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My research

Changing-look AGN

AGN are typically classified based on their spectra. Type 1 AGN have broad lines in their spectra, while Type 2 AGN do not. Most AGN unification theories attribute these types to the orientation of the AGN system relative to the viewer.

Changing-look AGN are AGN which gain or lose broad-line emission over timescales of months or years. We still don't know what causes this behavior, but popular theories include changes in the fuel available to the AGN, instabilities in the accretion disk, or transient events like the tidal disruption of stars.

Post-starburst galaxies

Galaxies in the local Universe can be broadly divided into star-forming spiral galaxies and quiescent/quenched (non-star-forming) elliptical galaxies. Galaxies must evolve from star-forming to quiescent by 'quenching,' or shutting down, their star formation. Most quiescent galaxies in the local Universe quenched slowly over billions of years, while quiescent galaxies in the early Universe (which had less time to shut down star formation) did so rapidly. To understand these early-Universe rapid quenching mechanisms, we can study so-called 'post-starburst galaxies,' or galaxies which experienced a burst of star formation followed by rapid quenching, in the local Universe.

My Papers

Modeling Star Formation Histories of Changing-look AGN Host Galaxies with Prospector

Changing-look active galactic nuclei, or CL-AGN, are AGN which appear to transition between Seyfert Type 1 and 2 over periods of months to years. Several mechanisms to trigger these transitions have been proposed, but we have yet to conclusively determine their cause. Recent studies suggest CL-AGN are hosted primarily in galaxies which are shutting down star formation (Dodd et al. 2021; Liu et al. 2021; Wang et al. 2023), which may indicate a link between galaxy quenching and changing look events. We use Prospector stellar population synthesis software (Leja et al. 2017; Johnson & Leja 2017; Johnson et al. 2021) to model non-parametric star formation histories for 39 CL-AGN host galaxies. We find that 43% of our gold sample CL-AGN at z < 0.15 are star forming, while 29% fall in the Green Valley of the stellar mass-sSFR diagram. At z > 0.15, 57% of CL-AGN in the gold sample are star-forming and 29% are in the Green Valley. CL-AGN hosts have similar star formation properties to the host galaxies of Seyfert 1 and 2 AGN at z < 0.15 and to Seyfert 2 AGN at z > 0.15. We find no statistically significant differences in the star formation properties of turn-on and turn-off CL-AGN. We also find no evidence for rapid quenching in the Green Valley CL-AGN. We conclude that CL-AGN state transitions are not associated with the formation history of CL-AGN host galaxies on large spatial scales, implying CL-AGN state transitions may instead result from nuclear-scale or accretion disk effects.

Merger Signatures are Common, but not Universal, in Massive, Recently Quenched Galaxies at z~0.7

We present visual classifications of merger-induced tidal disturbances in 143 massive post-starburst galaxies at z ~ 0.7 identified in the SQuIGGL⃗ E Sample. This sample spectroscopically selects galaxies from the Sloan Digital Sky Survey that have stopped their primary epoch of star formation within the past ~500 Myr. Visual classifications are performed on Hyper Suprime-Cam imaging. We compare to a control sample of mass- and redshift-matched star-forming and quiescent galaxies from the Large Early Galaxy Census and find that post-starburst galaxies are more likely to be classified as disturbed than either category. This corresponds to a factor of 3.6 times the disturbance rate of older quiescent galaxies and 2.1 times the disturbance rate of star-forming galaxies. Assuming tidal features persist for ≲500 Myr, this suggests merging is coincident with quenching in a significant fraction of these post-starbursts. Galaxies with tidal disturbances are younger on average than undisturbed post-starburst galaxies in our sample, suggesting tidal features from a major merger may have faded over time. This may be exacerbated by the fact that, on average, the undisturbed subset is fainter, rendering low-surface-brightness tidal features harder to identify. However, the presence of 10 young (≲150 Myr since quenching) undisturbed galaxies suggests that major mergers are not the only fast physical mechanism that shut down the primary epoch of star formation in massive galaxies at intermediate redshift.