EVN observations of the farthest and brightest ULIRGs in the local Universe: the case of IRAS 23365+3604

TL;DR

This study uses high-resolution radio observations from EVN, MERLIN, and VLA to analyze the compact and extended radio emission in the ULIRG IRAS 23365+3604, revealing complex nuclear activity and magnetic field estimates.

Contribution

First high-resolution, multi-epoch radio imaging of IRAS 23365+3604, combining EVN, MERLIN, and VLA data to study its nuclear structure and emission components.

Findings

Nuclear region is about 200 pc with two main emission components.

Detected both ongoing non-thermal activity and old supernova remnant populations.

Magnetic fields estimated at 18 μG galactic and 175 μG nuclear scales.

Abstract

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 ~5.0 GHz remains quite compact as seen at different resolutions, whereas at ~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 18 \mu G at galactic, and 175 \mu G at nuclear scales, which are typical for galaxies in advanced mergers.