Research

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Background

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 pretty much every galaxy in the universe contains one of these SMBHs in their center. For very nearby galaxies we can measure SMBH properties such as mass through, e.g., dynamical methods. For more distant SMBHs we have to get clever and use relationships between SMBH properties and host galaxy properties to infer the black hole mass function. A solid understanding of these objects is important for the study of cosmology, galaxy evolution, general relativity, and many other fields of astrophysics.

What is the Gravitational Wave Background?

It is theorized that two supermassive black holes (SMBH) can become gravitationally bound to one another at the center of a galaxy forming a binary. In the final stages of inspiral, before merging, the system loses energy and momentum through gravitational radiation which propogates away via gravitational waves. The gravitational wave background (GWB) is the superposition of all the gravitational waves emitted from SMBH binaries. The GWB is a constant, stochastic signal and four different pulsar timing array collaborations have found strong evidence for this signal. 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

It's hard to find galaxy characteristics that don'e correlate with SMBH mass! Some commonly used relations include that between SMBH mass and galaxy bulge luminosity, stellar mass, or velocity dispersion. These three relations are well studied for nearby galaxies and can help us predict what sort of masses we expect the SMBH population to have at high redshift. I mainly focus 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.

Galaxy Scaling Relations

Certain characteristics of each galaxy will be related to other properties of that same galaxy. When we find that the same properties are correlated for a large number of galaxies we can define a scaling relation that can be applied to other galaxies. For my work I use the mass fundamental plane (MFP) which tells us that stellar mass, velocity dispersion, and half-light radius all lie on a relatively thin plane in 3D space. This three-way relation is such that, for any massive galaxy, if you have estimates for two of the properties in the MFP, you can predict the third with reasonable accuracy.

A rotating gif demonstrating the 3D relation between galaxy stellar mass, half-light radius, and velocity dispersion by Rachel Bezanson. Image is linked to image source.

My Projects

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?

I am working with holodeck to determine how evolving scaling relations affect our predictions and interpretation for the GWB. At the moment I am exploring evolving Mbh-Mbulge scatter. I also plan to adapt holodeck so that I can test the impact of using the M-σ relation on the GWB predictions.

Recent Publications

Inferring Mbh-Mbulge Evolution from the Gravitational Wave Background

Find the paper here, pictured above is Figure 8.

This figure demonstrates that, the high amplitude of the nanohertz gravitational wave background (GWB) requires either an increased number density of galaxies, relative to observations, (y-axis) or a high redshift black hole / galaxy mass ratio that is increased relative to the local Mbh-Mbulge relation (x-axis).

Models that can reproduce the GWB spectrum require a high number density of the most massive black holes. These models imply a black hole density that is greater than predicted from the observed galaxy population and the inferred black hole mass function from the Mbh-Mbulge relation.

This project tested the impact of an evolving version of the Mbh-Mbulge relation on the interpretation of the GWB. If the black hole / galaxy mass was higher in the past, then the local Mbh-Mbulge relation would under-predict black hole masses (and therefore the GWB amplitude) for these distant black holes. An evolving Mbh-Mbulge relation can reproduce these massive black holes needed to explain the GWB. The evolution I find is also consistent with observational constraints for redshifts 2 < z < 7.

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