My research interests focus on various aspects of active galactic nuclei structure and physics. Specifically, I am interested in using observations and scaling relations of local active galaxies to investigate the properties of their high redshift analogs.
REVERBERATION MAPPING
Active galactic nuclei (AGNs) are compact regions at the centers of galaxies whose supermassive black holes are actively accreting matter. The M-sigma relation tells us that the mass of the central black hole is an essential parameter in the study of galaxy evolution (plot on right from McConnell & Ma 2013). For quiescent galaxies, the BH mass can be measured by stellar or gas dynamical modeling. However, in active galaxies where the light of the AGN is significantly brighter than that emitted by the host galaxy, an alternative method is needed.
Enter reverberation mapping (RM), a technique that can be used to measure BH masses in broad-lined (type 1) AGNs. In short, the broad emission lines in these AGNs come from ionized gas orbiting very close to the BH. As the AGN accretion disk (continuum) varies in flux due to inhomogeneous accretion, the broad lines also echo the changes in total flux, and the lag between the line and continuum light curves represent the light travel time from the ionizing source to the line-emitting gas. This lag gives us the average size of the broad-line region (BLR); the width of the broad line gives us the velocity dispersion of the gas; combining the two we can use the Virial Theorem to find the central black hole mass.
At high enough redshifts, it’s not only impossible to measure the BH mass using dynamical methods, but RM also becomes very difficult due to a variety of reasons. So to estimate BH masses for very distant objects, we have to use the BH mass scaling relation derived from the local sample of RM AGNs and extrapolate it out to high-z. The radius-luminosity (R-L) relation describes the observed correlation between BLR radius (R) and AGN continuum luminosity (L), and is anchored on the sample of local AGNs with reverberation mapped BH masses. Using this relation, we can estimate the BH mass in a distant AGN with just a single spectrum. The spectrum will give us the velocity dispersion of the gas (line width) as well as AGN luminosity, leading to a value for the BLR size using the R-L relation. We can then estimate the BH mass using these values.
Because the R-L relation is anchored on the local sample of AGNs with reverberation mapped BH masses, and it’s currently the only method with which we can probe BH masses of high-z quasars, RM of nearby galaxies is extremely important for studying BH evolution over cosmic time scales. Currently, we only have the R-L relation constrained for low luminosity AGNs, and only for the H-beta broad line, and to a lesser extent, the C IV broad line. My goal is to work toward better constraints for the R-L relation by adding more points to the sample at higher AGN luminosities, as well as explore the R-L relation for other UV lines, which will be needed for single-epoch BH mass estimates for quasars in the very distant universe.
MY CONTRIBUTIONS
My Ph.D. research focused on RM of local AGNs. I led a nine-month long RM campaign back in 2012 to study the Kepler-field AGN KA1858+4850, the second brightest AGN in the Kepler field. The project is completed and the results have been published in the Astrophysical Journal.
I was also involved in the AGN Space Telescope and Optical Reverberation Mapping (AGN STORM) project. This project aims to study in detail the BLR in the Seyfert galaxy NGC 5548, and uses simultaneous observations from HST COS, Swift, and ground-based photometric and spectroscopy. I led the spectroscopic analysis component of this project and my paper has been accepted for publication in the Astrophysical Journal.
OTHER RESEARCH INTERESTS
During my first Ph.D. project, I built a set of IDL codes that quickly and efficiently extract AGN light curves from large amounts of inhomogeneous photometric data. In this context, I’ve developed an interest in automating data reduction pipelines as well as working with larger astronomical datasets.