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

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 1.5×10 10 M ⊙), based on our fully sampled CO(1-0) observations at the NRAO 12m telescope. Currently, much of the understanding and modeling of the star formation in Arp 244 has been derived from a much lower molecular gas mass. It is imperative to reconsider all 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) L IR /M (H 2) ≈ 4.2L ⊙ /M ⊙ , 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 > ∼ 10 4 cm −3. Using the high-resolution BIMA plus the NRAO 12m telescope, full-synthesis CO(1-0) images, and our VLA continuum maps at 20cm, we estimate the local SFE indicated by the ratio map of the radio continuum to CO(1-0) emission, down to kpc scale. Remarkably, the local SFE stays roughly 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 20cm emission is a fairly good indicator of star formation down to kpc scale in Arp 244 because of the well-known correlation between the FIR and the radio continuum emission. These starburst regions are confined exclusively in the dusty patches seen in the HST optical images near the CO and FIR peaks where presumably the violent starbursts are 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 …