Abstract

It is generally believed that most massive galaxies host supermassive black holes (SMBHs) at their centers, with masses ranging from 10^6 -10^{10} M☉. During phases of rapid accretion, these SMBHs emit electromagnetic radiation across the entire spectrum, resulting in luminous active galactic nuclei (AGN). An open question is whether SMBHs can grow to exceed 10^{10} M☉ through continued accretion and successive mergers, potentially forming hypermassive black holes (HMBHs).  The broad line region (BLR) consists of dense gas clouds located within sub-parsec distances of the central SMBH. The clouds are photoionized by continuum radiation emitted by the accretion disk and hot corona, producing emission lines. These clouds move at several thousand km/s under the influence of the SMBH's gravitational potential, producing Doppler-broadened emission lines. Standard accretion disk theory predicts that, at constant Eddington ratio, the bolometric luminosity of AGN scales linearly with black hole mass, while simultaneously cooling the inner disk as T ∝M^{-1/4}. As a result, the spectral energy distribution (SED) becomes softer, shifting the thermal peak of the spectrum to  lower frequencies and reducing  the fraction of ionizing photons relative to the continuum. The softer spectrum, in combination with the Doppler broadening, suggests that higher SMBH mass leads to weaker but broader emission lines. The goal of this project is to determine how AGN spectral properties such as equivalent width, line luminosity, and line profile scale with black hole mass and whether these mass-dependent spectral signatures can serve as diagnostics of black hole mass from single-epoch spectra.  This thesis couples the spectral energy distribution model AGNSED \citep{Kubota_Done_2018} with the photoionization codes CLOUDY \citep{cloudy} and the multi-cloud BLR modeling code BELMAC \citep{saraz_2025} to compute broad emission-line spectra for 16 emission lines spanning six different black hole masses. We construct a  Keplerian disk model to reproduce low-ionization lines and a biconical outflow model to reproduce high-ionization lines, in order to examine the effect of black hole mass on spectral properties. We then combine the Keplerian disk and biconical outflow component into a two-zone BLR model to simultaneously reproduce both high- and low-ionization lines.  We find that the Keplerian disk model reproduces the equivalent widths of low-ionization lines more reliably than the biconical outflow model against the composite quasar spectrum developed by \cite{composite}. The two-zone model reproduces the full low- and high-ionization line spectrum simultaneously across all 16 lines. The equivalent widths of most emission lines decrease with increasing black hole mass as the ionizing photon fraction declines, while the line profiles broaden as v ∝M^{1/4}. At M = 10^{11} M ☉, high-ionization lines show an inverse equivalent width trend attributed to a transition to failed outflow kinematics. Together the equivalent width and the inter-percentile velocity at 50% (IPV50) define a mass diagnostic plane in which HMBHs with M >= 10^{10} M ☉ occupy a distinct lower-right region, providing a single-epoch diagnostic for identifying HMBH candidates.

Publication Date

8-2026

Document Type

Thesis

Student Type

Graduate

Degree Name

Astrophysical Sciences and Technology (MS)

Department, Program, or Center

Physics and Astronomy, School of

College

College of Science

Advisor

Andrew Robinson

Advisor/Committee Member

Michael Richmond

Advisor/Committee Member

Manuela Campanelli

Campus

RIT – Main Campus

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