Understanding the Properties of Obscured Radio AGN and Faint Submillimeter Galaxies at z-2

Author: ORCID icon orcid.org/0000-0002-9471-8499
Patil, Pallavi, Astronomy - Graduate School of Arts and Sciences, University of Virginia
Whittle, Mark, AS-Astronomy, University of Virginia

Cosmic noon marks the period in cosmic history when galaxies were in their most active state, with intense star-formation and maximum black-hole fueling. In order to fully understand the evolution of galaxies, it is essential to study both active galaxies and star-forming galaxies during this period, which in redshift spans 1 < z < 3. The work I present in this thesis, in four main chapters, targets these two classes of galaxies and identifies a number of important processes that drive change in galaxies across this epoch. Chapter 1 provides a brief outline of the context of the work presented in this thesis.
In Chapter 2, I present new sub-arcsecond-resolution Karl G. Jansky Very Large Array (VLA) imaging at 10 GHz of 155 ultra-luminous (L_{bol} ~ 10^{11.7-14.2} L_\odot) optically obscured quasars with redshifts z ~ 0.4-3. The sample was selected to have extremely red mid-infrared (MIR)-optical color ratios based on data from Wide-Field Infrared Survey Explorer (WISE) along with a detection of bright, unresolved radio emission from the NRAO VLA Sky Survey (NVSS) or Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) Survey. Our high-resolution VLA observations have revealed that the majority of the sources in the sample (93 out of 155) are compact with angular scales <0.2" (<1.7 kpc at z~2). The radio luminosity, linear extent, and lobe pressure of these sources are similar to those of young radio AGN, but their space density is considerably lower. Application of a simple adiabatic lobe expansion model suggests a range of properties from more compact to more extended lobes of relatively young ages (<10^3 - 10^6 years), relatively high ambient ISM density (~10^{-4}-10 cm^{-3}), and relatively modest lobe expansion speeds (~0.003c-0.1c). Folding together the rarity of the sources and their young dynamical age, I estimate that perhaps 20% of our class of AGN evolve through compact phases (such as Gigahertz Peaked Spectrum, GPS, and Compact Steep Spectrum, CSS, sources) to become large scale FR-I/II radio sources.
In Chapter 3, I present broadband radio spectra of the entire sample, constructed from our 10 GHz observations and archival radio data. About 57% of the sample exhibits peaked or curved radio spectra suggesting low-frequency absorption. At least 30% of the sample belongs to the class of peaked spectrum sources such as CSS, GPS, and High-Frequency Peakers (HFP). The sample also has, on average, steeper high-frequency radio spectra than classic optically thin synchrotron sources. A comparison of the magnetic energy density and the MIR photon energy density suggests that the spectral steepening could arise from inverse Compton scattering off the intense MIR photon field. Extrapolating the high-frequency spectra into the submm cannot produce our measured ALMA fluxes, which must therefore originate from another component, most likely cold dust. Returning to the peaked sources, assuming synchrotron self-absorption dominates, I derive source sizes and magnetic field strengths for the emitting regions, finding compact sources (a few to 10s of parsecs) with relatively strong magnetic fields (10-100 mG), consistent with very young radio sources. While the MIR properties of the sample are quite different from almost all other classes of AGN, I was unable to find any statistically meaningful correlations between the radio spectral properties and the MIR properties.
Chapter 4 aims to identify an important new class of submm galaxies (SMGs) with fluxes below the more well-known bright SMGs. These are thought to dominate the extragalactic background submm light but have remained undetected in most previous submm surveys. I present a catalog of 26 faint SMGs in the deep X-ray XMM-LSS field identified serendipitously within archival Band 6 and 7 ALMA observations, cross-matched with other deep multi-band surveys of this field. Of the 26 SMGs in our sample, 15 are identified here for the first time. To further characterize these galaxies, I provide 13-band photometry for the entire catalog using the Tractor deblending package and calculate photometric redshifts (median z~2.66) and rest-frame colors. I find that our faint SMGs have bluer colors than bright SMGs, and the UVJ color-color plot places them on the main sequence of star-forming galaxies. Our results help broaden our understanding of the important population of SMGs and endorse the importance of archival ALMA data in the serendipitous identification of new populations.
In Chapter 5, I summarize ongoing projects that target sub-samples of the primary sample of young radio AGN. In the first, I used multi-frequency multi-resolution imaging from the VLA, VLBA, and e-MERLIN to study 12 compact sources. In the second, I used multi-frequency VLA observations to study 20 well-resolved sources. In each, my aim is to understand the physical nature and evolutionary stage of the radio source and relate these to the host properties. In the third, I used ALMA to study the molecular gas content of three sources. The fourth is a pilot NuTSAR study of a single deeply obscured source in hard X-rays to identify the intrinsic AGN emission. Finally, I discuss the next generation Very Large Array (ngVLA) and its role in studying the life-cycles of z~2 radio AGN and their broader connection to galaxy evolution.
In the final chapter, I summarize the results presented in my dissertation.

PHD (Doctor of Philosophy)
Active Galactic Nucleus, Obscured Quasars, Faint Submillimeter Galaxies, Cosmic Noon, Radio Continuum Observations, Compact Radio AGN
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