Date of Award

May 2021

Document Type


Degree Name

Doctor of Philosophy (PhD)


Physics and Astronomy

Committee Member

Marco Ajello

Committee Member

Dieter Hartmann

Committee Member

Bradley Meyer

Committee Member

Mark Leising

Committee Member

Xian Lu


Blazars are an extreme class of active galactic nuclei (AGN) whose bright and violently variable panchromatic emission is ascribed to the presence of collimated relativistic jets closely aligned to our line of sight. The flux enhancement due to relativistic beaming makes them visible to high redshifts (z>2), rendering them extraordinary beacons to explore the early universe. Among them are the so-called ‘MeV blazars’, which are the most extreme objects of this class and whose inverse Compton (IC) peak lies at MeV energies. Found even beyond z = 5, their large jet power, high bolometric luminosities (Lbol > 10^48 erg s^-1), and black hole mass (MBH>10^9 Msun) set them apart.Since their emission is highly beamed, the detection of a single source implies the existence of 2Gamma^2 (i.e. ~450 for bulk Lorentz factor, Gamma~15) quasars with similar properties, at the same z, with jets pointed elsewhere. In other words, the discovery of each new source allows us to constrain the size of the parent population and to quantify the supermassive black-hole space density at z > 3. Moreover, blazars are the dominant source class to make up for the high-energy backgrounds, from MeV to gamma-ray. It is, therefore, paramount to perform population studies of this intriguing source class to understand how the most powerful jets and massive black holes have evolved in the history of time and how much they are contributing to the total electromagnetic output of our universe.

My Ph.D. thesis work is presented here and has been divided into two main branches. First, find high-redshift sources through hard X-rays and gamma-ray monitoring, and subsequently perform broadband (near-infrared to gamma-ray) data analysis and modeling (Marcotulli 2017, Marcotulli 2020a). This work determined key parameters, such as the bulk Lorentz factor (Gamma) found to systematically exceed the accretion one, hinting at an extra energy reservoir (e.g., black hole spin) and confirming a trend found for lower redshift blazars. Moreover, the black hole masses are all estimated to be MBH> 10^9 Msun. On the other hand, to understand the preferred evolutionary paths of these jets and their contribution to the high-energy extragalactic astrophysical backgrounds, I derived the source count distribution of blazars at gamma-rays, and their luminosity function at hard X-rays. This work (Marcotulli 2020b) enabled me to determine that gamma-ray blazars likely evolve in density (i.e. sources are more numerous at earlier times) and they can only contribute up to ~60% of the total extragalactic gamma-ray background. At hard X-rays, the blazar luminosity function (Marcotulli et al. in prep) confirms that bright X-ray blazars evolve in luminosity (i.e. sources are more luminous at earlier times) and peak at z>4, when the universe was ~1 billion years old. This provides further evidence that jet activity may be linked to fast black hole accretion early on in the universe. This study also predicts that these sources may contribute up to 100% of the MeV background.



To view the content in your browser, please download Adobe Reader or, alternately,
you may Download the file to your hard drive.

NOTE: The latest versions of Adobe Reader do not support viewing PDF files within Firefox on Mac OS and if you are using a modern (Intel) Mac, there is no official plugin for viewing PDF files within the browser window.