the near-infrared (J, H, and K′) images. These sources make up about 60% of the total N-band flux from He 2-10 and, we suspect, a similar fraction of the total far-infrared flux measured by IRAS. The inferred spectra of the UD H II regions are strikingly similar to those of Galactic ultracompact H II regions. We have modeled the radio and IR spectra of these UD H II regions under the assumption that they are "scaled-up" Galactic ultracompact H II regions. From this model, the bolometric luminosity of the brightest cluster alone is estimated to be ∼2 × 109 L☉. The total mass of the dust and gas in this UD H II region is Mshell ≈ 107 M☉. We have also used the observed spectra to place constraints on the masses and ages of the stellar clusters enshrouded within the UD H II regions. For the brightest UD H II region, we find that the stellar mass must be Mcluster ≳ 2.5 × 106 M☉, and the age must be ≲4.8 × 106 yr, with the most probable age being ≲3.6 × 106 yr. If we assume that the region is pressure confined and enforce the requirement that the star formation efficiency must be less than ∼90%, we find that the age of this stellar cluster must lie within a very narrow range, 4 × 105 < τ < 5 × 106 yr. All the clusters within the UD H II regions in He 2-10 are estimated to have ages less than about 5 × 106 yr and masses greater than about 5 × 105 M☉. We find that the logarithmic ratio of the radio to far-infrared flux densities, q, for the UD H II regions in He 2-10 is ∼4; q ≈ 2.6 for both He 2-10 as a whole and NGC 5253, another nearby starburst known to host UD H II regions. These values of q are significantly larger than the average q = 2.35 found for normal galaxies but are comparable to the values of q found for ultraluminous infrared galaxies. We suggest that large q values for starburst galaxies may indicate that a significant fraction of the far-infrared flux may arise from thermal dust emission from UD H II regions. Finally, the possibility that all the far-infrared flux from He 2-10 and other starburst galaxies may be produced by regions completely obscured at wavelengths as long as K′ suggests that the well-known correlation between ultraviolet continuum slope and infrared-to-ultraviolet flux ratio in starbursts cannot be due entirely to reprocessing of ultraviolet radiation by dust in a foreground screen geometry. In fact, the dust that reddens the ultraviolet continuum slope must be largely decoupled from the dust that produces the large infrared fluxes in some starbursts.