de Vos, Kelly
(2025)
The Far-Reaching Impact of Cluster Environments on the Nuclear Activity and Star Formation of Galaxies.
PhD thesis, University of Nottingham.
Abstract
This thesis presents interlinked studies on radio and optical observational data, with the aim of understanding environmental effects on the star formation and nuclear activity of galaxies. These studies have been facilitated by the recent availability of large, deep radio surveys, enabling detailed investigations of galaxy properties across diverse environments. By combining these datasets with optical observations, this work explores how local and global environmental processes, such as ram pressure, galaxy density, and cluster-centric distance, influence the evolution of galaxies.
In Chapter 3, we investigate where, how, and why star formation quenching begins in the outskirts of galaxy clusters. Using the Low-Frequency Array (LOFAR) Two-metre Sky Survey, we analyse the de-projected radial distribution of star-forming galaxies (SFGs) out to 30R500, where R500 is the radius within which the average cluster density is 500 times the critical density of the Universe. We find that the SFG fraction begins to decline from the field fraction at 10R500, well beyond the cluster's virial radius. This decline is influenced by both large-scale (cluster-centric distance) and local (nearest-neighbour density) environments. Galaxies in high-density local environments show lower SFG fractions, but for high-mass galaxies -- and to a lesser extent, low-mass ones -- such environments can also shield SFGs from external quenching mechanisms in cluster outskirts. For galaxies in low-density local environments, quenching due to global environment dominates and acts independent of stellar mass. These results reveal a complex interplay between galaxy mass, local density, and global cluster-centric distance in regulating star formation.
In Chapter 4, we examine how the fraction of LOFAR-identified radio active galactic nuclei (AGN) varies with cluster-centric radius, analysing their projected and de-projected distributions out to 30R500. The AGN fraction shows distinct trends: a ~25% increase above the field fraction in the outskirts (~10R500), a ~20% suppression near ~0.5R500, and a sharp rise to over three times the field fraction in the cluster core. These regions reveal differences in host galaxy stellar mass and AGN radio luminosity. In the core, AGN preferentially reside in massive galaxies, while regions with higher AGN fractions generally host more luminous radio AGN. These findings highlight a dynamic relationship between environmental mechanisms and intrinsic galaxy properties in driving AGN activity.
In Chapter 5, we investigate the details of the interactions between radio galaxies and both the intracluster medium (ICM) and the intergalactic medium (IGM) by analysing 208 narrow-angle tail (NAT) radio sources detected by the LOFAR Two-metre Sky Survey. Within 7R500 of the cluster centre, NAT tails show a strongly anisotropic distribution, predominantly bending radially away, consistent with galaxies on inbound orbits. Closer to the cluster core (<0.5R500), we observe an excess of NATs with jets bent toward the centre, suggesting these sources fade after passing pericentre. For NATs with spectroscopic redshifts, this radial alignment persists out to 10R500, well beyond the virial radius. The presence of aligned NATs at such large distances implies significant deceleration of the inflowing intergalactic medium, sufficient to produce ram pressure capable of bending jets and potentially triggering radio emission.
In Chapter 6, we investigate the presence and effects of ram pressure on star formation outside galaxy clusters, through analysis of H-alpha emission in galaxies in the cluster Abell 1682 and its surrounding environment. Utilising data we obtained using the Wide Field Camera at the Isaac Newton Telescope, we identify and characterise 39 H-alpha-emitting sources across a field of view spanning ~7.4 x 7.4 Mpc. By comparing the spatial offsets between H-alpha and i-band emission, we detect a statistically significant preference for enhanced H-alpha-emission on the leading edges of galaxies, consistent with a scenario that suggests enhanced star-formation due to gas compression by ram pressure. Surprisingly, these effects are observed not only in the expected filamentary regions but throughout the cluster outskirts, as far as 7.7 Mpc from the cluster centre. These tentative findings challenge current models of gas density in cluster environments and suggest that the intracluster medium may extend further than previously thought, or that multiple filaments may contribute to the observed effects.
| Item Type: |
Thesis (University of Nottingham only)
(PhD)
|
| Supervisors: |
Hatch, Nina
Merrifield, Michael |
| Keywords: |
galaxies, stars, galaxy clusters, radio sources |
| Subjects: |
Q Science > QB Astronomy |
| Faculties/Schools: |
UK Campuses > Faculty of Science > School of Physics and Astronomy |
| Item ID: |
80988 |
| Depositing User: |
de Vos, Kelly
|
| Date Deposited: |
31 Jul 2025 04:40 |
| Last Modified: |
31 Jul 2025 04:40 |
| URI: |
https://eprints.nottingham.ac.uk/id/eprint/80988 |
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