Abstract

During cosmic timescales, supermassive binary black holes (SMBBHs) form by galaxy mergers, where each galaxy hosts a supermassive black hole (SMBH) at its center. By studying SMBBHs, we can gain insights into galaxy evolution and black hole growth. However, the typical separation between black holes in SMBBHs is usually below 1 pc, making them difficult to resolve using direct imaging or photometry. To be able to distinguish binary black holes (BBHs) from typical AGN powered by single black holes (SBHs), we conducted this research project to develop a new diagnostic method to identify a unique feature of SMBBHs, which can be used to distinguish AGN powered by BBHs from those powered by SBHs. The basic idea of this method is that BBHs have different configurations compared with those of SBHs, such as the circumbinary disk enveloping the whole binary system, the minidisks around each black hole, and the streams between the circumbinary disk and minidisks. It is these different configurations of accretion disks of black holes that lead to the difference in the spectral energy distributions (SEDs), which in turn will differently photoionize the broad line region (BLR) and produce different strengths of broad emission lines. Thus, it is these differences in strengths of broad emission lines that we expect to see between two different configurations of black holes, either binary or single black holes. By identifying these differences in line strengths, we can distinguish between binary and single black holes. In order to achieve this goal of distinguishing BBHs from SBHs, we organized our project in the steps below: (1) obtained BBH SEDs from \cite{gutierrez_em_2022}, (2) generated SBH SEDs using XSPEC modeling code OPTXAGNF, (3) produced single-cloud models, which represent the BLR, input BBH and SBH SEDs into the models, and simulated the single0cloud response with a photoionization code Cloudy, (4) built cloud-ensemble models, which represent a more realistic BLR, input BBH and SBH SEDs into the models, and simulated the emission-line response within those clouds using a broad emission line mapping code BELMAC. In step (3), we have explored the differences in line ratios between BBH and SBH for a representative single-cloud photoionization model. The emission lines we used here are: Si IV $\lambda 1400\text{\AA}$, C III] $\lambda 1909\text{\AA}$, C IV $\lambda 1549\text{\AA}$, Mg II $\lambda 2798\text{\AA}$, and Ly$\alpha$ $\lambda 1216\text{\AA}$. It turned out that differences do exist, but they are too small to be identified in observational data. Furthermore, we have investigated the line equivalent widths predicted by SBH and BBH models respectively. By doing so, we found some apparent differences between BBHs and SBHs in some specific emission lines: Ly$\alpha$, CIV, and H$\alpha$. However, these differences vanish at the highest mass of black holes ($10^9 M_\odot$). In step (4), we continued the investigation of the equivalent width between SBH and BBH BLR cloud-ensemble models and found that some emission lines show the difference between BBHs and SBHs, such as CIV in the case of BH mass $10^7 M_\odot$, $U\propto r^{-2}$, and $\log{n/ 1 \text{ cm}^{-3}} = 10.5, 11.0$. For the highest mass in the case of $U\propto r^{-2}$, the results are similar to the one in single-cloud models: no difference is shown between BBHs and SBHs across all emission lines. Most importantly, in this step, we found that for CIV $\lambda 1549\text{ \AA}$ in the case of a black hole mass of $10^7 M_\odot$, $s=-2$, and $\log{n / 1\text{ cm}^{-3}} = 10.5$, only the SBH EW falls inside the range of the observed range in SDSS DR7 Quasar Catalog while the BBH EW falls outside the range and becomes an outlier. This is what we want to find to distinguish BBHs from SBHs in the observational data.

Publication Date

7-2024

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

Manuela Campanelli

Advisor/Committee Member

Michael Richmond

Campus

RIT – Main Campus

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