The relationship between galaxy environment and the quenching of star formation

Socolovsky, Miguel (2019) The relationship between galaxy environment and the quenching of star formation. PhD thesis, University of Nottingham.

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In this thesis, we explore the impact of environment on the star formation properties of galaxies across 6.5 Gyrs of cosmic time, corresponding to the redshift range 0.5<z<3.0. In order to accomplish this task, we study the dependence of various galaxy properties, such as the stellar mass function, galaxy structure, star formation rate and passive fraction, as a function of environmental density and galaxy type. The work presented here is entirely based on the deep photometric data of the 8th and 11th data releases (DR8 and DR11 respectively) of the UKIDSS Ultra Deep Survey (UDS). The galaxy classification we use is based on a Principal Component Analysis (PCA) technique which allows us to identify star-forming and passive galaxies, and also post-starburst galaxies (PSBs), using only broad-band photometry. Both the UDS catalogues and the PCA technique are described in Chapter 2 of this thesis.

We characterise galaxy environment using two different methods. The first method is used at low redshift (z<1.0) and is based of a friends-of-friends algorithm, which allows us to identify overdensities (galaxy clusters and groups) in the UDS field. This method was thoroughly optimised to run on the UDS data, and the output tested using the extensive spectroscopic redshifts that are available in the UDS field, and by comparing to previous overlapping cluster studies. The method also generates a field sample with the same redshift distribution as the cluster sample, allowing for the comparison between these two environments.

A second method, based on fixed apertures, was also implemented in order to study the environmental trends in a more self-consistent way across the broad redshift range 0.5<z<3.0.

Our first study focuses on the redshift range 0.5<z<1.0 using the UDS DR8 data and the cluster finder described above.

The study of the stellar mass function reveals clear differences between the cluster and field environments, with a strong excess of low-mass PSB galaxies in clusters compared to the field. Cluster environments also show a corresponding deficit of young, low-mass star-forming galaxies, which also show a sharp radial decline towards the cluster centres. By comparing mass functions and radial distributions, we conclude that young star-forming galaxies are rapidly quenched as they enter overdense environments, becoming PSBs before joining the red sequence. Our results also point to the existence of two environmental quenching pathways operating in galaxy clusters, on different timescales. Fast quenching acts on galaxies with high specific star formation rates (SSFRs), operating on timescales shorter than the cluster dynamical time (<1 Gyr). In contrast, slow quenching affects galaxies with moderate SSFRs, regardless of their stellar mass, and acts on longer timescales (>1 Gyr). Of the cluster galaxies in the stellar mass range 9.0<log(M/Msun)<10.5 that were quenched during this epoch, we find that 73% were transformed through fast quenching, while the remaining 27% followed the slow quenching route.

We extend our analysis through the usage of effective radii and Sersic indices as tracers for galaxy structure, determined using the deep K-band imaging from DR11. We find that the high-SSFR galaxies that survive into the cluster environment have, on average, larger effective radii than those in the field. We suggest that this trend is driven by the most compact star-forming galaxies being preferentially quenched in dense environments. We also find that PSBs in clusters have stellar masses and effective radii that are similar to the missing compact star-forming population, suggesting that these PSBs are the result of size-dependent quenching. We propose that both strong stellar feedback and the stripping of the extended halo act together to preferentially and rapidly quench the compact and low-mass star-forming systems in clusters to produce PSBs. We test this scenario using the stacked spectra of 124 high-SSFR galaxies showing that more compact galaxies are more likely to host outflows. From these results we conclude that a combination of environmental and secular processes is the most likely explanation for the appearance of PSBs in galaxy clusters.

Finally, we study the evolution of environmental quenching across a wider redshift range (0.5<z<3.0) by using the deeper data of the UDS DR11. We find that galaxy star-formation properties (passive fraction, SFR and SSFR) strongly correlate with environment until z=2.0. Furthermore, we find evidence suggesting that the quenching effects of environment and stellar mass are not independent from each other, as galaxies with M*<10^10.3Msun and M*>10^10.3Msun show slightly different environmental trends. At low stellar masses there is an enhancement of both star-forming galaxies with low SSFRs and PSBs in dense environments at z<1.5. In contrast, at higher masses the low-SSFR galaxies are strongly depleted in dense environments since z~1.75. We also find that massive galaxies are more sensitive to environment, i.e. their star-forming properties are more affected than those of low-mass galaxies at the same environmental overdensity level. From this we conclude that the action of secular processes in massive galaxies might be aiding the environmental effects, leading to faster quenching. In other words, secular and environmental processes may join forces to drive the evolution of the most massive galaxies in the distant Universe.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Almaini, Omar
Hatch, Nina
Keywords: Galaxy evolution, high redshift, environmental quenching
Subjects: Q Science > QB Astronomy
Faculties/Schools: UK Campuses > Faculty of Science > School of Physics and Astronomy
Item ID: 55763
Depositing User: Socolovsky, Miguel
Date Deposited: 19 Jun 2019 13:01
Last Modified: 07 May 2020 13:02

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