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Master's Thesis Analysis of the NVSS Catalog and Number Counts

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Master's Thesis Analysis of the NVSS Catalog and Number Counts Master's Thesis Analysis of the NVSS Catalog and Number Counts by Jonas Reckmann, B.Sc. A Thesis submitted to Bielefeld University Faculty of Physics in Partial Fulllment of the Requirements for the Degree of Master of Science on October 19, 2013 Supervisor: Prof. Dominik Schwarz CONTENTS 3 Contents 1 Introduction 4 2 Radio Objects 5 2.1 Active Galactic Nuclei . 5 2.1.1 Radio Galaxies . 6 2.1.2 Quasars . 8 2.1.3 Seyfert Galaxies . 8 2.2 Starburst Galaxies . 9 3 The VLA and the NVSS 10 3.1 The Very Large Array . 10 3.2 The NRAO VLA Sky Survey . 11 4 Cosmology and Number Counts 13 4.1 Euclidean Number Counts . 13 4.2 Cosmological Volume Element . 13 4.3 Luminosity distance . 14 4.4 Radio source counts . 15 5 Optimal Data-based Binning 17 6 Preparing the Data 20 6.1 Division of the Catalog . 20 6.2 Setting Flux Limits . 21 7 Comparing Data Sets 22 8 Number Counts 29 9 Conclusion 35 10 Appendix 36 1 INTRODUCTION 4 1 Introduction In the 1930s Karl Jansky detected the rst radio signals in the sky, coming from the constellation of sagittarius in the center of the Milky Way. Since then radio astronomy made its way to a major eld in astronomy. This development is also a result of the fact that radio astronomy allows observa- tions that can help to understand the early history of the universe. Due to their long wavelenghts radio waves do not get blocked by interstellar dust as waves in the optical spectrum would be. This allows radio telescopes to look signicantly further away than optical ones. As the age of observed objects decreases the more distant they are, radio astronomy allows observations of the light of young galaxies. Hence, radio signals can promote our knowledge about early stages of the universe. The aim of this thesis is to develop an optimal way of presenting the features of the NVSS catalog data with histograms to work out a comparison of dierent parts of the catalog. Furthermore, a model is tted that describes these features to get an estimate of parameters that are relevant to describe radio galaxies and their distribution. This thesis starts with a short overview on the most important sources of radio waves in our universe. This is followed by some general information about the NVSS catalog and about the telescope it was obtained with. It will be continued with a brief introduction to the cosmology of number counts and the mathematical concept of optimal binning used in this thesis. This is followed by preparing dierent data sets and using the discussed method of binning to prepare them for further processing and to visualize them. Then the dierent data sets are compared to each other in order to see if they have notable dierences. In the last section a model of number counts will be tted to the data and evaluated how good it describes the data. 2 RADIO OBJECTS 5 2 Radio Objects The look into the sky with radio telescopes reveals many dierent types of radio objects. In this thesis the focus lies on galaxies observed at radio wave- lenghts. The most general two categories that are observed are galaxies with an active galactic nucleus (AGN) and star forming galaxies. This Chapter gives a brief overview of the dierent types of galaxies that emit strong radio signals. 2.1 Active Galactic Nuclei Most of the stronger radio sources in the sky are galaxies with an AGN. The nucleus is a high concentration of mass in the central region of the galaxy, which is usually assumed to be a black hole. The nucleus generates an accretion disc due to the angular momentum of infalling material, which may be gas from the interstellar medium or stars in its vincinity. As nearby mass falls onto the nucleus gravitational energy is released. Around the accretion disc there is an opaque torus of inowing material which makes it impossible to observe the central regions of the galaxy from the sides. The last part in this simple model of an AGN are two jets of high energetic particles ejected from the core, perpendicular to the accretion disc. The mechanism that creates these jets is still unknown. Figure 1: A simple model of an active galactic nucleus[BGS10]. 