Research
Background
What are Supermassive Black Holes?
Black holes are extremely compact objects. Supermassive black holes (SMBH) are black holes that have masses a million times the mass of the sun or more. 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 farther SMBHs we have to get clever and use relationships between SMBH properties and host galaxy properties. 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. Evenetually the binary will merge, but just before that the system loses energy and momentum through several mechanisms, the last of which is gravitaional radiation also known as gravitational waves. The gravitational wave background (GWB) is the superposition of all the gravitational waves emitted from SMBH binaries. The GWB is a constant, random signal and several different pulsar timing array collaborations such as NANOGrav announced in mid-2023 that they detected this signal.
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
No really, 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 stellar mass, luminosity, and/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 M-M and M-σ relations respectively.
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 Work
General Interests
I focus on supermassive black holes (SMBH). I am particularly interested in how SMBH mass is related to various properties of the galaxy they reside in such as stellar mass and velocity dispersion. I apply this understanding to aid in the interpretation of the gravitational wave background (GWB). I plan to continue working with SMBHs and the GWB, the next steps include: investigating and constraining the potential for redshift evolution in SMBH - galaxy scaling relations, jointly analyzing the GWB and quasar populations to at high redshift, testing z > 1 evolution of the galaxy mass fundamental plane, and much more!
What am I doing now?
I am extending my most recent project by characterizing redshift evolution of SMBH-galaxy scaling relations. I am working with holodeck to determine how evolving scaling relations affect our predictions and interpretation for the GWB.
Recent Publications
Find the paper here
The goal of this project was to determine if one's choice of SMBH mass - galaxy scaling relation had a measureable effect on the predictions for the GWB amplitude. I investigated the SMBH mass populations predicted from both host stellar mass and velocity dispersion and compared the distributions (see, e.g., above).
Frequently, galaxy stellar mass is used when applying scaling relations outside of the local universe because other properties like velocity dispersion are resource intensive to measure for large volumes of galaxies. From SDSS and 3D-HST+CANDELS (0 < z < 3), I used the mass fundamental plane of galaxies to infer velocity dispersions for each galaxy from it's stellar mass and half-light radius, this cirvumventing the spectral limitations.
When predicting masses from velocity dispersion, I found, among other things, that there were higher numebers of the highest mass SMBHs than when using stellar mass. This is important because the GWB amplitude is impacted most by the most massive SMBHs and so my findings suggest that, using different scaling relations predicts significantly different GWB amplitudes.