PhD thesis defense by Katarzyna Magdalena Dutkowska

Title: Tracing star formation at high redshift using the Milky Way as a template


A key question in astrophysics is how most stars form, both in the nearby and the more distant Universe, near the peak of cosmic star formation. All stars form in molecular clouds and numerous surveys have provided detailed molecular inventories of such clouds within the Galaxy. Thanks to state-of- the-art facilities such as the Atacama Large Millimeter/submillimeter Array (ALMA), we are now routinely observing the distant Universe in molecular line emission. As we observe these molecules across the Universe, we start to fill the informational gap between high-z galaxies and the Milky Way.

How can we effectively compare these regimes and utilize our Galactic knowledge? We know that most stars form in giant molecular clouds and that these form clusters, where all stellar masses are present. Furthermore, relatively few high-mass stars easily outshine the entire low-mass population in a cluster. Moreover, the younger the protostar, the deeper it is embedded in gas and dust. Therefore, if we want to observe the entire forming population of stars, we need to use reliable tracers of active star formation that are common and bright enough to be easily observed and not obscured by dust extinction. One of the best tracers in our Galaxy also observed in the distant Universe is water: emission from this molecule lights up in the outflows driven by the youngest accreting protostars, and water emission thus serves as a tracer of forming stars. Furthermore, a linear empirical relation exists between the mass of the accreting star and the water intensity, making water emission a low-contrast tracer of active and current star formation. With this in mind, we can use what we know about the local star formation to constrain spatially unresolved star-formation processes in more distant galaxies and understand how molecular line emission can be used to quantitatively trace active star formation at different cosmic times.

The work presented in this thesis marks the first steps of creating an observationally based tool simulating molecular emission from star-forming regions in galaxies. The result of this work is the “galaxy-in-a-box” model, simulating water emission associated with the earliest and most active stages of clustered star formation. The initial results from running the galaxy-in-a- box model demonstrate that water emission is sensitive to the star-formation efficiency and the age of clusters. Further investigations have shown that a straightforward application of Galactic star-formation laws is not enough to match the observations, which can overestimate the amount of ongoing star formation while underestimating the expected emission.

To probe the origin of water emission further, water emission in shocked regions was studied, as this is where most of the Galactic water emission originates. This part included radiative transfer modeling of results from sophisticated shock models using the Paris-Durham shock code. The initial results show that water emission is highly dependent on the density of the medium, as well as the velocity of shock waves. The former can be especially important in the high-redshift regime, where the interstellar medium is denser. These results carve out a path for a novel use of molecular emission as a star formation tracer across cosmic times.

Supervisor: Lars Egstrøm Kristensen

PhD committee: Jes Jørgensen, Angela Adamo, Mario Tafalla