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

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 Atacama Large Millimeter/submillimeter Array. We report on the discovery of a long (3 kpc), thin (aspect ratio 30/1), filament of CO gas that breaks up into roughly 10 individual knots. Each individual knot has a low internal velocity dispersion (≈10 km s −1 ); the dispersion of the ensemble of knots in the filament is also low (≈10 km s −1 ). At the other extreme, we find that the individual clouds in the supergiant molecular cloud 2 region discussed by Wilson and collaborators have a large range of internal velocity dispersions (10 to 80 km s −1 ), and a large dispersion among the ensemble (≈80 km s −1 ). Other large-scale features observed in CO emission, and their correspondence with historical counterparts using observations in other wavelengths, are also discussed. We compare the locations of small-scale CO features with a variety of multiwavelength observations, in particular broad-(BVIJH) and narrow-band data (H α and Pa β ) taken with the Hubble Space Telescope, and radio (3.6 cm) continuum observations taken with the Karl G. Jansky Very Large Array. This comparison leads to the development of an evolutionary classification system that provides a framework for studying the sequence of star cluster formation and evolution-from diffuse supergiant molecular clouds (SGMCs) to proto, embedded, emerging, young, intermediate/old clusters. The relative timescales have been assessed by determining the fractional population of sources at each evolutionary stage. The main uncertainty in this estimate is the identification of four regions as candidate protoclusters (i.e., strong compact CO emission but no clearly associated radio emission). Using the evolutionary framework, we estimate that the maximum age range of clusters in a single GMC 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.

A Herschel/PACS Far-infrared Line Emission Survey of Local Luminous Infrared Galaxies

“…While there is not a clear sequence where galaxies with increasingly larger IR S and G/n H are found to be in progressively more advanced interacting stages, we find that most (∼80%) late-stage mergers ) have IR S > 5 × 10 10 L e kpc −2 and lie along the slope in Figure 12. This lack of a simple correlation with merger stage is consistent with the idea that star-forming clumps and stellar clusters can form and be quickly destroyed by powerful stellar feedback in gas-rich starbursts and mergers on relatively short (10-20 Myr) timescales (Whitmore et al 2014;Linden et al 2017;Oklopčić et al 2017)-significantly shorter than the timescale of the entire merging process (over several hundred Myr). Thus, rather than a single or small number of giant, long-lived, and relatively normal H II regions, it may be more accurate to think of the nuclear starburst in an LIRG as a collection of dense, dusty, and young star-forming regions that are individually short-lived but constantly replenished, like "fireworks" triggered by a galactic merger.…”

“…Our results indicate that this dusty phase, in which we are able to see a significant number of star-forming regions that are likely matter bounded and still embedded in their molecular cocoon (e.g., Kawamura et al 2009;Miura et al 2012;Whitmore et al 2014), seems to be associated primarily with the period when the overall starburst region is also most compact (above IR * S ). A mechanism often proposed to produce this compaction of the gas and dust in galaxies are major mergers (Sanders et al 1988;Hopkins et al 2008).…”

PHOTOIONIZATION MODELS FOR THE SEMI-FORBIDDEN C iii] 1909 EMISSION IN STAR-FORMING GALAXIES

“…In reality, at ages 3 Myr, the surrounding neutral gas and dust may strongly obscure optical and UV emission from the starburst (e.g. Kobulnicky & Johnson 1999;Gilbert & Graham 2007;Whitmore et al 2014;Sokal et al 2015), before feedback clears away some of this natal material. Therefore, the peak C iii] emission may occur instead at the slightly older ages corresponding to the Wolf-Rayet phase, consistent with the C iii] detections at low redshift (Rigby et al 2015).…”

The Scaling Relations and Star Formation Laws of Mini-Starburst Complexes

“…They are the birthplaces of massive OB stars and Young Massive Clusters (YMCs), thus are important in maintaining the chemical, energy, and mass balance of hosting galaxies (Bressert et al 2012). This is especially true for starburst galaxies such as the Antennae merger system, which undergoes starburst events and contains many YMCs and mini-starburst MCCs (Herrera et al 2012;Whitmore et al 2014).…”