2 RADIO OBJECTS 6 2.1.1 Radio Galaxies The most luminous radio galaxies can exceed power of 1038W [VKB88] in the radio frequency range. The strong radio emission from these galaxies comes from the jets, created in the nucleus, and structures that form when the jets start to dissipate, called lobes. The highly energetic and relativistic particles from the jets emit synchroton radiation as they interact with mag- netic elds along their path. Interaction with the surrounding interstellar and intergalactic medium creates radio hot spots along the jet and in the lobes. Figure 2: The radio galaxy Cygnus A. The electrons in the jets travel great distances at nearly the speed of light and form hot spots in the radio lobes as they are collide with the intergalactic medium. [source: http://images.nrao.edu/110] Many radio galaxies are highly symmetric regarding their straight jets and lobes, but some are asymmetric with bent jets. It is believed that this is due to the high velocities the galaxies have while moving through the intergalactic medium. Another common form of asymmetry in radio galaxies is a missing counter jet. This can be the result of relativistic beaming, where the jet moving in the direction of the observer appears much brighter than the other up to the point of an invisible counter jet. 2 RADIO OBJECTS 7 Radio galaxies are divided into two classes, the Fanaro-Riley Class I (FR I) and the Fanaro-Riley Class II (FR II)[FR734]. The jets in FR II sources move at supersonic speed and thus emitting most of their energy in the lobes, making them brighter in the outside regions. The speed of the jets in FR I sources becomes subsonic much earlier and most of their energy is emitted near the core region which results in more diuse structures and more core centered brightness distribution. Figure 3: Top: FR I galaxy M84 with a strong radio emission at the center that decreases towards the outer regions. Bottom: FR II galaxy 3C175 with two bright radio lobes. Only one the right lobe has a jet that connects it to the center of the host galaxy[Sch06]. 2 RADIO OBJECTS 8 2.1.2 Quasars Quasi-Stellar Objects (QSOs) or quasars are very luminous high energy AGN with broad emission lines. As quasars normally outshine their host galaxies they appear to be point-like at optical wave-lengths. Quasars can be highly energetic radio sources and the size of the core can be measured. As these radio-loud quasars have a core, which has a diameter of a fraction of a parsec, they must have a very high brightness temperature. The high energies of quasars can be explained by the angle it is observed at. They could be AGNs with a jet pointed directly towards the observer. 2.1.3 Seyfert Galaxies Seyfert galaxies are spiral galaxies with a bright core and broad and nar- row emission lines. They have similar appearance to quasars but are much less luminous.The brightness of the core region of Seyfert galaxies can be compared to the brightness of a Milky-Way like galaxy. As they are faint radio sources they could be radio-quiet quasars viewed from another angle. Seyfert galaxies are distinguished by type 1 and type 2. Type 1 Seyfert galaxies have broad and narrow emission lines, while type 2 seyfert galaxies only show the narrow emission lines. 2 RADIO OBJECTS 9 2.2 Starburst Galaxies Starburst galaxies have a peculiarly high star formation rate, up to 100 times higher than normal galaxies like the milky way. To achieve this there has to be a large amount of cold gas occupying a small region of space. Typically this is the result of a merger between two gas-rich galaxies. Due to the high rate of star formation, starbursts normally contain a large amounts of dust, which absorbs the UV emissions of the numerous young stars and re-emits them in the infrared. This makes them highly luminous in the infrared part of the spectrum. The radio emission from starburst galaxies typically has power of 1033W [BGS10] and is the result of massive stars ionizing the interstellar medium with their solar winds and electrons accelerated to rela- tivistic speeds in supernova remnants. Radio emissions from normal galaxies usually have power about 1030W [BGS10]. Figure 4: The starburst galaxy M82. The dark spots are supernova remnants [BGS10]. 3 THE VLA AND THE NVSS 10 3 The VLA and the NVSS 3.1 The Very Large Array Figure 5: The Very Large Array. [source: http://images.nrao.edu/Telescopes/VLA/298] The Very Large Array (VLA) is a radio observatory located on the Plains of San Augustin, New Mexico. It was built in 7 years, inaugurated in 1980, upgraded in 2011 and renamed in 2012 to Karl G. Jansky Very Large Array . The VLA is an interferometer consisting of 27 radio antennas which are each 25 meters in diameter. The antennas are arranged on three 21km long rail tracks resulting in a y-shaped conguration.As the antennas can be moved along the rail tracks, the VLA can operate in four main and several hybrid congurations resulting in dierent baselines. The frequency range of the VLA is 74 MHz to 50 GHz with a minimal angular resolution of 0.04 arcseconds at 43 GHz.
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