"A New View of Galaxy Morphology" by Timothy Blake Miller

Date of Award

Fall 2023

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Astronomy

First Advisor

van Dokkum, Pieter

Abstract

A galaxy’s morphology provides crucial constraints on its formation history. Most studies rely on forward modeling procedure utilizing the S\ersic profile to measure morphological parameters and focus on the half-light radius of galaxies measured at optical wavelength. In this P.hD I investigated how common assumptions made when measuring galaxy structure bias the interpretation. First I consider the size-mass distribution using the radii that contain 20% and 80% of the light, as opposed to the traditional half-light radius. We find qualitatively different relations and suggest that the 20% radius is related to the star-formation history of galaxies while the 80% radius is correlated with the halo. The Sersic profile provides a convenient paramaterization, however it cannot fully capture the complexity of real galaxies. Using data from the Dragonfly telescope, which is optimized for low-surface brightness sensitivity, I measure the luminosity of galaxies using a non-parametric technique to compare to the Sersic profile. We find two Sersic components are needed to match the total fluxes measured by Dragonfly but, even so, Dragonfly tends to measure bluer colors. As a consequence this led me to develop an alternative method based on multi-gaussian expansion, called imcascade. By representing the light profile as a series of gaussians and incorporating Bayesian inference it provides much greater flexibility. This method is implemented in Python and is available as an open source software package. I apply imcascade to HST images of galaxies at z=1-2 to study color gradients at this epoch and test their effect on the measurements of the half-light radii. These galaxies display strong negative gradients in their optical colors (i.e. redder in the center) which vary with stellar mass, redshift and galaxy type. By using this color to account for mass-to-light ratio gradients, I study the relationship between the half-mass and half-light radii. The ratio of half-mass to half-light radii decreases from 0.8 at z=2 down to 0.5 at z=1. This leads to a slowly evolving half-mass size to stellar mass relation with a shallow slope.Finally, By applying imcascade to early JWST data I measure near-IR color gradients for a sample of star-forming galaxies at a redshift of two to investigate the physical causes. I find strong gradients in their NIR and optical colors indicating strong central dust attenuation as the main physical driver of color gradients. This is markedly different than in the local universe where color gradients are caused by gradients in stellar age. In this work I suggest that we are witnessing dust-obscured bulge growth at cosmic noon.

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