Weighing supermassive black holes

Thesis

Supermassive black holes (SMBHs) drastically influence the evolution of their host galaxies, as evident in the tight correlations between SMBH mass and galaxy properties. Yet, many fundamental questions about how SMBHs form, grow, and co-evolve with galaxies are unanswered, due to uncertainties in local SMBH mass measurements and the lack of direct dynamical measurements of SMBH masses beyond the local Universe.

In this thesis, I first present three metrics that evaluate the precision and accuracy of SMBH mass measurements with molecular gas or megamaser dynamics. Applying them to all existing SMBH mass measurements using the two methods, I demonstrate the importance of high angular (and thus spatial) resolution observations to reduce SMBH mass uncertainties and show that Atacama Large Millimeter/sub-millimeter Array (ALMA) observations can provide the most precise and accurate SMBH mass measurements across large samples of galaxies.

Then, I present measurements of the SMBH masses of the local lenticular galaxies NGC 383 and NGC 1574 using high-resolution ALMA observations of the CO(2-1) emission line. Both measurements improved the precision and accuracy of the SMBH masses compared to lower-resolution measurements. In particular, the NGC 383 measurement is the highestresolution and thus the most accurate SMBH mass measurement using the molecular gas method to date.

Next, I discuss the prospects for the first cold-gas dynamical SMBH mass measurements at high redshifts, by utilising strong gravitational lensing as a magnifying glass. I also present candidate targets for such measurements, along with their available ALMA data. Additionally, I address a challenge for such measurements – accurately modelling the stellar mass distribution in dusty galaxies – by developing a neural network that efficiently deprojects stellar mass profiles.

Finally, I present an upper limit on the SMBH mass in the lensed sub-millimetre galaxy ID141 at redshift 𝑧 = 4.24, demonstrating the potential of gas-dynamical modelling of strongly lensed galaxies to directly constrain the masses of high-redshift SMBHs and their correlations with galaxy properties.

Overall, this thesis establishes high-resolution observations as the key to reduce the uncertainties of local SMBH mass measurements with ALMA. It also establishes the method to obtain the first high-redshift dynamical measurements of SMBH masses from cold gas kinematics.

Authors: Zhang, H

• DOI: 10.5287/ora-9r8898r9q
• Type of award: DPhil
• Awarding institution: University of Oxford
• Language: English
• Keywords: interferometry; interstellar medium; galaxy evolution; galaxy dynamics; gravitational lensing; supermassive black holes; submillimeter astronomy
• Subjects: Astrophysics