ALMAGAL I. The ALMA evolutionary study of high mass protocluster formation in the Galaxy. Presentation of the survey and early results

Context. A large fraction of stars form in clusters containing high-mass stars, which affect the local and galaxy-wide environment. Aims. Fundamental questions about the physics responsible for fragmenting molecular parsec-scale clumps into few thousands of au cores are still open, that only a statistically significant investigation with ALMA can address: what are the dominant agents at work in determining the cores’ demographics, mass and spatial distribution as a function of the physical properties of the hosting clumps, their evolutionary stage, and the different Galactic environments in which they reside? To what extent is fragmentation driven by clumps dynamics or mass transport in filaments? Methods. With the ALMAGAL project we observed 1.38 mm continuum and lines toward more than 1000 dense clumps in our Galaxy with M ≥ 500 M⊙, Σ ≥ 0.1 g cm−2 and d ≤ 7.5 kpc. Two different combinations of ACA and 12-m array setups were used to deliver a minimum resolution of ∼1000 au over the entire sample distance range. The sample covers all evolutionary stages from infrared dark clouds (IRDCs) to Hii regions from the Galactic Bar’s tip to the outskirts of the Galaxy. With a continuum sensitivity of 0.1 mJy ALMAGAL enables a complete study of the clump-to-core fragmentation process down to M ∼ 0.3 M⊙ Galaxy-wide. The spectral setup includes several molecular lines to trace the multiscale physics and dynamics of gas, notably CH3CN, H2CO, SiO, CH3OH, DCN, HC3N, SO, among others. Results. We present an initial overview of the observations and the early science product and results produced in the ALMAGAL Consortium, with a first characterization of the morphological properties of the continuum emission detected above 5σ in our fields. We use perimeter − vs − area and Convex Hull − vs − area metrics to classify the different morphologies. We find that more extended and morphologically complex (significantly departing from roundish or generally convex) shapes are found toward clumps that are relatively more evolved and with higher surface densities. Conclusions. ALMAGAL will be a game changer for a number of specific issues in star formation: clump-to-core fragmentation processes, demographics of cores, core and clump gas chemistry and dynamics, infall/outflow dynamics, disks detection, only to mention those covered by a first generation of papers that will closely follow the present publication.