The nuclear PAH emission of merger system NGC 1614: rings within rings

TL;DR

This study uses spatially resolved L-band observations to reveal a ring of PAH emission in NGC 1614, indicating outward propagating star formation without evidence of an active nucleus, shedding light on galaxy merger evolution.

Contribution

It provides the first spatially resolved L-band view of PAH emission in NGC 1614, linking PAH features to star formation propagation in a merging galaxy.

Findings

Detected a 200 pc PAH emission ring around the nucleus.

Identified an outward propagating star formation ring.

Ruled out the presence of an obscured AGN.

Abstract

It is important to understand the interplay between nuclear star-formation and nuclear activity when studying the evolution of gas rich galaxy mergers. We present here new spatially resolved L-band integral field unit observations of the inner kpc of the luminous IR galaxy NGC 1614. A broad ring of 3.3 um PAH emission is found at a distance of approximately 200 pc from the core. This ring overlaps with a previously established star-forming ring detected with Pa_alpha and radio continuum observations, but peaks outside it, especially if determined using the PAH equivalent width. Using the characteristics of the PAH emission and the ionised gas emission we argue that NGC 1614 features an outward propagating ring of star formation, where the equivalent width of the PAH emission localises the regions where the current star formation is just expanding into the molecular gas outward of the nucleus. The core itself shows a highly luminous, slightly resolved (at ~80 pc) L-band continuum source. We find no evidence of AGN activity and rule out the presence of an obscured AGN using L-band diagnostics. Furthermore, we detect the likely companion galaxy from archival HST/ACS imaging. The star formation and dynamical characteristics of the system are consistent with a relatively major merger just after its second passage. An outstanding question is how a gas-rich advanced merger such as this one, with strong LIRG level nuclear starburst and major merger-like tidal features, has not yet developed an active nucleus.