ALMA Observations of the Antennae Galaxies: I. A New Window on a Prototypical Merger

TL;DR

This paper presents high-resolution ALMA CO (3-2) observations of the Antennae galaxies' overlap region, revealing a filament of gas with knots of low velocity dispersion, and develops an evolutionary classification for star cluster formation.

Contribution

It introduces a detailed observational analysis of molecular gas structures and proposes a new evolutionary framework for star cluster development in merging galaxies.

Findings

Discovery of a 3 kpc filament of CO gas with low internal velocity dispersion.

Estimation of star cluster formation timescale as approximately 10 million years.

Identification of different stages in star cluster evolution based on multi-wavelength data.

Abstract

We present the highest spatial resolution (~0.5") CO (3-2) observations to date of the "overlap" region in the merging Antennae galaxies (NGC 4038/39), taken with the ALMA. We report on the discovery of a long (3 kpc), thin (aspect ratio 30/1), filament of CO gas which breaks up into roughly ten individual knots. Each individual knot has a low internal velocity dispersion (~10 km/s), and the dispersion of the ensemble of knots in the filament is also low (~10 km/s). At the other extreme, we find that the individual clouds in the Super Giant Molecular Cloud 2 region discussed by Wilson and collaborators have a large range of internal velocity dispersions (10 to 80 km/s), and a large dispersion amongst the ensemble (~80 km/s). We use a combination of optical and near-IR data from HST, radio continuum observations taken with the VLA, and CO data from ALMA to develop an evolutionary classification system which provides a framework for studying the sequence of star cluster formation and evolution, from diffuse SGMCs, to proto, embedded, emerging, young, and intermediate/old clusters. The relative timescales have been assessed by determining the fractional population of sources at each evolutionary stage. Using the evolutionary framework, we estimate the maximum age range of clusters in a single SGMC is ~10 Myr, which suggests that the molecular gas is removed over this timescale resulting in the cessation of star formation and the destruction of the GMC within a radius of about 200 pc. (abridged)