Taylor, Elizabeth
(2025)
Not Dead Yet: High Redshift Recently Quenched Galaxies As Probes Of Galaxy Evolution.
PhD thesis, University of Nottingham.
Abstract
In this thesis, we explore the roles that galaxy environment, stellar mass and outflows play in the quenching of star formation, by studying recently quenched galaxies at cosmic noon and beyond (0.5 < z < 4.4). We utilise photometric data from the UKIDSS Ultra Deep Survey (UDS), the deepest NIR survey over ∼1 deg2, which has sufficient volume to enable samples to be split by stellar mass, redshift and environment. Additionally, we use spectroscopic data for galaxies within the UDS field, from deep observations at ESO VLT (UDSz, VANDELS) and JWST NIRSpec (EXCELS), to study galactic scale winds in the interstellar medium.
We conduct the first study of how the relative quenching probability of galaxies depends on environment over the redshift range 0.5 < z < 3. By constructing the stellar mass functions for quiescent and post-starburst (PSB) galaxies in high-, medium-, and lowdensity environments, we find an excess of quenched galaxies in dense environments out to at least z ∼ 2. Using the growth rate in the number of quenched galaxies, combined with the star-forming galaxy mass function, we calculate the probability that a given star-forming galaxy is quenched per unit time. We find a significantly higher quenching rate in dense environments (at a given stellar mass) at all redshifts. Massive galaxies (M∗ > 1010.7 M⊙) are on average 1.7 ± 0.2 times more likely to quench per Gyr in the densest third of environments compared to the sparsest third. Finally, we compare the quiescent galaxy growth rate to the rate at which galaxies pass through a PSB phase, finding that the PSB route can explain ∼ half of the growth in the quiescent population at high stellar mass, and potentially all of the growth at lower stellar masses.
We then use optical spectra of galaxies selected from the UDS at z > 1 to explore the transition in outflow properties along a quenching time sequence. To do this, we perform a stacking analysis of Mg II absorption profiles, investigating outflow velocities as a function of time since the last major burst of star formation (tburst). We find evidence for high-velocity outflows (vout ∼ 1000 - 1500 kms−1) in a star-forming progenitor population and for recently quenched galaxies with tburst < 1 Gyr. The oldest galaxies (tburst > 1 Gyr) show no evidence for significant outflows. Our samples show no signs of AGN in optical observations, however the presence of significant outflows in the older quenched galaxies (tburst > 0.6 Gyr) is difficult to explain with starburst activity alone, and may indicate energy input from episodic AGN activity as the starburst fades.
Finally, we complement our ground-based stacking analysis using the JWST NIRSpec EXCELS survey. We select a sample of 9 post-starburst galaxies in the redshift range 1.8 < z < 4.4, and utilise the high resolution spectra to establish, for the first time, if outflows are prevalent in post-starburst galaxies at earlier epochs than cosmic noon. The large wavelength coverage and unique sensitivity of JWST allows us to compare outflow properties for both the Mg II and Na D absorption profiles. We first perform a stacking analysis of our sample, finding ∼ 1600 kms−1 and ∼ 290 kms−1 outflows from the magnesium and sodium profiles, respectively. We use these outflow velocities to estimate lower limits on the mass outflow rates, of Mout, MgII ∼ 1 M⊙yr− and Mout, NaD ∼ 3 M⊙yr−1, higher than the average SFR of our sample. We then investigate galactic scale winds for individual objects at z ∼ 3, finding a wide range of outflow velocities, with some galaxies also showing signs of inflowing gas. These results suggest that outflows may play a key role in the quenching of galaxies at these epochs, by removing the gas needed to fuel star formation.
| Item Type: |
Thesis (University of Nottingham only)
(PhD)
|
| Supervisors: |
Almaini, Omar
Merrifield, Michael
Maltby, David |
| Keywords: |
galaxies, star formation, stellar mass, quenching |
| Subjects: |
Q Science > QB Astronomy |
| Faculties/Schools: |
UK Campuses > Faculty of Science > School of Physics and Astronomy |
| Item ID: |
80292 |
| Depositing User: |
Taylor, Elizabeth
|
| Date Deposited: |
31 Jul 2025 04:40 |
| Last Modified: |
31 Jul 2025 04:40 |
| URI: |
https://eprints.nottingham.ac.uk/id/eprint/80292 |
Actions (Archive Staff Only)
 |
Edit View |
Tools
Tools
