Luke Zoltan Kelley

theoretical astrophysicist

Lindheimer Prize Postdoctoral Fellow, CIERA (Northwestern University)
Astrophysics Working Group Chair, NANOGrav Collaboration

About Me

curriculum vitae (CV)

I am currently a postdoctoral scientist at CIERA (Northwestern University). I previously earned my PhD from Harvard University in 2018 under the supervision of Prof. Lars Hernquist. As an undergraduate, I worked with Prof. Enrico Ramirez-Ruiz at UC Santa Cruz, not far from my hometown of Berkeley, CA. My research is in astrophysics theory at the intersection of high-energy transients and cosmological environments. My goal is to utilize both gravitational wave and electromagnetic observations to understand the formation and evolution of the most energetic objects in the universe.

My focus has been primarily on multi-messenger astrophysics with low-frequency gravitational waves: those produced by binaries of massive black holes (MBHs). This new class of gravitational waves will soon be detected by pulsar timing arrays like NANOGrav, which will revolutionize our understanding of MBH and galaxy coevolution within the next decade. I also study stellar tidal disruption events, the detailed structure of active galactic nuclei, LIGO sources, and the reciprocal relationship of explosive transients and galactic environments.

Research

publication list [NASA ADS]

Massive Black Hole (MBH) Binary Evolution

Two MBH are brought together during the merger of their host galaxies, which occurs on megaparsec scales. Only at about milliparsec separations can an MBH binary emit gravitational waves and eventually coalesce. The only way for the binary to traverse this vast range of scales is through environmental interactions, like dynamical friction and stellar scattering, with the host galaxy. This process typically takes billions of years and only a fraction of binaries coalesce at all.

During this process, a number of important processes are underway. Detailed circumbinary-disk models have recently shown that accretion onto the binary can favor the secondary, which can significantly alter the masses and dynamics. The spin of both MBHs also evolve over time, and if the spins are significant at the time of coalescence, can produce large recoil 'kicks' of the remnant---possibly ejecting it from the galaxy. Precesion in the GW waveform might also be detectable with LISA.



Low-frequency Gravitational Waves (GWs)

Once massive black hole binaris (MBHB) reach milliparsec scales, their GW emission becomes detectable by 'pulsar timing arrays'. There are generally three classes of signals: a stochastic 'GW Background' produced by the superposition of many binaries emitting across the Universe (purple lines), 'continuous GW' sources which are loud enough to resound above the background (red dots), and 'bursts' when the binaries finally coalesce.

The GW background encodes a huge amount of information about massive black holes, about their binary evolution, and even about their host galaxies. The low-frequency turnover in the background's spectrum, for example, tells us about how much binaries typically interact with their local stellar and gaseous environments. Continuous GW sources are also very exciting, particularly because we might be able to detect electromagnetic counterparts of those binaries, making them powerful multimessenger sources.



Electromagnetic (EM) Counterparts

When massive black holes are accreting enough material to be observable, we call them active galactic nuclei (AGN). There are many possible EM signatures of AGN in binaries, particularly photometric periodic-variability and offset spectroscopic emission lines. In both cases large populations of candidate binaries have been identified, however, there is growing evidence that many (or even most) of them are false positives, due to the high intrinsic variability of single-AGN systems.

Determining robust EM signatures is very important as the detection of both GW and EM signatures from MBH binaries opens up a huge, new discovery space to constrain their binary evolution, host-galaxy interactions, and even to construct cosmological distance measurements.



Structure of Active Galactic Nuclei (AGN)

One of the biggest discoveries in MBH physics was 'AGN unification', when people realized that the many different types of AGN (e.g. 'Type I' with broad lines, vs. 'Type II' only narrow lines) were actually the same basic structure but viewed from different observer angles. Much of the details of AGN structure, however, is completely unknown: we know the pieces, but we don't understand exactly how they're put together. Ongoing and upcoming surveys are delivering a vast amount of observational data that can be used to understand the detailed structure of AGN. I'm particularly excited about what time-dependent information (e.g. photometric variability and changing broad emission-lines).



Short Gamma Ray Bursts (SGRBs) and LIGO-Virgo Sources

SGRBs are now known to be produced by the merger of neutron star (NS) binaries, which also produce (high-frequency) gravitational waves detectable with ground-based interferometers like LIGO-Virgo. These systems go through an exotic evolution: requiring two supernovae, typically a 'common envelope' phase, and then GW driven coalescence. During the supernovae, and associated mass-loss, the binary system can recieve a large velocity 'kick', which can launch the system into wide orbits around the host galaxy, or even eject the system altogether.

I am very interested in the information that SGRBs and their host-galaxy environments can tell us. The location of mergers relative to the host galaxy encode information about the binary's formation, its lifetime, and its binary evolution. The explosive emission from the merger also serves as a precision probe of the local galaxy, and all of the intervening matter between it and Earth.



Stellar Tidal Disruption Events (TDEs)

One of the biggest discoveries in MBH physics was 'AGN unification', when people realized that the many different types of AGN (e.g. 'Type I' with broad lines, vs. 'Type II' only narrow lines) were actually the same basic structure but viewed from different observer angles. Much of the details of AGN structure, however, is completely unknown: we know the pieces, but we don't understand exactly how they're put together. Ongoing and upcoming surveys are delivering a vast amount of observational data that can be used to understand the detailed structure of AGN. I'm particularly excited about what time-dependent information (e.g. photometric variability and changing broad emission-lines).