Identifying and Distinguishing Quenching Galaxies with Spatially Resolved Star Formation

Abstract

This thesis is concerned with understanding galaxy evolution, in particular how galaxies have their star formation shut down, or "quenched." The process of quenching transforms actively star forming galaxies into passive, quiescent, or otherwise dormant galaxies with very low levels of star formation. Many physical mechanisms have been proposed to describe quenching, and can be divided into internal processes or external processes due to environment. These quenching mechanisms should result in different spatial signatures when viewing the distribution of star formation in a galaxy, but the spatial and temporal evolution of these different processes remains unclear. Therefore, spatially resolved observations of star formation are necessary to investigate quenching mechanisms, and will prove critical for understanding galaxy evolution in the next decade.

The first section of this thesis is concerned with understanding galaxy quenching in simulations. Using the high-resolution IllustrisTNG simulation, quenching signatures for 361 galaxies with log(M/Msun) ⩾ 9.5 by z = 0 are investigated, and four morphological metrics that collectively describe the spatial distribution of star formation within these galaxies are developed. The evolution of the metrics is studied through the galaxy quenching episodes, relative to a control sample of normal star forming galaxies which show no evolution. Two prominent modes are found which describe the evolution of quenching, where the first is consistent with inside-out quenching, and numbers 78 galaxies, while the second is consistent with outside-in quenching, numbering 185 galaxies. Inside-out quenched galaxies are generally centrals in the field, and are more massive than outside-in quenched galaxies, which tend to be satellites in denser environments like groups and clusters. Inside-out quenched galaxies take ∼2.5 Gyr to quench, while outside-in quenched galaxies take ∼1.5 Gyr to quench, and begin quenching ∼1 Gyr after being accreted onto a more massive structure.

The second section of this thesis concentrates on adapting the morphological metrics from the simulation for use with mock observations. Using the sample of simulated galaxies, high-resolution mock observations consistent with Cosmological Advanced Survey Telescope for Optical and UV Research (CASTOR) and Nancy Grace Roman Space Telescope (NGRST) imaging are produced for nearly 70,000 galaxies at z = 0.5, covering the ultraviolet to the near-infrared. These mock observations are additionally consistent with Hubble Space Telescope imaging. A flexible hybrid star formation history is used to fit the resulting spectral energy distributions from the mock photometry. Spatially resolved star formation rate and stellar mass are accurately recovered when compared with the simulation, and with observational adaptations to the morphological metrics, these parameters are likewise recovered to high fidelity.

The third section of this thesis investigates galaxy quenching for observed galaxies in the Hubble Frontier Fields at z ≈ 0.4, and builds on the previous sections. The spatially resolved spectral energy distributions are fit for a sample of 1437 galaxies with log(M/Msun) ⩾ 8 and SFR ⩾ 0.001 Msun/yr, to determine star formation rate and stellar mass radial profiles. The observational morphological metrics are then calculated using these profiles, and are compared with the simulated galaxies from IllustrisTNG. A population consistent with an inside-out quenching mechanism is found, and numbers 129 galaxies, as well as a population consistent with an outside-in quenching mechanism, numbering 70 galaxies. Inside-out quenching galaxies are more massive than outside-in quenching galaxies, and are more often found in clusters, while the outside-in population often reside in the field. The inside-out population makes up ∼30% of the sample in clusters at high stellar mass [log(M/Msun) ⩾ 10.5]. Both populations show little dependence on cluster infall time, except the high mass inside-out population, which commonly has older inferred infall times. These results also suggest that the quenching prescription within simulations is incomplete, as it does not predict inside-out quenching for high-mass satellites.

Collectively, this thesis highlights the need for spatially resolved observations to investigate different quenching mechanisms, in both simulations and observations, where simulations can help form a framework which can be adapted to observations. Fortunately, upcoming large-scale galaxy surveys will provide millions of galaxies suitable for spatially resolved analyses, in order to better understand galaxy quenching and evolution.