Feeding and Feedback in Merging Galaxies

The fate of galaxies is tied to the galactic nuclei, but how the central supermassive black hole governs its host’s growth is a fundamental key question that currently lacks clarity in both physical mechanisms and timescales. I am driven to resolve the detailed physics of how SMBHs grow and merge, particularly in black hole pairs as gravitational wave precursors. As a co-leader of a major Keck program, KOALA, targeting merging massive black holes at scales comparable to their spheres of influence, my work has demonstrated how nuclear molecular outflows driven by dual or obscured supermassive black holes governs the surrounding fuel for star formation – a process known as feedback that regulates the life cycle of galaxies.

Growth Pathway of Supermassive Black Holes

As nature’s challenge to observers, how AGN are triggered is difficult to pinpoint. Supermassive black holes often grow most rapidly in an obscured state, such as in the dusty nuclei of major merger of gas-rich galaxies. I have started a new NASA Astrophysics Data Analysis Program project based on high-resolution archival data from Keck, ALMA, and other major observatories that will provide a multiwavelength baseline for understanding the nature of obscured accreting supermassive black holes in high spatial resolution observations. This work will address how and when active galactic nuclei are triggered, and the role they play in regulating the structure and dynamics of gas in their immediate vicinity. Results from the project will pave the way for elucidating the obscured state of supermassive black holes in nearby galaxy nuclei, such as those targeted by our GOALS team's approved JWST ERS-1328 program and my Cycle 1 GO Program-1717. I have thus far mentored two high school students and one masters student on the resolved SED part of this project; stay tuned for details!

Gas Physics around Accreting SMBHs

Black hole scaling relations often times provide the only available tool for measuring their masses in the distant universe. The calibrations for such mass estimates rest upon presumptions that distant quasars are similar to low-redshift reverberation-mapped AGN at low luminosities and Eddington ratios, which they are not. To test the robustness with which we can extend local virial assumptions about broad line region (BLR) gas dynamics to distant quasars, I led the latest spectroscopic campaign of the Lick AGN Monitoring Project with the main intent of investigating luminosity-dependent trends in BLR gas dynamics using velocity-resolved reverberation results.

Next-Generation Extremely Large Telescopes

The excitement brought about by the prospect of a 30-meter class telescope and an advanced suite of instruments cannot be overstated. As a SMBH enthusiast, I have been an active member of the TMT SMBH International Science Development Team: developing the AGN fueling and feedback section in the Detailed Science Case and providing quantitative constraints for IRIS and MICHI in the science flow-down documents for engineers. As part of the IRMS Science Team, I have written its exposure time calculator for its Operational Concept Definition Document. I have since broadened my enthusiasm to the GMT since becoming a contributor to the US ELT Program within the Galaxy Evolution and Supermassive Black Hole working group, providing my vision of achieving transformative science with the ELTs in a variety of Astro2020 Decadal White Papers.

Star-forming Galaxies in the Early Universe

While the local universe provides the ideal laboratory for understanding detailed physics via high-resolution studies of individual galaxy systems, much of star formation activity peaked at higher redshifts. Although spatially-resolved studies are currently unfeasible at higher redshifts, a powerful technique to understand the nature of galaxies since the epoch of reionization is through the combination of spectroscopic and morphological characterization, particularly in the modern era of large surveys. In our study of 300 Lyα emitters at 3 < z < 7 in the GOODS and COSMOS fields, we have quantitatively analyzed and found a correlation between the Lyα spectral line profile and rest-frame UV morphology. Our finding suggests that these high-redshift galaxies are not likely to be intrinsically round and symmetric disks, but probably host galactic outflows traced by Lyα emitting clouds.

In my earliest undergraduate research days, I had worked on understanding the fundamental plane of moderate redshift galactic bulges in GOODS and the properties of Extremely Red Objects in the DEEP fields. These experiences have sparked my interests in the extragalactic sciences and triggered my journey therein ever since.

Distance to M33 via Stellar Atmosphere of Blue Supergiants

While the cosmological distance ladder is extended to very high redshifts with hitherto unknown precision, we are confronted with the fact that distances to many nearby galaxies where individual stars and HII regions can be resolved are disturbingly uncertain. The Triangulum Galaxy M33 suffers from such a dilemma where the distance moduli obtained with different techniques differ by as much as 30% in linear distance. Applying the flux-weighted gravity–luminosity relationship to the quantitative spectral analysis of blue supergiants, we computed a new distance modulus of 24.93±0.11 mag for M33. The significance of this work is three-fold: 1. Our distance has been widely adopted; it refines the FGLR calibration as a basis for wide application of this distance determination method. 2. Our distance has been compared to that from HST observations of Cepheids and with the tip of the red giant branch using HST ACS images to constrain reddening assumptions. 3. Our measurement of metal abundance and metallicity gradient have notable implications for those who study the resolved stellar properties in galaxies.