Arp 220 6 and 33GHz images

This similarity might be due to the fast cooling of cosmic rays electrons caused by the presence of a strong (~mG) magnetic field in this system. We estimate extremely high molecular gas surface densities of 2.2_-1.0_^+2.1^x10^5^M_{sun}_/pc^2^ (east) and 4.5_-1.9_^+4.5^x10^5^M_{sun}_/pc^2^ (west), corresponding to total hydrogen column densities of N_H_=2.7_-1.2_^+2.7^x10^25^cm^-2^ (east) and 5.6_-2.4_^+5.5^x10^25^cm^-2^ (west).

The implied gas volume densities are similarly high, n_H2_~3.8_-1.6_^+3.8^x10^4^cm^-3^ (east) and ~11_-4.5_^+12^x10^4^cm^-3^ (west). We also estimate very high luminosity surface densities of {Sigma}_IR_~4.2_-0.7_^+1.6^x10^13^L_{sun}_/kpc^2^ (east) and {Sigma}_IR_ ~ 9.7_-2.4_^+3.7^x10^13^_L_{sun}/kpc^2^ (west), and star formation rate surface densities of {Sigma}_SFR_~10^3.7+/-0.1^M_{sun}_/yr/kpc^2^ (east) and {Sigma}_SFR_~10^4.1+/-0.1^M_{sun}_/yr/kpc^2^ (west).

These values, especially for the western nucleus are, to our knowledge, the highest luminosity surface densities and star formation rate surface densities measured for any star-forming system. Despite these high values, the nuclei appear to lie below the dusty Eddington limit in which radiation pressure is balanced only by self-gravity. The small measured sizes also imply that at wavelengths shorter than {lambda}=1mm, dust absorption effects must play an important role in the observed light distribution while below 5GHz free-free absorption contributes substantial opacity.

According to these calculations, the nuclei of Arp 220 are only transparent in the frequency range ~5-350GHz. Our results offer no clear evidence that an active galactic nucleus dominates the emission from either nucleus at 33GHz.