EVN observations of the farthest and brightest ULIRGs in the local Universe: The case of IRAS23365+3604

Abstract

We present high-resolution, high-sensitivity radio images of the ultra-luminous infrared galaxy (ULIRG) IRAS23365+3604. We performed contemporaneous observations at 1.7 and 5.0GHz, in three epochs separated by 1 year from each other, with the European Very Long Baseline Interferometry (VLBI) Network (EVN). We also present complementary Multi-Element Radio Linked Interferometry Network (MERLIN) at 1.6 and 5.0GHz, and archival Very Large Array data, taken at 1.4 and 4.9GHz. We find that the emission at ∼5.0GHz remains quite compact as seen at different resolutions, whereas at ∼1.7GHz, high-resolution imaging reveals some extended structure. The nuclear region has an approximate linear size of 200pc 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 the presence of an active galactic nucleus), and (ii) the other with a steep spectrum, likely dominated by an old population of radio emitters, such as supernova remnants. Radiative losses are important, so re-acceleration or replenishment of new electrons is necessary. We estimate a magnetic field strength of ∼ 18μG at galactic and ∼ 175μG at nuclear scales, which are typical for galaxies in advanced mergers. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.