Molecular Gas and the Modest Star Formation Efficiency in the “Antennae” Galaxies: Arp 244 = NGC 4038/9

Abstract

We report here a factor of 5.7 higher total CO flux in Arp 244 (the Antennae galaxies) than that previously accepted in the literature (thus a total molecular gas mass of 1.5 × 1010 M☉), based on our fully sampled CO(1-0) observations at the NRAO 12 m telescope. Currently, much of the understanding and modeling of the star formation in Arp 244 has been derived using a much lower molecular gas mass. It is imperative to reconsider everything, as the high molecular gas mass might provide sufficient fuel for ultraluminous extreme starburst in Arp 244 once the merging advances to late stage. Our observations show that the molecular gas peaks predominately in the disk-disk overlap region between the nuclei, similar to the far-infrared (FIR) and mid-infrared (MIR) emission. The bulk of the molecular gas is forming into stars with a normal star formation efficiency (SFE) LIR/M(H2) ≈ 4.2 L☉/M☉, the same as that of giant molecular clouds in the Galactic disk. Additional supportive evidence is the extremely low fraction of the dense molecular gas in Arp 244, revealed by our detections of the HCN(1-0) emission, which traces the active star-forming gas at density 104 cm-3. Using the high-resolution BIMA + NRAO 12 m telescope, full-synthesis CO(1-0) images and our VLA continuum maps at 20 cm, we estimate the local SFE indicated by the ratio map of the radio continuum to CO(1-0) emission, down to kiloparsec scale. Remarkably, the local SFE stays roughly the same over the bulk of the molecular gas distribution. Only some localized regions show the highest radio-to-CO ratios that we have identified as the sites of the most intense starbursts with SFE 20 L☉/M☉. Here we have assumed that the 20 cm emission is a fairly good indicator of star formation down to kiloparsec scale in Arp 244 because of the well-known correlation between the FIR and the radio continuum emission. These starburst regions are confined exclusively to the dusty patches seen in the Hubble Space Telescope optical images near the CO and FIR peaks where the violent starbursts are presumably heavily obscured. Nevertheless, recent large-scale star formation is going on throughout the system (e.g., concentrations of numerous super-star clusters and MIR hotspots), yet the measured level is more suggestive of a moderate starburst (SFE 10 L☉/M☉) or a weak to normal star formation (SFE ~ 4 L☉/M☉), not necessarily occurring at the high concentrations of the molecular gas reservoir. The overall starburst from the bulk of the molecular gas is yet to be initiated as most of the gas further condenses into a kiloparsec scale in the final coalescence.