Molecular Gas in NUclei of GAlaxies (NUGA): XII. The head-on collision in NGC1961

TL;DR

This study investigates the peculiar morphology of NGC1961 through high-resolution CO mapping and numerical modeling, proposing a head-on collision with a smaller galaxy as the cause, which impacts the galaxy's structure and potential nuclear activity.

Contribution

It introduces a detailed collision scenario explaining NGC1961's features, supported by high-resolution observations and numerical models, highlighting the galaxy's warped disk and the impact on its dynamics.

Findings

Collision with a 1:4 mass ratio companion explains morphology

NGC1961's disk is more edge-on than previously thought

Collision may induce gas inflow to the active nucleus

Abstract

We present high-resolution CO(1-0) and CO(2-1) maps of the LINER 2 galaxy NGC1961. This galaxy is unusual among late-type (Sc) disk galaxies in having a very large radial extent and inferred dynamical mass. We propose a head-on collision scenario to explain the perturbed morphology of this galaxy- both the off-centered rings and the inflated radius. This scenario is supported by the detection of a steep velocity gradient in the CO(1-0) map at the position of a southwest peak in radio continuum and near-infrared emission. This peak would represent the remnant of the disrupting companion. We use numerical models to demonstrate the plausibility of the scenario. While ram pressure stripping could in principle be important for shocking the atomic gas and produce the striking head-tail morphology, the non detection of this small galaxy group in X-ray emission suggests that any hot intragroup medium has too low a density. The collision with the companion of mass ratio 1:4 produces a lopsided wave, which accounts for the sharp boundary observed in the atomic gas on the southern side. We argue that NGC1961 has a strongly warped disk, which gives the false impression of a nearly face-on system; the main disk is actually more edge-on, and this error in the true inclination has led to the surprisingly high dynamical mass for a morphologically late-type galaxy. In addition, the outward propagating ring artificially enlarges the disk. The collision de-stabilizes the inner disk and can provide gas inflow to the active nucleus.