Exploring the interplay between star formation and active galactic nuclei and the role of environment in galaxy evolution




Florez, Jonathan

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One of the central goals of extragalactic astronomy is to understand how galaxies grow their stellar mass and central black holes, the connection between star formation and active galactic nuclei (AGN), and the impact of environment on this growth. In this thesis, I utilize multiwavelength surveys that are both deep and wide, advanced computational codes that model the spectral energy distributions of galaxies with and without AGN, as well as state-of-the-art simulations of galaxy evolution in order to explore how galaxy properties are impacted by their surrounding environment and AGN activity. These studies explore galaxies over a redshift range of 0.015 < z < 0.023 (lookback time of ~ 0.2 to ~ 0.3 Gyr), and over a redshift range of 0.5 < z < 3.0 (lookback time of ~ 5 to ~ 12 Gyr). The large-area surveys used here provide some of the largest and most statistically robust samples to-date of rare massive galaxies (with stellar mass M [subscript *] > 10¹¹ M☉) and extremely luminous AGN (with X-ray luminosity L [subscript X] > 10⁴⁴ erg s⁻¹) out to z ~ 3, thereby limiting the effects of cosmic variance and Poisson statistics. I analyze the observed stellar masses and star formation rates of galaxies as a function of environment and AGN activity, compare the empirical results to theoretical models of galaxy evolution, and discuss the implications of such comparisons. This work will provide significant guidance and constraints to the future development of theoretical models of galaxy growth. In Chapter 2 (Florez et al. 2021, ApJ, 906, 97) I measure the environmental dependence, where environment is defined by the distance to the third nearest neighbor, of multiple galaxy properties inside the Environmental COntext (ECO) catalog. I focus primarily on void galaxies at redshifts z = 0.015 - 0.023, which I define as the 10% of galaxies having the lowest local density. I compare the properties of void and non-void galaxies: baryonic mass, color, fractional stellar mass growth rate (FSMGR), morphology, and gas-to-stellar-mass ratio. The void galaxies typically have lower baryonic masses than galaxies in denser environments, and they display the properties expected of a lower mass population: they have more late-types, are bluer, have higher FSMGR, and are more gas rich. I also control for baryonic mass and investigate the extent to which void galaxies are different at fixed mass. I find that void galaxies are bluer, more gas-rich, and more star forming at fixed mass than non-void galaxies, which is a possible signature of galaxy assembly bias and other environmental processes. Furthermore, I show that these trends persist even at fixed mass and morphology, and I find that voids host a distinct population of early-types that are bluer and more star-forming than the typical red and quenched early-types. In addition to these empirical observational results, I also present theoretical results from mock catalogs with built-in galaxy assembly bias. I show that a simple matching of galaxy properties to (sub)halo properties, such as mass and age, can recover the observed environmental trends in the local galaxy population. In Chapter 3 (Florez et al. 2020, MNRAS, 497, 3273) I investigate the relation between AGN and star formation activity at 0.5 < z < 3 by analyzing 898 galaxies with high X-ray luminosity AGN (L [subscript X] > 10⁴⁴ erg s⁻¹) and a large comparison sample of ~ 320,000 galaxies without such AGN. My samples are selected from a large (11.8 deg²) area in Stripe 82 that has multi-wavelength (X-ray to far-IR) data. The enormous comoving volume (~ 0.3 Gpc³) at 0.5 < z < 3 minimizes the effects of cosmic variance and captures a large number of massive galaxies (~ 30,000 galaxies with M [subscript *] > 10¹¹ M☉) and high X-ray luminosity AGN. While it is typical for studies of galaxy evolution to discard AGN host galaxies, I fit the SED of galaxies with and without high X-ray luminosity AGN with Code Investigating GALaxy Emission (CIGALE) and include AGN emission templates. I find that without this inclusion, stellar masses and star formation rates in AGN host galaxies can be overestimated, on average, by factors of up to ~ 5 and ~ 10, respectively. The average star formation rate of galaxies with X-ray luminous AGN is higher by a factor of ~ 3 to 10 compared to galaxies without X-ray luminous AGN at fixed stellar mass and redshift, suggesting that high star formation rates and high AGN X-ray luminosities may be fueled by common mechanisms. The vast majority (> 95%) of galaxies with X-ray luminous AGN at z = 0.5 - 3 do not show quenched star formation: this suggests that if AGN feedback quenches star formation, the associated quenching process takes a significant time to act and the quenched phase sets in after the highly luminous phases of AGN activity. In numerical simulations and theoretical models of galaxy evolution, AGN and star formation activity are closely linked and AGN feedback is invoked to regulate galaxy growth. However, few empirical tests exist on how well the models and simulations implement the growth and interplay between AGN and star formation. To address this issue, in Chapter 4 (Florez et al. submitted) I compare the hydrodynamical simulations IllustrisTNG and SIMBA, and the semi-analytical model SAG to the empirical results on AGN and star formation at cosmic noon reported in Chapter 3. The main results of my comparisons are: (i) SAG and IllustrisTNG both qualitatively reproduce the empirical result that galaxies with high X-ray luminosity AGN have higher mean star formation rates, at a given stellar mass, than galaxies without such AGN. SAG, however, strongly over-produces the number density of high X-ray luminosity AGN by a factor of 10 to 100, while IllustrisTNG shows a lack of high X-ray luminosity AGN at high stellar mass (M [subscript *] > 10¹¹ M☉) at z ~ 2. (ii) In SIMBA, the mean star formation rate of galaxies with high X-ray luminosity AGN is lower than the star formation rate of galaxies without such AGN. Contrary to the data, many high X-ray luminosity AGN in SIMBA have quenched star formation, suggesting that AGN feedback, or other feedback modes in galaxies with such AGN, might be too efficient in SIMBA. I discuss the implications of these results for our understanding of the evolution of galaxies and the growth of their stellar masses and black holes across cosmic time.



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