General methodology: a multi-scale approach. Linking together a wide range of scales and object classes is at the very heart of ECOGAL. For the theory side we will achieve this by combining adaptive mesh refinement (AMR) with improved zooming techniques. Our first experience in the context of the turbulent ISM and on Galactic disk scales  is very encouraging. This is not just a technical challenge, more importantly, this approach requires a broad understanding of the relevant physics at each scale. This is far beyond the capacity of any single research group, and it is only possible with the combined expertise and knowledge of all ECOGAL team members. This synergy is also needed for the observations. Exploring the Galactic ecosystem requires us to combine measurements from many different instruments at many different wavelengths, from space-borne satellites and Earthbound telescopes, from single dishes and interferometers. Again this cannot be achieved by any single group alone and requires the concerted effort of all teams.

We address three primary scales: (1) the disk of the Milky Way and its population of molecular clouds, (2) molecular cloud structure, with embedded stellar clusters and cores, (3) protoplanetary disks and their population properties. In a joint effort, we will combine the results and data obtained for each scale and, by doing so, gain insight into the complete hierarchy of structures that make up our Galaxy. 

Disk of the Milky Way and its population of molecular clouds. In ECOGAL we will build up a large statistical database of the ISM conditions and star-formation in different environments of our Galaxy. Our team has full access to survey data at many different wavelengths that trace a wide range of different physical conditions in the ISM. We specifically mention the Herschel Infrared Galactic Plane Survey (HiGAL) and associated databases, including line surveys with ground based telescopes, The HI/OH/Recombination line survey of the Milky Way, THOR, and SDSS-V, a new panoptic spectroscopic all-sky survey at infrared and optical wavelength starting in 2021. Complementary to that, the team in Heidelberg will perform numerical simulations of the full disk of the Milky Way with the moving mesh code Arepo with time-dependent chemistry, sink particles to model star cluster formation, and different forms of stellar feedback. This technique also allows us to zoom to specific regions of interest with increased resolution. As additional test of our models linking galactic and molecular cloud scales, we will also simulate other type of galaxies (mergers, elliptical galaxies, high-redshift disks) and investigate their cloud population. We will try to reproduce the observed variations in molecular line ratios and overall star formation efficiency. We will post-process the simulation data to produce synthetic observations for the direct comparison with the available survey data. Our data analysis efforts and the comparison between observed and simulated maps will be the combined effort of all teams in ECOGAL. 

Molecular cloud structures:  proto-stellar clusters and molecular dense cores. The work on the structure of molecular clouds and the formation of star clusters will start with a detailed comparison between existing data from the Herschel satellite and numerical simulations. The numerical models will follow the build-up and early evolution of star clusters from initial conditions provided from the large-scale simulations. They are the responsibility of the team in Paris-Saclay and bridge the gap between molecular clouds and dense protostellar cores. A key aspect is the generation of synthetic observations, using the radiative transfer tools developed in Heidelberg, which allows us to perform the same analysis steps on model and observational data. At the beginning of the project we will extend the observational database with a high-resolution survey of 15 nearby high-mass clouds as part of the accepted ALMA-IMF large program and with a survey of the dust polarization structure in 100 cores, with the NOEMA array, constraining the role of magnetic field in the evolution and fragmentation of clumps. These data will be compared with molecular cloud simulations with dedicated initial conditions. We will also extend the ALMA-IMF approach to explore the full extent of the Galactic ecosystem, and will submit an ALMA large program (led by S. Molinari) to observe about one thousand molecular clouds with embedded star clusters. This will provide a detailed description of their morphological and kinematical parameters and help us to understand how star formation proceeds in different regions of the Galaxy.

Disk populations and histories. To infer a reliable population of protoplanetary disks, necessary to eventually understand how planets form, the close collaboration of all four teams is crucial. We will start with the analysis of existing protoplanetary disk surveys that are available to our teams from our ongoing programs or from the ALMA archive. We have direct access to data from Calypso, Solis, and ALMA FAUST large program and surveys of nearby clouds. We will compare with a well-designed set of numerical simulations that will use initial conditions both from observed and simulated cluster-forming clouds. These will make intense use of adaptive mesh refinement, which allows us to cover the scale range between cluster and accretion disk. Synthetic maps using the radiative transfer tools provided by Heidelberg will enable this comparison. We will adopt a wide range of initial and boundary condition and study to what extent the cluster environment does influence disk formation and evolution. In a second step, we will use ALMA to perform a large survey program to extend our local knowledge of disk population properties to sampling clusters in the inner Galaxy, in dense spiral arm regions and in the outer disk. This will provide a systematic assessment of the properties and temporal evolution of dust and gas in large disk populations as a function of environment and location in the Galaxy. We focus on three main aspects of the problem: physical parameters of dust and gas, and the competition between stellar winds and accretion. All these are crucial to understand disk evolution and their ability to give birth to planets. For the last step, we will employ ‘effective’ planet formation models and generate statistically well-constrained ensembles of the planet population associated with the observed and simulated disk populations. Putting all together will allow us learn how planet formation is affected by the environment, and ultimately, to answer the question of how common are the conditions that led to the formation of our own solar system.