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Characterizing properties of the interstellar medium to better understand how stars form in galaxies

Periodic Reporting for period 1 - eGALISM (Characterizing properties of the interstellar medium to better understand how stars form in galaxies)

Reporting period: 2017-02-01 to 2019-01-31

The universe hosts a zoo of galaxies with different sizes and shapes. Our Galaxy, the Milky Way, is a rather large and massive spiral galaxy, with 250 billion stars and about one new star forming every year. At early epochs of the universe, galaxies were smaller but forming stars at higher rates. How actively stars are forming in galaxies is an important aspect of galaxy evolution as it determines the chemical make-up and the dynamical state of galaxies. Understanding the mechanisms that regulate the star-formation activity of galaxies and how those mechanisms vary with the evolutionary stage of a galaxy are active areas of research and necessary steps to address fundamental questions such as: When and where do stars form in galaxies? What is fueling star formation? How does the star-formation activity shape galaxies through cosmic times?

There are multiple size scales in the process of star formation. On the largest, galaxy-wide scales, the material in galaxies - the interstellar medium (99 percent gas and 1 percent dust) - needs to cool. It then contracts and fragments into clouds down to smaller scales until the collapse of clouds on the smallest scales. While the small-scale processes are best studied in our own Milky Way, the current main challenge to understand star formation in external galaxies is to quantify the amount, composition, and state of the material on the large scales.

To this end, the goal of my project was to better characterize the properties of the interstellar medium of galaxies that is linked to the formation of new stars. My project relied on an innovative approach, combining the analysis of a large set of new multi-wavelength observations from space and ground-based telescopes and building state-of-the-art object-specific spectral synthesis models. With this, I have constrained, for the first time, the cooling, physical conditions, topology, masses, and star-formation efficiencies in a range of galaxies, which are key parameters that link the interstellar medium to the star-formation activity in galaxies.
The work performed during the project was divided into three key parts.

First, I investigated the properties of the material in galaxies that is coldest and densest. That material was expected to relate in a simple, linear fashion to the formation of new stars. With the EMPIRE collaboration, we have obtained exquisite maps the of dense gas emission in a sample of nearby spiral galaxies. I have lead and participated in studies showing that the relation between dense gas and star formation is not as straightforward as expected. We found variations in the gas opacity, abundance, dense gas fraction, star-formation efficiency as a function of location within a galaxy and as a function of spatial scales. This highlights the need for a full characterization of the interstellar medium (multiple parameters and not just density) in galaxies.

Second, I investigated the properties of the material around stellar clusters in a sample of 30 nearby dwarf galaxies. These galaxies are chemically young, meaning that they have not been polluted significantly by elements produced by the previous generation of stars. For this, I have built self-consistent models of the main gas phases in those galaxies (see image attached) and found systematic differences in the physical conditions and topology as a function of the chemical enrichment and star-formation activity of galaxies. This provides a very useful method to interpret multi-phase observations of galaxies and has potential implications for the evolution of the first galaxies of the universe that are also chemically young.

Third, I investigated the cooling of the interstellar medium in a range of galaxies, from local dwarf galaxies and spirals galaxies to typical galaxies at redshifts 1-2. I have collaborated with researchers at the host institute on several studies that analyzed new, sensitive observations of infrared and submillimeter emission lines and demonstrated their utility as tracers of the star-formation rate and of the molecular gas reservoir. This is very useful for faint galaxies where acquiring multi-wavelength observations is challenging and this can help guide future observing efforts.

All observations and models related to these studies were made publicly available through publications and dedicated data repositories. Overall, the work carried out during my project resulted in 14 publications, including 2 as the lead author and 4 as second author that will soon be submitted. I have presented my results in 8 international conferences and created several websites (personal homepage, EU project page, institute's group page) for dissemination.
My project relied on both state-of-the-art models and observations, in particular from telescopes with significant European investment. It has established a method to interpret the observations of the interstellar medium in unresolved galaxies which will both lead the way for future studies on the topic and make predictions for future observing campaigns and space missions. It has brought new knowledge on the physics of nearby galaxies, a topic that is also linked to the research fields of galaxy evolution and early universe which are of large interest to the public.
Illustration of the multi-component model of the galaxy Haro3