Nair, Dhanya G. (2019). Global Millimeter VLBI Array Survey of Ultracompact Extragalactic Radio Sources at 86 GHz. PhD thesis, Universität zu Köln.

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Ultracompact extragalactic radio sources, representing the inner parsec and sub-parsec scales of Active Galactic Nuclei (AGN), form the best laboratory to test extreme physics and understand our universe. Studies of ultracompact extragalactic radio sources have revealed the morphology and kinematics of AGN from sub-parsec to Mega-parsec scales, the powerful energy emission ranging from radio to gamma rays, the galaxy evolution, the important role of accretion disk and super massive black hole in the formation, acceleration and collimation of relativistic jets and many other interesting facts about AGNs. A substantial fraction of these studies has been enabled by the advent and continued developments of the technique of very long baseline interferometry (VLBI) which combines radio telescopes across the entire Earth into a single instrument. VLBI observations at 86 GHz (wavelength, ~3 mm) reach a resolution of about 50 microarcsec, and can probe the collimation and acceleration regions of relativistic outflows in the vicinity of super massive black holes in AGN. In this context, a high resolution 86 GHz VLBI survey of a representative sample of ultracompact extragalactic radio sources provides the best statistical tool, to investigate the underlying physics of AGN and their relativistic jets. This dissertation presents results from a large global VLBI survey of compact radio sources carried out in 2010-2011 with the Global Millimeter VLBI Array. The main goals of the survey are to extend VLBI imaging surveys at 86 GHz to fainter sources, to develop a comprehensive sample of ultracompact radio sources imaged at 86 GHz, and to study the morphology and the distribution of brightness temperatures. The survey data reaches a typical baseline sensitivity of 0.1 Jy and a typical image sensitivity of 5 mJy/beam. A total of 162 unique compact radio sources have been observed in this survey. All the sources are detected and imaged,signifying a nearly twofold increase in the total number of sources ever imaged with VLBI at 86 GHz. 138 sources are imaged for the first time with VLBI at 86 GHz through this survey. Gaussian model fitting of the visibility data is applied to represent the structure of the observed sources and to estimate the flux densities and sizes of distinct emitting regions (components) in their jets. These estimates are used to calculate the brightness temperature (Tb) at the jet base (core) and in the next closest regions (jet components) downstream of the core. These model fit based estimates of Tb are compared to the estimates of brightness temperature limits made directly from the visibility data, demonstrating a good agreement between the two methods. The apparent brightness temperature estimates for the jet cores in our sample ranges from 1.1 x 10^9 K to 5.5 x 10^12 K, with the mean brightness temperature of 1.8 x 10^11 K. The apparent brightness temperature estimates for the nearest jet components in our sample ranges from 5.8 x 10^7 K to 4.1 x 10^11 K. A population model with a single intrinsic value of brightness temperature, To, is applied to reproduce the observed distribution. It yields To = (3.77^{+0.10}_{-0.14}) x 10^11 K for the jet cores, implying that the inverse Compton losses dominate the emission. In the nearest jet components, To = (1.42^{+0.16}_{-0.19}) x 10^11 K is found, which is slightly higher than the equipartition limit of ~5 x 10^10 K expected for these jet regions. A correlation between To and Lorentz factor, gamma_j inherent to the population model description precludes fitting for To and gamma_j simultaneously. We find that a relation To[K] ~(7.7 x 10^8) x (gamma_j)^2.7 is implied for this modelling framework by the survey data. The approach used to derive the intrinsic brightness temperature at 86 GHz through population modelling is applied to the database of VLBI survey at lower frequencies (2, 8, 15 GHz).This enables us to investigate the multi-frequency aspects of Tb. The intrinsic brightness temperature obtained for cores at 86 GHz is found to be lower than that at 2 GHz, 8 GHz and 15 GHz. Such a decrease of To will provide a hint in favor of the decelerating jet model or particle-cascade model. A decreasing To would also have consequences for higher frequency VLBI. This has to be further investigated by compiling the jet speeds, beta_app with observed Tb. The intrisic To and the observed Tb in core and jets show that the brightness temperature drops by approximately a factor of ~(2-10) already on sub-parsec scales in the jets. This evolution might occur with the inverse Compton, synchrotron, and adiabatic losses subsequently dominating the energy losses. A basic evolutionary scheme in which the changes observed in the jet emission are caused by the adiabatic energy losses in relativistic shocks is investigated. For objects with sufficient structural detail detected, the adiabatic energy losses are shown to dominate the observed variations of brightness temperature along the jet implying that the jet components can be viewed as adiabatically expanding relativistic shocks. Under the equipartition condition between the magnetic field energy and particle energy density, the absolute distance of the VLBI core from the central engine can be estimated. The brightness temperature measurements obtained from this survey at 86 GHz are combined with the database of VLBI survey at lower frequencies (2, 8, 15 GHz) to study the brightness temperatures in source frame in the sub-parsec scale of the ultracompact radio sources. From the vicinity of the central engine, the brightness temperature increases slowly and then rises with a steeper slope and then slow down. This gives a strong implication that the jets are accelerated and collimated by the magnetically driven force, as predicted by the relativistic inner jet models for the magnetic acceleration.

Item Type: Thesis (PhD thesis)
Translated title:
Globale Millimeter VLBI-Array-Vermessung von ultrakompakten extragalaktischen Radioquellen bei 86 GHzGerman
CreatorsEmailORCIDORCID Put Code
URN: urn:nbn:de:hbz:38-93721
Date: 22 February 2019
Language: English
Faculty: Faculty of Mathematics and Natural Sciences
Divisions: Faculty of Mathematics and Natural Sciences > Department of Physics > Institute of Physics I
Subjects: Physics
Uncontrolled Keywords:
Active Galactic NucleiEnglish
Extragalactic astronomyEnglish
Compact radio sourcesEnglish
Radio InterferometryEnglish
Relativistic jetsEnglish
VLBI surveysEnglish
Brightness temperature of jetsEnglish
Population ModelEnglish
Date of oral exam: 8 March 2018
NameAcademic Title
Eckart, AndreasProf. Dr.
Zensus, J. AntonProf. Dr.
Refereed: Yes


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