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Title: Observing high redshift galaxy clusters at sub-millimetre wavelengths
Author: Smith, Connor Michael Anthony
ISNI:       0000 0004 9354 5308
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2020
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This thesis aims to investigate high redshift galaxy clusters, focusing on Far Infrared (FIR) and Sub-millimeter (sub-mm) observations. The main focus is to observe the potential environmental effects on cluster galaxies, and see if and how these processes effect the evolutionary path a galaxy is on. Using SCUBA-2 and Herschel data I investigate the star forming properties of two mature, high redshift galaxy clusters, CLJ1449 (at redshift two) and JKCS041 (at redshift 1.8). Using high-resolution Atacama Large Millimetre Array (ALMA) and Jansky Very Large Array data I identify potentially confused galaxies, and use the Bayesian inference tool XID+ to estimate fluxes for them. Using archival optical and near-infrared data with the energy-balance code CIGALE I calculate star formation rates and stellar masses for all our cluster members. For both clusters I estimate star formation rates (SFRs) between 12-1600M/yr over the entire 3Mpc radial range. In the central 0.5Mpc region I find SFRs of 800 +/- 200 M/yr for CLJ1449, and 770 +/- M/yr for JKCS041. When converted to a SFR-density this gives (1.2 +/- 0.3)x10^4 and (1.3 +/- 0.1)x10^4 M/yr Mpc^3 for CLJ144 and JKCS041 respectively, which are five orders of magnitude greater then expected field values. This indicates that there is indeed a reversal in the SFR-density relation in both of these clusters, and the majority of the star formation is occurring in the densest regions of the cluster (opposed to what happens in local clusters). When I look at the SFR-stellar mass relation I find a deviation between the two clusters. For CLJ1449, I find that all the cluster galaxies either exist on the galaxy main sequence, or just above it in a region dominated by star burst galaxies. There does appear to be a radial trend, with those galaxies closest to the cluster core showing the lowest stellar masses, which could indicate environmental influences quenching these galaxies. For JKCS041, the majority of galaxies lie below the main sequence, with very few galaxies exhibiting star-burst behaviour. This could show that the galaxies within are being quenched and the environment is having a profound effect. When comparing these clusters to other high redshift clusters, I see that CLJ1449 seems to follow expected scaling relations for clusters. JKCS041 on the other hand deviates significantly. Since the mass of JKCS041 is significantly larger then other clusters at this redshift, it could show some of the first direct evidence that a clusters mass has an effect on the galaxies that reside within it. Using band 3 ALMA spectral scans, I searched for a potential proto-cluster in an over-density of red Herschel sources. I used a match filter technique to try and identify any potential spectral lines in the ALMA data, and identified 57 detections with a S/N greater than 4 (six of these are associated with continuum sources). Assigning redshifts to all the sources, I identified an over-density at redshift 3.64, which I take as the redshift of the cluster. Using velocity dispersions I assign proto-cluster membership, with 10 sources being a part of the structure. From the cluster members I estimate a velocity dispersion for the cluster of ~570 km/s, which corresponds to a mass of ~2.5x10^13M. This means this protocluster is the progenitor to a Coma like cluster. Calculating gas masses (directly from the CO spectra) and stellar masses (from Spitzer IRAC data) I find gas fractions of ~35%. These gas fractions fall below expected scaling relations, and could show that these galaxies are undergoing a period of extreme star formation. These galaxies also have gas fractions that fall below what is seen in field galaxies. This could indicate that environmental processes are already at work. With those sources with continuum measurements, high SFRs were found (greater then 1500M/yr), and they exhibit behaviour similar to sub-mm galaxies. This extreme star formation cannot be supported for long, and will start to passively evolve after ~50 Myr, and could be the beginning of a red sequence of galaxies.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QB Astronomy