A Radio Perspective on the Wet Merger Remnant NGC 34

TL;DR

This study uses VLA radio observations to analyze the neutral hydrogen and radio continuum in NGC 34, revealing gas dynamics, tidal interactions, and potential AGN activity, shedding light on the evolution of a gas-rich merger remnant.

Contribution

First detailed radio study of NGC 34's neutral hydrogen and continuum, revealing gas kinematics, tidal features, and possible AGN or starburst-driven outflows.

Findings

Detection of HI in tidal tails and nearby galaxies.

Broad HI absorption indicating complex gas motions.

Diffuse radio lobes suggesting past AGN activity or starburst winds.

Abstract

We present VLA observations of the neutral hydrogen and radio continuum of NGC 34 (= NGC 17 = Mrk 938). This object is an ideal candidate to study the fate of gas in mergers, since, as shown by an optical study done by Schweizer & Seitzer (2007), it is a gas-rich ("wet") merger remnant of two disk galaxies of unequal mass hosting a strong central starburst and a weak AGN. We detect HI emission from both tidal tails and from nearby galaxies, suggesting that NGC 34 is actually part of a gas-rich group and might have recently interacted with one of its companions. The kinematics of the gas suggests this remnant is forming an outer disk of neutral hydrogen from the gas of the northern tail. We also detect broad HI absorption (514 +/- 21 km/s wide) at both negative and positive velocities with respect to the systemic velocity. This absorption could be explained by the motions of the tidal tails or by the presence of a circumnuclear disk. In addition, we present radio-continuum images that show both nuclear (62.4 +/- 0.3 mJy) and extra-nuclear emission (26.5 +/- 3.0 mJy). The extra-nuclear component is very diffuse and in the shape of two radio lobes, spanning 390 kpc overall. This emission could be a signature of an AGN that has turned off, or it could originate from a starburst-driven superwind. We discuss the possible scenarios that explain our observations, and what they tell us about the location of the gas and the future evolution of NGC 34.