I study the evolution of galaxies and the processes which drive that evolution. I analyze both observations (primarily radio-through-optical) and N-body/hydrodynamic simulations to understand the growth of galaxies and their black holes, and how feedback processes may eventually halt that growth. Broadly speaking, I work in the following areas:

Beyond direct probes of the activity in starbursts and AGN, these research topics also impact predictions for gravitational wave and neutrino sources and I am interested in multi-messenger astronomy.

Here is a list of potential student projects.

The Interstellar Medium of Galaxies


Stars are thought to form from molecular hydrogen, which cannot be directly observed in the cold interstellar clouds that are forming stars. Instead we use tracer molecules such as CO, HCN, and HCO+ to estimate the mass and physical conditions in the molecular gas. I am undertaking a series of molecular observations of galaxies to probe the link between these tracers and the activity in the centers of the galaxies. The figure to the left shows the galaxy-integrated ratio of the luminosity in the 1–0 transition of the HCN and HCO+ molecules as a function of the dominance of an active galactic nucleus in the mid-infrared, from my 2015 paper (linked below). In the paper, we were unable to find a single physical process which explained the relationship of the global HCN/HCO+ ratio.

Distributed star formation in the dwarf galaxy merger dm1647+21.

A map of Hα emission in the dwarf galaxy merger dm1647+21, showing the distributed yet enhanced starburst. From Privon et al. (2017). Click to enlarge.

I am now pursuing resolved observations of these molecular lines with the Atacama Large Millimeter/submillimeter Array to further understand the emission from these tracer molecules. This is being supported with X-ray observations from NuSTAR to assess the reliability of using elevated HCN/HCO+ ratios as a tracer of deeply buried AGN.

Lastly, I am leading an optical integral field unit campaign on samples of Luminous Infrared Galaxies (LIRGs) and dwarf galaxy pairs to study the properties of their atomic ISM. This includes studying the effects of star formation, shocks, and AGN on the excitation of the atomic gas, as well as searching for winds being driven by star formation and AGN.


Dynamical Models of Galaxy Mergers

The redistribution of stars and consumption of gas by star formation and AGN during galaxy interactions and mergers have a strong influence on the properties of the galaxies involved, driving significant morphological transformation. I am matching dynamical models to observations of local mergers in order to facilitate detailed comparisons between numerical models of star formation and the actual star formation taking place in these systems. This modeling effort will accurately timestamp individual systems, something not previously available for a sample of galaxy mergers.

The image to the left shows the optical image of an ongoing galaxy interaction, between NGC 5257 and NGC 5258, as seen by the Hubble Space Telescope. Below the HST image is a N-body simulation of a galaxy merger which has been matched to the observed morphology and kinematics of the system. This model was presented in my 2013 paper listed below and is representative of the modeling of galaxy mergers.


Dwarf Galaxy Interactions

Facilities: VLT/MUSE, ALMA

Little is known about interactions between dwarf galaxies, compared to more massive galaxies. However, by number, mergers between dwarfs are expected to be more frequent than mergers between massive galaxies, but it is unclear how these interactions will affect the star formation rates, stellar masses, and gas content of dwarfs. I am part of the TiNy Titans (TNT) survey of local dwarf galaxies. Our SDSS-selected sample of isolated dwarf galaxy pairs and a matched sample of non-isolated pairs and individual dwarfs is being used to study the effect of interactions and mergers on dwarf galaxies. Additionally, we are using large-volume cosmological simulations to obtain theoretical predictions for these interactions. I am pursuing optical integral field unit observations of dwarf-dwarf mergers and dynamical models for individual pairs.


Active Galactic Nuclei

Facilities: ALMA, NuSTAR, Spitzer

While it only takes a relatively small amount of gas to fuel a supermassive black hole, the energy output can potentially affect the entire host galaxy. I am interested in the interactions between radio jets and the ambient host galaxy, as well as the broadband emission from AGN and their hosts. Studying the broadband emission can disentangle the power sources of these systems: star formation vs AGN activity.

Additionally, I am interested in studying the rare examples of “dual AGN” – galaxy mergers where both supermassive black holes are simultaneously accreting. These systems may provide critical information on the role of AGN feedback in galaxy evolution. I am part of an international team, working on the optical integral field unit and millimeter spectral line observations of these objects.

The figure to the left shows the radio-through-mid-infrared spectral energy distribution (SED) of the radio galaxy Cygnus A. Additionally, the figure shows model fits to the SED, which I used to disentangle the AGN and starburst contribution to the far-infrared emission. I found that the bulk of the infrared could be explained with dust heated by the AGN, but the rest of the infrared is due to ongoing star formation.


Multi-messenger Astronomy

In 2015 the first detection of gravitational waves (GW) from the merger of two black holes ushered in an era of GW astronomy. Similarly, in 2018 the first extragalactic neutrino detection was associated with a flaring AGN (a blazar). Together, these discoveries have introduced new observing opportunities for understanding the cosmos. My research on galaxy mergers and the growth of black holes has implications for future GW detection with the pulsar timing arrays and the Laser Interferometer Space Antenna; my dynamical models provide an independent estimate of the galaxy merger rate at low-redshift and the black hole growth studies are important for the total masses and mass ratios of future black hole binaries. On the neutrino side, starburst galaxies have been proposed as a source for the extragalactic neutrino background. The neutrino production rates are sensitive to the cosmic ray production rates and the gas densities of the ISM; these can both be estimated from GOALS data, including my studies of the molecular ISM.