Embedded Files
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?
My current project involves comparing electromagnetically-measured quasar luminosity functions to the gravitational wave-inferred black hole mass functions. I will constrain the distribution of Eddington ratios and active fractions are necessary to unify these two constraints.
Recent First-Author Publications
Recent First-Author Publications
Gravitational Wave measurement of the Mbh-Mbulge Intrinsic Scatter at High Redshift
A plot of intrinsic scatter verus redshift. Figure shows several models fit to gravitational wave data compared to electromagnetic measurements. Image is linked to image source.
Find the paper here, pictured above is Figure 5.
This figure demonstrates how gravitational wave-based models for the Mbh-Mbulge scatter evolution (lines) are in good agreement with measurements from electromagnetic data (individual markers).
The observed GWB spectrum is higher in amplitude than model predictions by a factor of 2-3. I evaluated the effect of a high-scatter supermassive black hole (SMBH) scaling relation (Mbh-Mbulge) on models of the nanohertz gravitational wave background (GWB). By implementing an intrinsic scatter of the Mbh-Mbulge relation, which is larger at higher redshift, but matches local observations, I found that the amplitude of GWB models increases to be consistent with the low-frequency end of the GWB spectrum. Models with positively evolving intrinsic scatter can reproduce the electromagnetically observed overmassive SMBHs at 4 < z < 6 without changing the Mbh-Mbulge normalization though we find that including moderate normalization evolution marginally improves fits to the GWB data. The results of this work imply that the Mbh-Mbulge relation we see today is not universal throughout cosmic time and that a diversity of seeding models and growth mechanisms may be at play in the early stages of SMBH-galaxy evolution.
Page updated
Report abuse