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Origins of the Molecular Cloud Structure

Periodic Reporting for period 3 - PROMISE (Origins of the Molecular Cloud Structure)

Période du rapport: 2019-03-01 au 2020-08-31

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 in 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 PROMISE project has 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 finding out how these giant gas clouds break up into smaller and smaller pieces, down to the units that give birth to individual stars, and in determining how gravity and turbulence affect the internal structure of gas clouds.

The most substantial goal of the PROMISE project is to derive a novel map picturing thousands of gas clouds in the centre plane of the Milky Way. The map will be the most detailed and largest map of such clouds so far; obtaining it has been made possible by novel methodology we have recently developed. The map and relevant data has now been completed and is being released to the community and used for new science projects (Kainulainen & Zhang in prep., Zhang & Kainulainen 20109, Astronomy & Astrophysics, 632, 68).

Related to the above work, we have also developed a new methodology to study how exactly the gas is moving in the interstellar gas clouds. The movement of gas can be studied with the help of the molecular line emission that originates from the clouds; the emission lines contain information on the velocity structure of the gas. Our novel technique takes advantage of machine-learning to identify detailed velocity patterns from such data. This is done automatically, so that large amounts of data can be analyzed efficiently, without human intervention. This enables analyses of huge amounts of data—such a tool is highly useful not only for our project, but also for astronomers working on other questions with similar data. We have used this methodology to create a new view of how molecular clouds in the Milky Way are distributed and how they move about (Riener et al. 2019, Astronomy & Astrophysics, 633, 14; Riener et al. 2020, Astronomy & Astrophysics, 640, 72; Henshaw et al. 2020, Nature Astronomy, arXiv:2007.01877).

The PROMISE project has made strong progress in studying how the internal structure of individual gas clouds is organized, i.e. how the gas clouds fragment to smaller and smaller units of gas. We have been extremely successful in the competition to use the most advanced technologies for this purpose, namely the Atacama Large Millimeter/submillimeter Array (ALMA). The PROMISE group is currently among the most active groups in the world studying this topic. We are in the process of finalising several papers reporting results of our ALMA programs.
The PROMISE project has so far been focused on deriving and obtaining observational data and analysing them. The main science results and the papers relevant for them are in preparation and expected to be finalised during the remainder of the project and in the near future following it. Specifically, the remainder of the project concentrates in the following items.

The PROMISE mapping of giant gas clouds in the Milky Way will be made available to the community. We will also finalise our first studies of the internal structure of a large number—thousands—of gas clouds, especially from the perspective of cloud fragmentation. This will be a major outcome of the PROMISE mapping: we will determine how exactly the cloud structures are built up, from large to small scales. We expect that 2-3 papers will be published reporting these results. The exploitation of the data also continues beyond the action.

We also expect the group's cloud fragmentation studies with ALMA to culminate in 2-3 papers and to continue beyond the action. In particular, these papers address a new scenario for how the internal structure of massive filamentary clouds evolves through fragmentation towards star formation.

The information resulting from the on-going analyses is vital for developing accurate models for how rapidly and how efficiently stars form in the interstellar gas clouds. Bringing our observational results in contact with the existing models of interstellar medium physics and star formation is the key goal of the second half of the project. We expect that our results will immediately affect those models and help in developing new ones. Such improvements would represent a major step forward in understanding the star formation process and they would mark a successful end of the PROMISE project.