Masters Thesis Defense by Elie Cueto

Title of the thesis: Advancing the modelling of star formation in the early Universe

Abstract: 

The James Webb Space Telescope (JWST) has uncovered an unexpectedly high abundance of ultraviolet (UV)-bright galaxies in the early Universe at z>10. Various explanations have been proposed, including bursty star formation, a top-heavy initial mass function (IMF) or a higher star formation efficiency. These different mechanisms have been explored in a number of different studies in the last few years, but to date we still lack simulations that explore the interplay of the underlying processes driving the luminosities of these UV-bright, early galaxies.
This work aims to bridge this gap by advancing our modelling of star formation in semi-analytic galaxy evolution models. We have built a model that uses the physical picture of stars forming in clouds, along with other key galaxy evolution processes to model bursty star formation in galaxies in the early Universe. We model star formation in molecular clouds, with stellar feedback mechanisms regulating star formation efficiency. We explore an evolving IMF based on the results of hydrodynamical simulations of star forming clouds which becomes more top-heavy in increasingly high-z, metal poor environments. With this we explore the interplay of bursty star formation, stellar IMF, and star formation efficiency, and their effects on the evolution of galaxies in the early Universe. We have explored different IMFs (a standard Salpeter and an evolving IMF) and star formation models (including and neglecting cloud-based star formation).
With our model we find that (i) stellar mass builds up more slowly towards lower redshifts with bursty compared to smooth star formation; (ii) at higher z the lower mass-to-light ratio of the more top-heavy evolving IMF results in less efficient star formation in individual clouds, but more efficient galaxy-wide star formation; (iii) increasing the burstiness of galaxy star formation does not significantly impact the UV luminosity functions (UVLFs) at z>12, but it suppresses the UVLFs at z<10; (iv) due to this suppression at lower redshifts, a higher star formation efficiency is required to reproduce the observed UVLFs at z<8, which also increases the abundance of bright galaxies at higher z; (v) the lower mass-to-light ratio of the evolving IMF compared to the Salpeter IMF results in increasingly brighter galaxies at higher z, which further works to increase the resulting UVLFs at higher redshifts, allowing us to simultaneously reproduce observations at z~5-7 and at z~11-13.
Our findings suggest that the combination of an evolving IMF and bursty star formation, along with the higher star formation efficiency in galaxies at higher z caused by our bursty star formation model, may be able to explain the high observed abundance of bright z>12 sources. In future work, we intend to implement our star formation model into a larger semi-analytical galaxy evolution framework, to explore how realistic dark matter assemblies affect our findings.

Censor: Thomas R. Greve (DTU)
Supervisors: Anne Hutter, Charlotte Mason

Online participation:

Link: https://ucph-ku.zoom.us/j/69453021311?pwd=2MUhxka9KMfuZsP7mxtYfG9ZSzkWii.1