The Role of Merger Stage on Galaxy Radio Spectra in Local
  Infrared-Bright Starburst Galaxies
  An investigation of the steep, high-frequency (i.e., ~12 GHz) radio spectra
among a sample of 31 local infrared-bright starburst galaxies is carried out in
light of their HST-based merger classifications. Radio data covering as many as
10 individual bands allows for spectral indices to be measured over three
frequency bins between 0.15-32.5 GHz. Sources having the flattest spectral
indices measured at ~2 and 4 GHz, arising from large free-free optical depths
among the densest starbursts, appear to be in ongoing through post-stage
mergers. The spectral indices measured at higher frequencies (i.e., ~12 GHz)
are steepest for sources associated with ongoing mergers in which their nuclei
are distinct, but either share a common stellar envelope and/or exhibit tidal
tails. These results hold after excluding potential AGN based on their low
6.2um PAH EQWs. Consequently, the low-, mid-, and high-frequency spectral
indices each appear to be sensitive to the exact merger stage. It is
additionally shown that ongoing mergers, whose progenitors are still separated
and share a common envelope and/or exhibit tidal tails, also exhibit excess
radio emission relative to what is expected given the far-infrared/radio
correlation, suggesting that there may be a significant amount of radio
emission that is not associated with ongoing star formation. The combination of
these observations, along with high-resolution radio morphologies, leads to a
picture in which the steep high-frequency radio spectral indices and excess
radio emission arises from radio continuum bridges and tidal tails that are not
associated with star formation, similar to what is observed for so-called
"taffy" galaxies. This scenario may also explain the seemingly low
far-infrared/radio ratios measured for many high-z submillimeter galaxies, a
number of which are merger-driven starbursts.
