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Origins of the Molecular Cloud Structure

Periodic Reporting for period 4 - PROMISE (Origins of the Molecular Cloud Structure)

Reporting period: 2020-09-01 to 2021-01-31

Our cosmic home—the Milky Way galaxy—continuously gives birth to new stars: on average, a couple of stars such as our own Sun are formed every year. These new stars form from the interstellar gas that is an indivisible component of the Milky Way. If a large-enough amount of this gas is located in a small-enough space, the gravity of the gas causes a gravitational collapse, followed by a formation of a new star. Exactly how does this process proceed is not yet well known. Specifically, it remains unknown how the different physical processes—such as gravity and turbulence—set the rate and efficiency with which the new stars form. As a result, our comprehensive understanding of how galaxies like Milky Way build up their stellar content remains lacking.

Overall, the question of “How do the stars form?” is linked to the fundamental question of our own origins in the Universe. Exploring and understanding such a topic has its main societal value in enabling and promoting critical, fact-based thinking and deep understanding of our own natural environment. On a more practical level, the study of the topic requires methods and technologies highly relevant in the modern society, for example, in the fields of data science, computer science, and engineering. Thus, the question “How do the stars form?” interfaces with the society by being a motivating question by its own right, by providing a route to develop skills that are in high demand in today’s society, and by providing a unique, curiosity-driven perspective to complex problem-solving.

The PROMISE project addresses fundamental open questions related to how the star formation process proceeds in the interstellar gas. The foremost goal of the project is to accurately map how the gas is distributed in a large number of star-forming gas clouds in the Milky Way. The exact distribution of gas determines its energetics, and from therein, where exactly the new stars can form. The PROMISE project focuses on exploiting novel and innovative observational techniques to map the gas distribution in thousands of gas clouds in the Milky Way in a detail that has not been available before. This enables PROMISE to make a step forward in understanding how the interstellar gas is distributed and how it is transformed into new stars in the Milky Way. The project was concluded successfully, resulting in the production of the ‘PROMISE galactic plane survey’ and several scientific results advancing our understanding of how new stars form in the Milky Way.
The PROMISE project made significant progress in studying the formation of new stars in three fronts: in deriving detailed maps of giant interstellar gas clouds in the Milky Way, in studying how such clouds are located and moving about in the Milky Way, and in finding out how the giant gas clouds break up into smaller and smaller pieces, down to the units that give birth to individual stars.

The main result of the PROMISE project is a detailed map of a large section of the Milky Way, portraying thousands of molecular clouds in extremely fine detail. The map is the most accurate and largest map of such clouds so far; obtaining it was made possible by the novel methodology we developed during the project. The map enables a wide range of studies in the future addressing how exactly the gas is distributed in the clouds and how does it evolve towards star formation.

Another main result of PROMISE is a new picture of how molecular clouds are distributed across the Milky Way and how do their internal movements look like. The distribution of gas and its movement can be studied with the help of the molecular line emission that originates from the clouds. We developed a new, machine-learning based methodology that enabled us to automatically analyze a large amount of data, without human intervention. We then used this method to determine where the gas emission we observe is coming from and used that information to build a picture of what the galaxy would look like when viewed from outside. Further, we described how this gas is moving about, finding a surprising result that the gas in the Milky Way disk seems to be “undulating” everywhere. The origin of these movements is still a mystery, but it is likely that understanding them in the future is important to understand how the gas collapses to forms new stars.

The PROMISE project also made important progress in studying how the internal structure of individual gas clouds is organized. Specifically, we studied how the giant gas clouds fragment to smaller and smaller units of gas, eventually forming stars. The PROMISE team was extremely successful in the competition to use the most advanced technologies for this purpose, namely the Atacama Large Millimeter/submillimeter Array (ALMA). These data helped us to pioneer how the gas fragmentation process happens. One main result we obtained was that the fragmentation process is more complex than previously thought of and that we do not quite understand why the clouds fragment like they do.

The results of the PROMISE project were disseminated in the scientific community via international conferences and collaborations. The members of the PROMISE team participated several key conferences in the field per year, disseminating the results and networking with the community. The project also organized mini-workshops and invited participants to join the team’s work. Overall, the project members published around 30 scientific publications either directly related to the PROMISE project or in its periphery. The exploitation of the project data also continues beyond the project—the scientific exploitation of the project’s products is expected to continue for years.
Seeds of stars falling into young star clusters revealed by ALMA
Gas spirals at the heart of a young star cluster
Details from the PROMISE galactic plane mapping survey