Optical and OH megamaser observations of the starburst galaxy IIZw 096

Abstract

Using the Multi-Element Radio-Linked Interferometer Network (MERLIN), we have acquired a high spatial resolution spectral image of the OH megamaser (OHMM) in the nearby starburst ‘galaxy’ known as IIZw 096. We have also acquired optical B, V, R, I and Hα images with the 1.5-m telescope in San Pedro Martir. By comparing the MERLIN observations with Hubble archive images, we are able to pinpoint the location of the OH maser emission in this object; the OHMM emission seems to be associated with a faint and reddish, compact-round structure (a few hundred parsecs) off-centre of a merging system of galaxies. From the colour and strong Hα emission, this object is consistent with the remains of the nucleus of a very perturbed galaxy in interaction. Assuming a Keplerian bounded system the mass necessary to produce a velocity range of ≈200 km s−1 for the OHMM is of the order of 109 M⊙, which is consistent with a massive black hole. Consequently, although the OHMM was previously suspected to be of starburst origin, our analysis suggests that an active galactic nucleus (AGN) could also be present in this merging system making it a composite source. IIZw 096 is a nearby example which shows how high spatial resolution data of the OHMM emission region, and supporting optical data, can be extremely helpful in determining the nature (starburst, AGN or composite) of such phenomenon.