We used the Submillimeter Array (SMA) to image 860 µm continuum and CO(3-2) line emission in the ultraluminous merging galaxy Arp 220, achieving a resolution of 0. ′′ 23 (80 pc) for the continuum and 0. ′′ 33 (120 pc) for the line. The CO emission peaks around the two merger nuclei with a velocity signature of gas rotation around each nucleus, and is also detected in a kpc-size disk encompassing the binary nucleus. The dust continuum, in contrast, is mostly from the two nuclei. The beam-averaged brightness temperature of both line and continuum emission exceeds 50 K at and around the nuclei, revealing the presence of warm molecular gas and dust. The dust emission morphologically agrees with the distribution of radio supernova features in the east nucleus, as expected when a starburst heats the nucleus. In the brighter west nucleus, however, the submillimeter dust emission is more compact than the supernova distribution. The 860 µm core, after deconvolution, has a size of 50-80 pc, consistent with recent 1.3 mm observations, and a peak brightness temperature of (0.9-1.6)×10 2 K. Its bolometric luminosity is at least 2 × 10 11 L ⊙ and could be ∼10 12 L ⊙ depending on source structure and 860 µm opacity, which we estimate to be of the order of τ 860 ∼ 1 (i.e., N H2 ∼ 10 25 cm −2 ). The starbursting west nuclear disk must have in its center a dust enshrouded AGN or a very young starburst equivalent to hundreds of super star clusters. Further spatial mapping of bolometric luminosity through submillimeter imaging is a promising way to identify the heavily obscured heating sources in Arp 220 and other luminous infrared galaxies.

Context. Deuterated ions, especially H 2 D+ and N 2 D + , are abundant in cold (∼10 K), dense (∼10 5 cm −3 ) regions, in which CO is frozen out onto dust grains. In such environments, the N 2 D + /N 2 H + ratio can exceed the elemental abundance ratio of D/H by a factor of 10 4 . Aims. We use deuterium fractionation to investigate the evolutionary state of Class 0 protostars. In particular, we expect the N 2 D + /N 2 H + ratio to decrease as temperature (a sign of the evolution of the protostar) increases. Methods. We observed N 2 H + 1−0, N 2 D + 1−0, 2−1 and 3−2, C 18 O 1−0 and HCO + 3−2 in a sample of 20 Class 0 and borderline Class 0/I protostars. We determined the deuteration fraction and searched for correlations between the N 2 D + /N 2 H + ratio and wellestablished evolutionary tracers, such as T Dust and the CO depletion factor. In addition, we compared the observational result with a chemical model. Results. In our protostellar sample, the N 2 H + 1−0 optical depths are significantly lower than those found in prestellar cores, but the N 2 H + column densities are comparable, which can be explained by the higher temperature and larger line width in protostellar cores. The deuterium fractionation of N 2 H + in protostellar cores is also similar to that in prestellar cores. We found a clear correlation between the N 2 D + /N 2 H + ratio and evolutionary tracers. As expected, the coolest, i.e. the youngest, objects show the largest deuterium fractionation. Furthermore, we find that sources with a high N 2 D + /N 2 H + ratio show clear indications of infall (e.g. δv < 0). With decreasing deuterium fraction, the infall signature disappears and δv tends to be positive for the most evolved objects. The deuterium fractionation of other molecules deviates clearly from that of N 2 H + . The DCO + /HCO + ratio stays low at all evolutionary stages, whereas the NH 2 D/NH 3 ratio is >0.15 even in the most evolved objects. Conclusions. The N 2 D + /N 2 H + ratio is known to trace the evolution of prestellar cores. We show that this ratio can be used to trace core evolution even after star formation. Protostars with an N 2 D + /N 2 H + ratio above 0.15 are in a stage shortly after the beginning of collapse. Later on, deuterium fractionation decreases until it reaches a value of ∼0.03 at the Class 0/I borderline.

We have observed the nucleus of the nearby luminous infrared galaxy NGC 4418 with subarcsec resolution at 860 and 450 µm for the first time to characterize its hidden power source. A ∼20 pc (0. ′′ 1) hot dusty core was found inside a 100 pc scale concentration of molecular gas at the galactic center. The 860 µm continuum core has a deconvolved (peak) brightness temperature of 120-210 K. The CO(3-2) peak brightness temperature there is as high as 90 K at 50 pc resolution. The core has a bolometric luminosity of about 10 11 L ⊙ , which accounts for most of the galaxy luminosity. It is Compton thick (N H 10 25 cm −2 ) and has a high luminosity-to-mass ratio ∼500 L ⊙ M ⊙ −1 as well as a high luminosity surface density 10 8.5±0.5 L ⊙ pc −2 . These parameters are consistent with an AGN to be the main luminosity source (with an Eddington ratio about 0.3) while they can be also due to a young starburst near its maximum L/M . We also found an optical color (reddening) feature that we attribute to an outflow cone emanating from the nucleus. The hidden hot nucleus thus shows evidence of both an inflow, previously seen with absorption lines, and the new outflow reported here in a different direction. The nucleus must be rapidly evolving with these gas flows.