Universität zu Köln

Ricci, Luca ORCID: 0000-0002-4175-3194 (2024). Jet formation and propagation in the nearby radio galaxy NGC 315. PhD thesis, Universität zu Köln.

PDF (Doctoral dissertation) Doctoral_dissertation_publication.pdf - Published Version Download (38MB)

Abstract

Since their discovery many decades ago, relativistic jets have intrigued and fascinated astronomers. Besides being crucial elements in shaping the Universe as we observe it today, jets are unique laboratories to test and broaden our knowledge in many fields of physics, such as plasma physics and cosmology. This thesis aims at investigating the relativistic jets launched from radio galaxies, a specific class of Active Galactic Nuclei (AGN), with the goal of understanding the jet formation and propagation phenomena, as well as the connection between accretion and ejection. Specifically, the work focuses on NGC 315, a giant radio galaxy whose vicinity and large mass of the central black hole offer a unique opportunity to address these subjects. The study combines two approaches: the analysis of Very Long Baseline Interferometry (VLBI) observations is accompanied by a theoretical study carried out by means of numerical simulations. These are performed using a relativistic magnetohydrodynamic (RMHD) code. The thesis is composed of three introductory chapters followed by three results chapters. In Chapter 1, I introduce the fundamental physical and astronomical concepts concerning AGN, which are central throughout the thesis. In Chapter 2, I describe the basic elements of plasma physics and provide an overview of the state-of-the-art knowledge regarding the theory of jet formation and propagation together with the currently open questions. In Chapter 3, I explain the fundamental concepts concerning the VLBI technique, and how to perform the analysis of VLBI data. Finally, Chapters 4, 5, and 6 present the series of works on NGC 315, with Chapters 4 and 6 focusing on the observational studies and Chapter 5 describing the theoretical one. From the analysis of a multi-frequency and multi-epoch VLBI data set, the extents of the jet collimation and acceleration regions in NGC 315 are suggested to coincide. Namely, the jet is observed to collimate within ~ 0.6 pc from its apex, and to accelerate up to v ~ 0.9c on the same scale. The co-spatiality of the two phenomena is in agreement with theoretical predictions for magnetically-driven outflows, in which the magnetic energy is progressively converted into kinetic energy of the bulk flow. Remarkably, a very steep spectral index down to ~ -2 is observed in the same region, in agreement with the findings for the jet in M 87. The spectral properties in the jet acceleration and collimation region are still poorly studied, and the possible physical processes leading to the steep spectrum, such as magnetic reconnection and cooling due to synchrotron losses, are discussed. Different and independent approaches considering the jet brightness temperature and turnover frequency along the jet indicate that the magnetic field strength increases (quasi-)linearly from parsec scales up to the jet apex. Such a behavior, together with measured strengths of ~ 0.1 G on parsec scales, implies magnetic fields of 100 - 1000 G in the nuclear region. The combination of the observational constraints, especially on the jet magnetization, together with theoretical models suggests that the jet in NGC 315 is launched by a maximally spinning black hole surrounded by a Magnetically Arrested Disk. The synergy between the observational results and the RMHD simulations further suggests that thermal acceleration, in addition to magnetic acceleration, plays a relevant role in increasing the jet speed on sub-parsec and parsec scales. In conclusion, the simulations hint at the necessity of winds launched from the accretion disk to favor the propagation of relativistic jets by delaying the growth of instabilities and by shielding the internal spine from the influence of the ambient medium.

Item Type:	Thesis (PhD thesis)
Translated title:	Title Language UNSPECIFIED English
Creators:	Creators Email ORCID ORCID Put Code Ricci, Luca lricci@mpifr-bonn.mpg.de orcid.org/0000-0002-4175-3194 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-719196
Date:	2024
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Physics > Institute of Physics I
Subjects:	Natural sciences and mathematics Physics
Uncontrolled Keywords:	Keywords Language Experimental physics English Astrophysics English Radio galaxies English Black holes UNSPECIFIED Jet physics UNSPECIFIED
Date of oral exam:	16 October 2023
Referee:	Name Academic Title Zensus, J. Anton Prof. Dr. Eckart, Andreas Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/71919