Dissertation: AGN–Host Galaxy Image Decomposition in the CEERS Survey

Title: by Dr Callum Dewsnap

Thesis defended on  October 23, 2025

Department of Physics & Astronomy, Western University

Thesis advisor: Dr. Sarah Gallagher & Dr. Pauline Barmby

Abstract:

The properties of galaxies that host active galactic nuclei (AGNs) offer clues to the conditions that enable accretion onto supermassive black holes. A galaxy’s morphology can provide significant information on the evolutionary processes underway. By modelling the surface brightness profiles of galaxies which host AGN, we can characterize several significant properties of the host galaxy. By including a sample of sources across a wide range of redshifts, we can investigate how both the supermassive black hole and galaxy evolve over time. Since AGN are among the most luminous objects in the universe, they can significantly contaminate the brightness profile of the galaxy that hosts them. Since AGN are so small relative to the host galaxy, they appear as a bright point source at the centre of the galaxy. So, our surface brightness model has two components: a Sérsic profile representing the host galaxy and a point source representing the AGN. The ability to disentangle the light of an AGN from its host galaxy is strongly dependent on the spatial resolution and image depth. As telescopes improve over time, it is important to confirm that our standard techniques are still capable of modelling the increasingly complex structure revealed. We perform rigorous tests of the Sérsic + point source model using AstroPhot and GALFIT and find that we can robustly determine the brightness of the AGN. However, we reveal degeneracies within the Sérsic profile, resulting in structural parameters that do not uniquely represent the host galaxy. We present a set of best practices for those who attempt similar fits. Applying the best practices acquired from our previous tests, we perform Sérsic + point source fits of 96 sources across up to 13 bands of James Webb Space Telescope and Hubble Space Telescope imaging using AstroPhot. We use both parametric (Sérsic) and nonparametric methods in tandem to best model the host galaxy. We use CIGALE to fit model spectral energy distributions to our resulting brightness measurements to estimate several physical measurements such as the stellar mass and star formation rate of the host galaxy, or the bolometric luminosity of the AGN.