We present high-resolution , high-sensitivity radio images of the ultra-luminous infrared galaxy ( ULIRG ) IRAS 23365 + 3604 .
We performed contemporaneous observations at 1.7 and 5.0 GHz , in three epochs separated by one year from each other , with the European very long baseline interferometry Network ( EVN ) .
We also present complementary Multi-Element Radio Linked Interferometry Network ( MERLIN ) at 1.6 and 5.0 GHz , and archival Very Large Array ( VLA ) data , taken at 1.4 and 4.9 GHz .
We find that the emission at \sim 5.0 GHz remains quite compact as seen at different resolutions , whereas at \sim 1.7 GHz , high resolution imaging reveals some extended structure .
The nuclear region has an approximate linear size of 200 pc and shows the presence of two main emission components : i ) one with a composite spectrum due to ongoing non-thermal activity ( probably due to recently exploded supernovae and AGN activity ) , ii ) another one with a steep spectrum , likely dominated by an old population of radio emitters , such as supernova remnants ( SNRs ) .
Radiative losses are important , so re-acceleration or replenishment of new electrons is necessary .
We estimate a magnetic field strength of \sim 18 \mu G at galactic , and \sim 175 \mu G at nuclear scales , which are typical for galaxies in advanced mergers .