Discovery of a Molecular Collision Front in Interacting Galaxies NGC 4567/4568 with ALMA

TL;DR

This study uses ALMA observations to identify and analyze a molecular collision front in interacting galaxies NGC 4567/4568, revealing a filamentary structure with bound molecular clouds that may lead to super star cluster formation.

Contribution

First detection of a molecular collision front in interacting galaxies with detailed physical characterization using ALMA.

Findings

Identified a molecular collision front with velocity dispersion of 16.8 km/s.

Discovered a 1800x350 pc filamentary molecular structure coinciding with a dark lane.

Found four gravitationally bound molecular clouds within the filament.

Abstract

We present results of $^{12}$CO(J = 1-0) imaging observations of NGC 4567/4568, a galaxy pair in a close encounter, with Atacama Large Millimetre/submillimetre Array. For the first time, we find clear evidence of a molecular collision front whose velocity dispersion is 16.8$\pm$1.4 km s$^{-1}$ at the overlapping region owing to high spatial and velocity resolution. By integrating over the velocity width that corresponds to the molecular collision front, we find a long filamentary structure with a size of 1800 pc $\times$ 350 pc at the collision front. This filamentary molecular structure spatially coincides with a dark-lane seen in R-band image. We find four molecular clouds in the filament, each with a radius of 30 pc and mass of 10$^{6} M_{\odot}$; the radii matching a typical value for giant molecular clouds and the masses corresponding to those between giant molecular clouds and giant molecular associations. All four clouds are gravitationally bound. The molecular filamentary structure and its physical conditions are similar to the structure expected via numerical simulation. The filament could be a progenitor of super star clusters.