A Multi-frequency VLA Continuum Investigation of Starburst & AGN Activity in Local U/LIRGs

The rates at which galaxies form stars have been declining since ~ 3 billion years after the Big Bang. Massive stars that drive the chemical enrichment of the Universe are now being produced at much lower rates in the Milky Way and most other galaxies in our local Universe, compared to galaxies at z ~ 1-2, when most stars formed. During this distant era of rapid growth, star formation took place primarily in Luminous and Ultra-luminous Infrared Galaxies (U/LIRGs), where UV and optical light from newly formed massive stars are largely absorbed by thick layers of dust in these systems and re-emitted at infrared wavelengths. At z ~ 0, U/LIRGs are relatively rare but they are the most extreme star-forming galaxies in the local Universe. While these local U/LIRGs do not fully resemble their high-z counterparts, they provide an excellent opportunity for a detailed assessment of the extreme activity that may have been taking place at the peak of cosmic star formation.

Many local U/LIRGs are observed to be interacting and merging gas-rich spiral galaxies, during which process molecular gas in the system is driven towards the galaxy centers, triggering intense nuclear starbursts and/or accretion of the supermassive black holes (i.e. Active Galactic Nuclei; AGN). Meanwhile, the large amounts of dust generated in the starburst prohibit a direct view of the most energetic regions in local U/LIRGs using UV/optical/near-IR observations.

In this dissertation, I characterize the starburst and AGN activity in a representative sample of 68 local U/LIRGs from the multi-wavelength Great Observatories All-sky LIRG Survey (GOALS), using 3 - 33GHz radio continuum observations from the GOALS "equatorial" Survey (GOALS-ES) conducted with the Karl G. Jansky Very Large Array (VLA), described in Chapter 2. These multi-frequency observations of the optically-thin radio continuum provide a highly-detailed and extinction-free view of the most energetic yet often dust-obscured star-forming and AGN activity in these extreme local systems.

Using the 33GHz radio continuum as a direct tracer of star formation rates (SFR), in Chapter 3 I studied the star-forming properties of nuclear rings in four GOALS-ES systems and found that these nuclear rings all contribute to more than 50% of the total star formation of their host U/LIRGs, and the individual regions residing in these rings have star formation rate surface densities that rival star-forming clumps observed at z = 1 - 4 with much larger sizes. Looking beyond the nuclear rings, in Chapter 4 I identified and characterized the size and luminosity of over 100 individual regions of compact 15 and/or 33GHz radio continuum emission in the full GOALS-ES sample on 100pc scales. On average, the individual star-forming regions identified in these U/LIRGs have 100 times higher star formation rates and surface densities compared to those in nearby normal star-forming disk galaxies that are of similar sizes. I also identified a sample of luminous galactic nuclei that appear to be forming stars near the maximal capacity as predicted by theoretical models for star-forming disks supported by dust-reprocessed radiation pressure. Several of these nuclei have been identified to host AGN, and the rest may be going through a key phase of heavily obscured co-evolution between supermassive black holes and nuclear star formation that is thought to precede the formation of luminous quasars. Detection of high brightness temperature radio cores indicative of AGN activity via follow-up VLBI observations will directly verify this scenario. Lastly in Chapter 5, by modeling the 3 - 33GHz spectral energy distribution observed in these local U/LIRGs on kpc scales, I find that compared to AGN, the radio continuum emission of star-forming regions and nuclei in these systems indeed has a higher contribution from thermal free-free emission from HII regions, particularly at 33 GHz, validating the usage of 33 GHz as a direct tracer of recent star formation. The 3 - 33 GHz radio continuum is also strongly correlated with mid- and far-IR dust emission observed on ~ 10 kpc scales for star-forming regions, especially at 70um, indicating their shared origin.