AGN–Host Galaxy Image Decomposition in the CEERS Survey

Dewsnap, Callum James

AGN–Host Galaxy Image Decomposition in the CEERS Survey

Files

Primary dewsnap_callum_phd_2025_thesis.pdf (30.82 MB)

Date

2025-10-23

Authors

Dewsnap, Callum James

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.

Summary for Lay Audience

Galaxies are massive collections of stars, gas, dust, and dark matter. Located at the centre of every galaxy is a supermassive black hole (10^6–10^10 solar masses). These supermassive black holes can form an accretion disk, where gas orbits in a disk around the black hole. As the gas accretes onto the black hole, enormous amounts of energy are released as light. This system is called an active galactic nucleus (AGN). Despite the small size (<0.001% the diameter of the host galaxy), the light released by the AGN can outshine the entire galaxy combined. Both the galaxy and the AGN have significant influence on one another. For example, the AGN produces outflows which can stifle star formation in the surrounding region, and the mass of the AGN is tightly correlated to the mass of the host galaxy’s bulge. To understand either the AGN or host galaxy individually, we must account for both. The shape of a galaxy (morphology) is tied to various mechanisms of galaxy evolution. By modelling how the brightness of the galaxy changes from its centre outwards, we can measure how the material that forms the galaxy is distributed. However, the bright, point-like AGN contaminates the brightness profile of the galaxy. Thus, we model the system with two components: a profile for the galaxy and a point source for the AGN. This is called AGN-host decomposition. 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 AGN-host decomposition and find that we are able to successfully separate the AGN’s light from the host galaxy. However, we find degeneracies in the host galaxy model, resulting in parameters that do not uniquely represent the galaxy. We define a set of best practices for future fits and then apply them on 96 different AGN hosts across 13 bands of James Webb Space Telescope and Hubble Space Telescope imaging. We use the resulting fits to extract physical information about the sources, such as the stellar mass or total AGN luminosity.