Understanding Radio-selected Thermal Sources in M33: Ultraviolet, Optical, Near-Infrared, Spitzer Mid-Infrared, and Radio Observations

We present ultraviolet, optical, near-infrared, Spitzer mid-infrared, and radio images of 14 radio-selected objects in M33. These objects are thought to represent the youngest phase of star cluster formation. We have detected the majority of cluster candidates in M33 at all wavelengths. From the near-IR images, we derived ages 2-10 Myr, K_S-band extinctions (A) of 0-1 mag, and stellar masses of 10³-10⁴ M_☉. We have generated spectral energy distributions (SEDs) of each cluster from 0.1 to 160 μm. From these SEDs, we have modeled the dust emission around these star clusters to determine the dust masses (1-10³ M_☉) and temperatures (40-90 K) of the clusters' local interstellar medium. Extinctions derived from the JHK_S, Hα, and UV images are similar to within a factor of 2 or 3. These results suggest that 11 of the 14 radio-selected objects are optically visible young star clusters with a surrounding H II region, that 2 are background objects, possibly active galactic nuclei (AGNs), and that 1 is a Wolf-Rayet star with a surrounding H II region.