Master Thesis Defense by Natalie Allen

Title: Understanding Galaxy Size and Mass Build-up in the first 2Gyrs with JWST

Abstract: One of the most fundamental ways of understanding galaxy evolution is by studying their sizes. Physical processes, such as feedback, star formation and environment, all impact the size of galaxies. Previously, HST has been used to study galaxy sizes, but due to its wavelength limitations, these results have been restricted to certain redshifts. Rest-frame optical wavelengths fall out of HST range at z ≥ 3, thus restricting galaxy size measurements to the rest frame-UV.

In this thesis we analyse galaxy structure at z ≥ 3 using the wavelength range and high resolution of the James Webb Space Telescope. We select a total number of 3244 sources at 3 ≤ z < 6 from the CEERS, PRIMER-UDS and PRIMER-COSMOS fields: 1711 at 3 ≤ z < 4, 1163 at 4 ≤ z < 5 and 370 at 5 ≤ z < 6. Using the HST and JWST coverage, we are able to measure reliable photometric redshifts and galaxy physical properties by fitting spectral energy distributions with Eazy and Bagpipes over 14 bands. Single Sersic models were fit to the z > 3 sample using GalfitM to measure effective radii as well as other Sersic model parameters.

We then analyse the structure of high-z galaxies by fitting the size-mass relationship to galaxies between 3 ≤ z < 6 and seeing how this evolves as well as its dependence on wavelengths. The linear fit of the size-mass relationship was fit using Linmix: a Bayesian approach to linear regression. Linmix allows for both x and y errors to be taken into account, as well as measuring
the intrinsic scatter of the measurements of the individual galaxies.

We find that galaxies at z ≥ 3 continue to have a positive size-mass relation. The slope and intrinsic scatter of size-mass fits continues to have no evolution, matching low-z HST studies. However, galaxies at logM∗ = 5 × 1010 show a faster decline in size than previous predicted. Although this evolution is faster, the sizes at z ≥ 3 are similar, showing slow down in growth at z ≥ 3. We also find that the rest-UV and rest-optical size-mass relationship for all redshift bins have no significant difference between each other. Showing that galaxies probed in rest-UV and rest-5000Å probe similar integrated light.

Our size-mass fit in the lowest mass bin shows some significant dependence on wavelength, while at higher redshift the size-mass relationships have no significant differences from F115W to F444W. We expect that there is either contamination in our sample or some morphological transition to bulge formation is occurring at 3 ≤ z < 4.

Supervisor:

  • Sune Toft, University of Copenhagen, Niels Bohr Institute

Co-Supervisor:

  • Pascal Oesch, University of Geneva and University of Copenhagen, Niels Bohr Institute

Censor:

  • Thomas Greve, DTU Space