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Background
Background
A cartoon gif of a supermassive black hole depicted as an owl with stars and compact objects in its orbit by Chris Smith. Image is linked to image source.
What are Supermassive Black Holes?
What are Supermassive Black Holes?
Black holes are extremely compact objects. Supermassive black holes (SMBH) are black holes that have masses at least a million times the mass of the Sun. We now know that roughly every massive galaxy in the universe hosts a SMBH in their center. For very nearby galaxies we can measure SMBH properties such as mass through direct methods such as Schwarzschild modeling or reverberation mapping. For more distant SMBHs we have to get clever and use relationships between SMBH properties and host galaxy properties to infer the SMBH masses. A solid understanding of these objects is important for the study of cosmology, galaxy evolution, general relativity, and many other fields of astrophysics.
A plot of characteristic strain verus frequency showing the sensitivity curves for different gravitational wave observatories / techniques. Image is linked to image source.
What is the Gravitational Wave Background?
What is the Gravitational Wave Background?
It is theorized that, following a galaxy merger, the central supermassive black holes (SMBH) can become gravitationally bound forming a binary. In the final stages of inspiral, before merging, the binary will lose energy and momentum through gravitational radiation which propagates away via gravitational waves. The superposition of the gravitational waves emitted by a population of SMBH binaries is expected to generate a gravitational wave background (GWB). In 2023, four different pulsar timing array collaborations found strong evidence for a common quadrupolar signal consistent with the expected GWB (but with a higher amplitude than predicted!). If the GWB is indeed produced by SMBH binaries, then we can use the data to infer information about the SMBH population. Especially when combined with electromagnetic data, the GWB is a powerful tool for studying black holes.
A figure showing the correlation between SMBH mass and galaxy velocity dispersion by Kayhan Gültekin. Image is linked to image source.
Black Hole Mass Correlates with So Many Galaxy Properties
Black Hole Mass Correlates with So Many Galaxy Properties
The locally constrained correlation between SMBH mass and galaxy properties imply a tight evolutionary link between SMBHs and their host galaxies. The three most commonly used scaling relations are between SMBH mass and galaxy bulge luminosity, stellar mass, or velocity dispersion. These three relations are well studied for nearby galaxies and can be extrapolated to help us predict what sort of masses we expect the SMBH population to have at high redshift. My work focuses on the latter two, also called the Mbh-Mbulge and Mbh-σ relations. I want to know whether these relationships have been the same throughout cosmic time or if they have changed. Changing relations have implications for SMBH-galaxy coevolution and would have an impact on inferred SMBH mass outside the local universe. My work has shown that extrapolating Mbh-Mbulge and Mbh-σ lead to different predicted number densities of SMBHs at the high- and low-mass regimes. I have also shown that the high amplitude of the gravitational wave background can be explained well by a Mbh-Mbulge that evolves with redshift.
My Projects
My Projects
General Interests
General Interests
I am focused on methods of inferring supermassive black holes (SMBH) mass from galaxy properties. In particular, many of my projects aim to establish constraints on how and when the local scaling relations were established. I use a mix of electromagnetic and gravitational wave data to place limits on how the Mbh-Mbulge and Mbh-σ relations evolve. Establishing how these scaling relations behave outside the local universe informs our models for feedback mechanisms in SMBH accretion and galaxy star formation. The ultimate goal of my work is to develop an accurate and robust model for the SMBH mass distribution across time, especially since z ~ 3.
What am I doing now?
What am I doing now?
I am currently applying for postdoc jobs. Meanwhile I am thinking about the best ways to use EM and GW data to differentiate between types of Mbh-Mbulge evolution.
Recent First-Author Publications
Recent First-Author Publications
A Unified Model for the Gravitational Wave Backround and Quasar Luminosity Functions
The top panel shows three best-fit quasar luminosity functions against data. Two fits match the data well, one falls short at the high-luminosity end. The lower panel shows the Eddington ratio distribution function (ERDF) for two models from the top panel. The well-fitting model has an ERDF that favors lower Eddington fractions than the other model. A vertical line indicates the Eddington limit.
Find the paper here, pictured above is adapted from Figures 3 and 4.
This figure compares the best-fit quasar luminosity functions (QLF) compared to data and the global fit A from Shen et al. (2020). The blue dashed line used an SMBH population that was inferred from the observed GSMF and extrapolated local Mbh-Mbulge relation. The red dotted line used an SMBH population that is inferred from the GWB and uses an Mbh-Mbulge relation that evolves positively with redshift. Here we see that the GWB-inferred SMBH population matches the QLF data as well or better than the fiducial model while also requiring less extreme Eddington fractions.
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