Searching for Post-Merger Recoiling Black Holes Open Access

Abstract

When galaxies merge, their central supermassive black holes will ultimately merge as well. During the merger, they can gain velocities with respect to their host galaxies. This occurs when the emission of gravitational wave radiation during the merger is non-isotropic. In this project, we search for a spectroscopic signature of recoiling black holes in quasars. Quasars have multiple Doppler broadened emission lines. These lines can be identified as originating in different parts of the quasar depending on if the line is broad or narrow. The broad lines originate from gas gravitationally bound to the black hole, and narrow lines from further out in the host galaxy. We use spectral data from the Sloan Digital Sky Survey, and fit the emission lines with the software pyQSOFit. We search for objects of interest by calculating the velocity of the broad line with respect to the narrow line. We fit approximately 50,000 quasar spectra, focusing on broad H𝛽 and the narrow forbidden line [OIII]. The resulting velocities are consistent with the predicted range of recoil velocities of post-merger black holes. A number of outliers, with distinct spectral features such as double-peaked broad lines, were identified within the sample.

Table of Contents

Contents

1 Introduction 1

1.1 What is an AGN? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 What is Gravitational Wave Radiation? . . . . . . . . . . . . . . . . . . . . . . 5

1.3 Quasar Mergers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.4 What are Kicks and How can we Identify Them? . . . . . . . . . . . . . . . . 8

2 Methods 13

2.1 Sloan Digital Sky Survey (SDSS) & Spectra . . . . . . . . . . . . . . . . . . . . 13

2.2 PyQSOFit & Fitting Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.3 Data Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.3.1 Manual Review of Spectra . . . . . . . . . . . . . . . . . . . . . . . . . 25

3 Results 29

3.1 Comparison with Expected Results . . . . . . . . . . . . . . . . . . . . . . . . 29

4 Analysis 31

4.1 Interesting Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4.2 Improved Velocity Plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

5 Next Steps 39

5.1 Cross Examination with Broad Lines . . . . . . . . . . . . . . . . . . . . . . . 39

5.2 Model Velocity Histogram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

6 Conclusion 42

7 Appendix 44

About this Honors Thesis

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School	Emory College
Department	Physics and Astronomy
Degree	B.S.
Submission	Honors Thesis
Language	English
Research Field	Physics, Astrophysics
Keyword	Spectroscopy Quasar AGN Black Hole Black Hole Merger
Committee Chair / Thesis Advisor	Bonning, Erin, Emory University
Committee Members	Tezzele, Marco, Emory University Bans, Alissa, Emory University

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