Wagner, Jan Florian (2014) RFI Mitigation for VLBI and Arrays - Water Megamasers in Active Galaxies. PhD thesis, Universität zu Köln.

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Abstract

This thesis studies the central parsec in active galaxies via spectral line observations of OH and water megamasers and millimeter-wave interferometry. The aims are four-fold: 1) measure the mass of the supermassive black hole in active galactic nuclei (AGN) through circumnuclear water maser observations, 2) measure the Hubble constant through circumnuclear water maser observations, 3) mitigate the radio frequency interference that degrades spectral line observations, and 4) probe the innermost regions of AGN by expanding a mm-wavelength interferometer. The current paradigm conjectures that AGN contain a supermassive black hole The high spatial resolution of very long baseline radio interferometry (VLBI) allows to track AGN-launched jets over pc to kilo-pc scales outside an AGN. Within AGN, water megamaser emission mapped by VLBI is an excellent tracer of the sub-pc structure, in particular of the accretion disk. Its rotation curve, traced by water masers, allows an accurate SMBH mass measurement, important for constraining the empirical relation between black hole mass and stellar velocity dispersion in the galactic bulge – current evidence suggest that this MBH–sigma -relation is not universal, indicating differences in SMBH feeding, accretion history and AGN evolution. Disk masers further yield accurate distances to nearby maser galaxies by an almost purely geometric means. This allows measuring the current rate of expansion of the Universe, described by the Hubble constant, H0, an important cosmological parameter. To constrain DE models the key goal of the Megamaser Cosmology Project (MCP; Braatz, NRAO) is to measure H0 to at least 3% precision. Relatively few (~5%) active galaxies have been detected in water masers. In this thesis I carried out a water maser survey in nearby galaxies, some with AGN, some also detected in the dissociation product of water, the neutral hydroxyl radical (OH). Four new water masers were detected, including OH-detected galaxies, such as NGC 4261 famous for its dusty torus. The MCP project by J. Braatz et al. conducts 22 GHz VLBI and single-dish observations of disk maser galaxies. I analyzed the data of three quite different maser galaxies with putative AGN (NGC 23, UGC 3193 and IC 2560). The maser data for NGC 23 suggest either an association with a possibly truncated low-luminosity jet, or exceptionally luminous nuclear /extranuclear star formation masers. The AGN disk water masers in UGC 3193 reveal a large accretion disk with two rings suggestive of a disrupted disk around a low central SMBH mass of about 0.6+-0.2 x 10^6 Msun The disk masers in IC 2560 on the other hand indicate a SMBH mass of 5.4^+0.9_-0.6 x 10^6 Msun, and undermassive in the common MBH-sigma -relation, consistent the trend seen in other water maser galaxies. The IC 2560 disk masers also allowed the Hubble constant to be measured, resulting in an H0 of 67.7^+11.6_-8:9 km/s/Mpc. Combined with recent MCP results (UGC 3789, NGC 6264) this yields a new maser-based Hubble constant of 68.4+-5.3 km/s/Mpc (8%). Spectral lines observations are often degraded by man-made radio frequency interference (RFI). Thus I also discuss two techniques for mitigating RFI. The first was implemented in a popular VLBI software correlator. I evaluate the theoretical and practical performance, also in a VLBI search for 1.6 GHz OH in tori of Cygnus A and NGC 1068. I find the mitigation method to be effective but limited in practice due to the particular software correlator architecture. The second method improves over two existing approaches for antenna arrays, including focal plane arrays. While current instrumentation did not allow an immediate application, e.g., in the maser survey, I find that the method shows great potential for future observations in presence of RFI. Lastly, our recent work on expanding a global sub-millimeter VLBI array has enabled extreme angular resolution, suffcient to resolve the SMBH/disk system and the jet launching region. I present the first 230 GHz VLBI observation of the core region of blazar 3C 279. The work allows extreme-resolution observations of, e.g., Sgr A* and the SMBHs in M87 that will lead to a better understanding of SMBH spin, accretion, and jet launching.

