Context. Thanks to its excellent 5100 m high site in Chajnantor, the Atacama Pathfinder Experiment (APEX) systematically explores the southern sky at submillimeter wavelengths, in both continuum and spectral line emission. Studying continuum emission from interstellar dust is essential to locating the highest density regions in the interstellar medium, and deriving their masses, column densities, density structures, and large-scale morphologies. In particular, the early stages of (massive) star formation remain poorly understood, mainly because only small samples of high-mass proto-stellar or young stellar objects have been studied in detail so far. Aims. Our goal is to produce a large-scale, systematic database of massive pre-and proto-stellar clumps in the Galaxy, to understand how and under what conditions star formation takes place. Only a systematic survey of the Galactic Plane can provide the statistical basis for unbiased studies. A well characterized sample of Galactic star-forming sites will deliver an evolutionary sequence and a mass function of high-mass, star-forming clumps. This systematic survey at submillimeter wavelengths also represents a preparatory work for Herschel and ALMA. Methods. The APEX telescope is ideally located to observe the inner Milky Way. The Large APEX Bolometer Camera (LABOCA) is a 295-element bolometer array observing at 870 μm, with a beam size of 19. 2. Taking advantage of its large field of view (11. 4) and excellent sensitivity, we started an unbiased survey of the entire Galactic Plane accessible to APEX, with a typical noise level of 50−70 mJy/beam: the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL). Results. As a first step, we covered ∼95 deg 2 of the Galactic Plane. These data reveal ∼6000 compact sources brighter than 0.25 Jy, or 63 sources per square degree, as well as extended structures, many of them filamentary. About two thirds of the compact sources have no bright infrared counterpart, and some of them are likely to correspond to the precursors of (high-mass) proto-stars or protoclusters. Other compact sources harbor hot cores, compact H ii regions, or young embedded clusters, thus tracing more evolved stages after massive stars have formed. Assuming a typical distance of 5 kpc, most sources are clumps smaller than 1 pc with masses from a few 10 to a few 100 M . In this first introductory paper, we show preliminary results from these ongoing observations, and discuss the mid-and long-term perspectives of the survey.

We report observations of 47 candidate protostars in two optically thick lines (H 2 CO 2 12 − 1 11 and CS 2 − 1) and one optically thin line (N 2 H + 1 − 0) using the IRAM 30-m, SEST 15-m, and Haystack 37-m radio telescopes. The sources were selected from the redness of their spectra (T bol < 200 K) and their near distance (d < 400 pc). Most of the sources have asymmetric optically thick lines. The observed distribution of velocity differences, δV = (V thick − V thin )/∆V thin , is skewed toward negative (blue-shifted) velocities for both the H 2 CO and CS samples. This excess is much more significant for Class 0 than for Class I sources, suggesting that we detect infall motions toward Class 0 and not toward Class I sources. This indicates a difference in the physical conditions in the circumstellar envelopes around Class I and Class 0 sources, but does not rule out the presence of infall onto Class I sources by e.g. lower opacity gas. Bipolar outflows alone, or rotation alone, cannot reproduce these statistics if the sample of sources has randomly oriented symmetry axes. We identify 15 spectroscopic infall candidates, of which 6 are new. Most of these infall candidates have primarily turbulent rather than thermal motions, and are associated with clusters rather than being isolated.

Using high-resolution SPH numerical simulations, we investigate the effects of gas on the in-spiral and merger of a massive black hole binary. This study is motivated by the very massive nuclear gas disks observed in the central regions of merging galaxies. Here we present results that expand on the treatment in a previous work by studying models in which the gas is in a disk. We run a variety of models, ranging from simulations with a relatively smooth gas disk to cases in which the gas has a more clumpy spatial distribution. We also vary the inclination angle between the plane of the binary and the plane of the disk, and the mass ratio between the MBHs and the gaseous disk. We find that, as in our previous work, in the early evolution of the system the binary separation diminishes mainly due to dynamical friction exerted by the background gas, and in the later stages the gaseous medium responds by forming an ellipsoidal density enhancement whose axis lags behind the binary axis. This offset produces a gravitational torque on the binary that causes continuing loss of angular momentum and is able to reduce the separation to distances at which gravitational radiation is efficient. The main difference is that between these two regimes we now find a new transition regime that was not apparent in our previous paper, in which the evolution is temporarily slowed down when neither of these mechanisms is fully effective. In the variety of simulations that we perform, we find that the coalescence timescale for the MBH binary varies between 5 ; 10 6 and 2:5 ; 10 7 yr for typical ULIRGs. For MBHs that satisfy the observed ''m-c '' relation, our simulations suggest that in a merger of galaxies that have at least 1% of their total mass in gas, the MBHs will coalesce soon after the galaxies merge.

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Using high-resolution SPH numerical simulations, we investigate the effects of gas on the inspiral and merger of a massive black hole binary. This study is motivated by both observational and theoretical work that indicate the presence of large amounts of gas in the central regions of merging galaxies. N-body simulations have shown that the coalescence of a massive black hole binary eventually stalls in a stellar background. However, our simulations suggest that the massive black hole binary will finally merge if it is embedded in a gaseous background. Here we present results in which the gas is assumed to be initially spherical with a relatively smooth distribution. In the early evolution of the binary, the separation decreases due to the gravitational drag exerted by the background gas. In the later stages, when the binary dominates the gravitational potential in its vicinity, the medium responds by forming an ellipsoidal density enhancement whose axis lags behind the binary axis, and this offset produces a torque on the binary that causes continuing loss of angular momentum and is able to reduce the binary separation to distances where gravitational radiation is efficient. Assuming typical parameters from observations of Ultra Luminous Infrared Galaxies, we predict that a black hole binary will merge within 10 7 yrs; therefore these results imply that in a merger of gas-rich galaxies, any massive central black holes will coalesce soon after the galaxies merge. Our work thus supports scenarios of massive black -2hole evolution and growth where hierarchical merging plays an important role. The final coalescence of the black holes leads to gravitational radiation emission that would be detectable out to high redshift by LISA. We show that similar physical effects, which we simulate with higher resolution than previous work, can also be important for the formation of close binary stars.