Spatially resolved star formation and quenching: insights from observations and simulations

Abstract

The cosmic star formation rate density is known to have declined by a factor of ~10 to present day after reaching its peak at z ~ 2. There were drastic differences in the star-forming conditions between z ~ 0 and z ~ 2, such that galaxies in the past were more star-forming and gas-rich. Given the complexity of the star formation histories of galaxies, it is not trivial to pinpoint the exact physical mechanisms responsible for the decline from z ~ 2 to z ~ 0. To do so, it is crucial to study the spatially resolved star formation activity of galaxies across a broad range of star-forming states - from those above to below the star-forming main sequence (SFMS). In this thesis, I use the Middle Ages Galaxy Properties with Integral Field Spectroscopy (MAGPI) survey to investigate how galaxies have fuelled, regulated and quenched their star formation activity at intermediate redshifts (z ~ 0.3).

In the first science chapter, I map radial trends in star formation for both the star-forming and passive populations in the MAGPI sample at z ~ 0.3, using a combination of the Halpha emission line and the 4000 Angstrom break as star formation rate indicators. I then compare these trends to those measured in the local Universe (z ~ 0) and at cosmic noon (z ~ 1 - 2), to capture how star formation has progressed within galaxies across ~10 Gyrs of lookback time. I capture evidence for potential evolution in radial trends for galaxies above the star-forming main sequence (SFMS), from uniformly (z ~ 1) to centrally (z ~ 0) enhanced star formation. Galaxies below the SFMS tend to quench inside-out at all redshift epochs probed.

In the second chapter, I use mock observations of galaxies from the EAGLE, Magneticum, and IllustrisTNG simulations to match MAGPI observational details including the average point spread function and pixel scale. Overall, there is no agreement in radial trends between MAGPI observations and the simulations, except for galaxies below the SFMS. All three simulations agree that central and satellite galaxies are predominantly subject to active galactic nuclei (AGN) feedback and environmental processes, respectively. Different degrees of SF suppression in the centres of central galaxies are captured across different simulations, which can be explained by differences in AGN feedback recipes.

In the final chapter, I contextualise the Halpha-based SFMS at z ~ 0.3 with other Halpha-based surveys in the literature, where I find no strong evidence for the evolution in either the slope or scatter of the Halpha-based SFMS out to z ~ 2. I also investigate the distribution of the Halpha-detected galaxies about the SFMS, particularly in the context of the role of galaxy interactions in determining a galaxy's offset from the SFMS. While galaxy interactions are common across the SFR - stellar mass plane, the fraction of galaxies in close (projected separations < 20 kpc) pairs is higher off the SFMS, than on the SFMS. In addition, 4 massive quiescent galaxies with nearby companions exhibit red optical colours with recent SF, along with Balmer absorption, suggesting that these galaxies may have been driven below the SFMS due to galaxy-galaxy interactions.

With the MAGPI survey, this thesis extends the science unravelled by large integral field spectroscopy (IFS) surveys of galaxies at z ~ 0 towards intermediate redshifts (z ~ 0.3) for the first time, for a well-representative sample of both the star-forming and passive populations. Moreover, this thesis complements these resolved observations with an apples-to-apples comparison to three distinct cosmological simulations, providing a unique testbed to understand the effects of different physical prescriptions on the enhancement and quenching of star formation within galaxies. Overall, this thesis illustrates the power and importance of resolved studies of galaxies, especially in bridging observations and simulations, to demystify the evolution of galaxies across cosmic time. Show more