Universität zu Köln

Nikonov, Aleksei ORCID: 0000-0002-4009-9186 (2025). Relativistic outflow in the galaxy M87: The first hundred parsecs. PhD thesis, Universität zu Köln.

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Abstract

Relativistic jets from Active Galactic Nuclei (AGN) are highly collimated plasma streams (or "jets") accelerated to nearly the speed of light, extending from the vicinity of supermassive black holes (SMBH) out to megaparsec distances, into the intergalactic medium. These jets play a crucial role in the formation and evolution of galaxies. Additionally, they act as cosmic laboratories or natural particle accelerators, producing high-energy cosmic rays and neutrinos and enabling unique tests of fundamental physical models. One of the most prominent examples of an AGN jet is the one found in the giant elliptical galaxy M87, located approximately 17 megaparsecs from Earth. Over a century ago, before modern AGN and jet theories were developed, Heber Doust Curtis discovered a "curious straight ray" in M87, now recognised as a powerful relativistic jet. The galaxy M87 harbours a supermassive black hole with a mass of around six billion solar masses—roughly a thousand times more massive than the one in our own galaxy. The proximity of M87 and the large mass of its central SMBH make it an excellent target for AGN studies. In the radio regime in particular, the technique of very long baseline interferometry (VLBI) uniquely enables the most detailed probes of the central regions in M87. Recently, the Event Horizon Telescope produced the first-ever image of a black hole "shadow" in M87, thereby, for this first time, directly imaging the vicinity of a cosmic black hole down to its event horizon scale. The superb linear resolution achieved in VLBI imaging of M87 allows for the detailed study of the collimation, acceleration, and internal structure of its relativistic jet. This dissertation describes an investigation of the structure of the relativistic jet in M87 in linear scales spanning almost four orders of magnitude, from 0.01 to 100 parsecs. The introductory chapter explains the fundamental physical and astronomical concepts related to AGN. The first key finding, presented in Chapter 2, involves the discovery of a helical structure in the parsec-scale jet of M87, interpreted as Kelvin-Helmholtz instability (KHI) threads developing in a supersonic flow with a Mach number of around 20 and an enthalpy ratio of approximately 0.3. This was determined through the analysis of full-track dual-frequency VLBI observations conducted by the Very Large Baseline Array (VLBA), together with the Very Large Array and Effelsberg telescopes at 8 and 15 GHz, achieving a record dynamic range of about 20 000:1. Based on the two VLBI images obtained at 8 and 15GHz, a discussion of the spectral and polarisation properties of the jet emission is presented. The 8-15 GHz spectral index map reveals a complex structure, with flattening observed in areas where the helical threads intersect, further supporting the KHI origin of the jet internal structure. Polarisation emission is detected in the jet up to distances of 20 milliarcseconds from the VLBI core, with Faraday rotation manifesting a transverse gradient across the jet. The Faraday-corrected polarisation map indicates that the electric field position angle (EVPA) changes as a function of distance from the jet axis, suggesting the presence of a helical magnetic field. HST-1, a knot within the conical kiloparsec-scale jet of M87, was first observed by the Hubble Space Telescope. It is the first feature to be resolved near the jet's "core" on arcsecond scales. HST-1 exhibits superluminal speeds up to 6c and significant flaring activity across multiple wavelengths, from radio to gamma rays. The transition from parabolic to linear expansion of the flow believed to occur on these scales suggests the potentially important role played by HST-1 in the jet’s collimation. However, due to its distance from the jet base and smearing effects limiting VLBI’s field of view, HST-1 remains a challenging feature to study in detail, with high-resolution VLBI data and spectral information still lacking. In Chapter 3, properties of the jet on hectoparsec scales are discussed, and quad-frequency observations of HST-1 at 2, 5, 8, and 15 GHz using the VLBA and EVN arrays are presented. The VLBI observations presented in Chapter 3 have allowed for accurate reconstruction of spectral index maps and, notably, for obtaining the first turnover frequency map of HST-1. The spectral index maps reveal a uniform structure with a steep spectrum slope of approximately -0.7, suggesting that HST-1 is likely part of an optically thin jet, rather than a standing shock. The turnover frequency maps also enabled the estimation of HST-1’s magnetic field strength, found to be of the order of one milligauss. Chapter 4 of the dissertation deals with specific issues pertaining to imaging sparsely sampled interferometric data. While the CLEAN algorithm is the standard method for image reconstruction in radio astronomy, the necessity of supervised reconstruction in the hybrid image involving CLEAN deconvolution can introduce biases, which may produce artefacts in the final image—an issue observed in the spectral index maps from Chapter 2. In order to alleviate these potential biases, in Chapter 4, a novel Bayesian image reconstruction algorithm, resolve, is applied to RadioAstron observations at 2 and 5 GHz. This method corrects for amplitude and phase gains for each antenna and polarisation mode by modelling the temporal correlation of the gain solutions. A variational inference method is used to estimate uncertainties in both the gain and the image. The resulting images fully reproduce the filamentary, helical structures observed earlier, providing further support for the KHI interpretation of the jet threads.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Nikonov, Aleksei anikonov@mpifr-bonn.mpg.de orcid.org/0000-0002-4009-9186 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-785248
Date:	2025
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
Uncontrolled Keywords:	Keywords Language M87, VLBI, galaxies, interferometry, dissertation, astronomy, astrophysics, jets, relativistic English М87, РСДБ, галактики, интерферометрия, диссертация, астрономия, астрофизика, джеты, релятивистские истечения Russian М87, ОУБР, галактиканы чинчийии, халлаан, астрономия, астрофизика, джеттар, плазма сүүрээн Yakut
Date of oral exam:	9 December 2024
Referee:	Name Academic Title Eckart, Andreas Prof. Dr. Tezkan, Bülent Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/78524