Isabella Chi Gieseler Cortzen MSc defense

Title: Examining the existence of two distinct modes of star formation: a study of the gas reservoirs and dust emission in star forming galaxies across cosmic time

Supervisors: Georgios Magdis and Sune Toft, DARK
Censor: Thomas Greve, University College London

Abstract: Observations indicate that the majority of star-forming galaxies (SFGs) across cosmic time follow a remarkably tight relation between the star formation rate (SFR) and stellar mass (Mstar) forming a "main-sequence" (MS) of galaxies. Outliers located above the MS, with higher SFR for a given Mstar are classified as starbursts (SBs) and while rare, are present at all redshifts. In the local universe these rare systems are usually identified as ULIRGs (Ultra Luminous Infrared Galaxies). In these systems, star formation is triggered by major mergers and is characterized by an enhanced star formation efficiency (SFE), defined as the ratio of the current start formation rate over the molecular gas mass (Mgas) of a galaxy. On the other hand, local galaxies populating the MS are typical spiral galaxies, which are more than 10 times less efficient in converting gas into stars compared to the local, merger-driven, ULIRGs. However, the increasing normalization in the SFR-Mstar plane with increasing z, makes distant galaxies with high SFRs (similar to that of local ULIRGs), part of the MS at earlier epochs. Recent studies have provided evidence, that these high-z MS galaxies, despite their high SFRs, have SFEs similar to that of local disks rather than that of local star-bursting ULIRGs, suggesting a minor role of mergers in the stellar built up of galaxies through cosmic time, and a bimodal star formation law (that connects SFR and Mgas) between MS and SB galaxies at all redshifts.

However, in these studies the molecular reservoir is indirectly traced by CO emission lines, and thus, the different SFEs between SBs and MS galaxies at all redshifts could be driven by the highly uncertain conversion factor, used to convert the observed CO luminosity to Mgas.

In this thesis we examine if there is evidence for the existence of two proposed modes of SF, when considering only direct observables, i.e. the LIR, which is a tracer of SFR, and LCO. For this purpose, we collected and homogenized CO line observations for 801 galaxies from the literature, covering a broad range of redshifts. The CO data were then coupled with multi-wavelength photometry available from infrared surveys carried out by various infrared space telescopes, that was used to model the spectral energy distribution of the galaxies and derive their physical properties such as LIR and dust mass (Mdust). With this data we examined possible variations of the LIR/LCO ratio, as well as of the physical properties, between SBs and MS galaxies. Our analysis provide hints, that the majority of MS galaxies indeed follow a unique relation in the LCO - LIR parameter space, whereas the SB galaxies systematically lie above it, supporting the scenario of two distinct modes of SF between SB and MS galaxies across cosmic time.

Link to thesis –>>here