Item Type: Thesis (PhD thesis)
Translated abstract:
AbstractLanguage
Diese Arbeit untersucht die inneren Parsec (pc) aktiver Galaxien anhand Spektrallinenbeobachtungen und Radiointerferometrie. Aktive Galaxien beherbergen eines der leuchtkräftigsten Objekte des Universums, den Aktiven Galaktischen Kern (engl. AGN), eine kompakte zentrale Region die extreme Leuchtstärken erreichen kann. Die Lehrmeinung vermutet ein supermassives Schwarzes Loch (engl. SMBH) im inneren sub-pc-Bereich eines AGN, mit Materiezufluss aus einer das SMBH umgebenden rotierenden Akkretionsscheibe (AS). Man vermutet außerdem, dass das SMBH/AS-System teilweise durch einen äußeren sub-10 pc Materie-“Torus” verdeckt wird. In einigen AGN stößt die innere Region auch einen relativistischen Materiestrom aus. Die Mechanismen die diesen “Jet” bilden sind umstritten, da die Region bisher nicht direkt beobachtet und bildgebend aufgelöst werden konnte. Mit Radiointerferometrie auf langen Basislinien (engl. VLBI) kann man z.B. der Jet-Materiefluss außerhalb des AGN über mehrere (kilo-)pc verfolgen. Die innere sub-pc -Region kann wiederum durch VLBI-Beobachtungen leuchtstarker Wassermaser (H2O-Emissionslinien) erfasst werden. Wassermaser werden oft in Jet-Nähe, aber auch in der sub-pc AS beobachtet. Die durch “Disk-Maser” abgetastete Rotationskurve der AS erlaubt eine genaue Bestimmung der Masse des zentralen SMBH. Dies liefert wichtige Daten zur empirischen Relation zwischen der SMBH-Masse und der stellaren Geschwindigkeitsdispersion im Zentralbereich der Galaxie (MBH-sigma -Relation). Sie scheint bekanntermaßen nicht universell zu sein. Dies deutet auf Unterschiede in der AGN- und SMBH-Evolution in verschiedenen Galaxientypen hin. Disk-Maser ermöglichen auch eine präzise und nahezu rein geometrische Entfernungsmessung zu Masergalaxien. Anhand der Distanz lässt sich die Expansionsrate des Universums bestimmen, also die Hubble-Konstante, H0, einem wichtigen kosmologischen Parameter. Anders als bei populären Methoden hängt die maserbasierte H0-Messung nicht von teils noch umstrittenen Details der Astrophysik ab. Mit einem exakten Wert für H0 lassen sich konkurrierende Dark Energy (DE) -ähnliche Modelle der theoretischen Physik ausschließen. Deshalb ist das Hauptziel des Megamaser Cosmology Project, für das ich drei recht verschiedene Masergalaxien analysiert haben, die Bestimmung von H0 auf eine Genauigkeit von mindestens 3%. Noch sind relativ wenige Masergalaxien bekannt. In dieser Arbeit habe ich eine Masersuche in aktiven Galaxien, und Galaxien mit Hydroxyl-Radikal (OH) -Spektrallinien (einem Dissoziationsprodukt von Wasser) durchgeführt. Vier neue Detektionen enthielten u.a. einen Disk-Maser, und einen Jet-Maser in der Galaxie NGC 4261, bekannt für ihren Torus. Unter dem MCP-Projekt von J. Braatz et al. habe ich drei recht verschiedene Masergalaxien mit möglichen AGN niedriger Leuchtstärke (NGC 23, UGC 3193 und IC 2560) analysiert. Die Maser in NGC 23 sind entweder mit einem gekürzten Jet von sehr niedriger Leuchtkraft assoziert, oder sind ungewöhnlich leuchtstarke Maser in inneren Sternformationregionen in NGC 23. Die Disk-Maser in UGC 3193 andererseits beschreiben eine weite, gewölbte und unterbrochene AS mit zwei Ringen, die eine SMBH-Masse von nur 0.6+-0.2 x 10^6 Msun umgeben. Die Disk-Maser in IC 2560 deuten wiederum auf eine SMBH-Masse von 5.4^+0.9_-0.6 x 10^6 Msun, untermassiv im Vergleich zu der MBH–sigma -Relation. Dieser Trend ist auch in anderen Masergalaxien sichtbar. Zusätzlich erlaubten die Disk-Maser in IC 2560 eine Messung der Hubble-Konstante von 67.7^+11.6_-8.9 km/s/Mpc. Mit den Resultaten des MCP aus zwei weiteren Galaxien (UGC 3789, NGC 6264) ist der maserbasierten H0-Wert 68.4+-5.3 km/s/Mpc (8%). Spektrallinienbeobachtungen leiden oft unter terrestrischen Störsignalen (engl. RFI). Diese Arbeit enthält deshalb auch zwei Methoden zur RFI-Abwehrung (engl. RFI mitigation). Die erste Methode wurde in einem bekannten VLBI-Softwarekorrelator implementiert und die theoretischen und praktischen Leistung wurde untersucht, auch in einer VLBI-Suche nach 1.6 GHz OH in den Tori von Cygnus A und NGC 1068. Die Methode an sich ist effektiv, aber wegen der Softwarearchitektur des Korrelators konnte die volle Leistungsfähigkeit nicht erreicht werden. Die zweite Methode, RFI-Subtraktion für Antennenarrays, stellt eine Verbesserung zweier Ansätze dar. Die aktuelle Antenneinstrumentation erlaubte zwar keine sofortige Anwendung der Methode, sie zeigte jedoch großes Potential für Beobachtungen mit zukünftigen Arrays und Multi-pixel-Empfängern in der Präsenz von RFI. Um zukünftig auch die innerste AGN-Region um das SMBH bildgebend aufzulösen, das Ziel der des GMVA und der Event Horizon Telescope (EHT) -Kollaboration das zu einem besseren Verständis des SMBH-Spins, Akkretion und der Jet-Entstehung führen wird, arbeiteten wir an einer Erweiterung eines globalen Sub-Millimeter-VLBI-Arrays. Im letzten Kapitel werden die Ergebnisse einer ersten extrem hochauflösenden Testbeobachtung von 3C 279 vorgestellt.German
Creators:
CreatorsEmailORCID
Wagner, Jan Florianjan.wagner@iki.fiUNSPECIFIED
URN: urn:nbn:de:hbz:38-56332
Subjects: Physics
Uncontrolled Keywords:
KeywordsLanguage
radio astronomyUNSPECIFIED
active galaxiesUNSPECIFIED
Faculty: Faculty of Mathematics and Natural Sciences
Divisions: Faculty of Mathematics and Natural Sciences > I. Physikalisches Institut
Language: English
Date: 26 June 2014
Date of oral exam: 26 June 2014
Referee:
NameAcademic Title
Eckart, AndreasProf. Dr.
Zensus, J. AntonProf. Dr.
Full Text Status: Public
Date Deposited: 29 Jul 2014 08:52
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