Galactic Centre observations with local mm-telescope arrays - Kölner UniversitätsPublikationsServer

Kunneriath, Devaky (2010). Galactic Centre observations with local mm-telescope arrays. PhD thesis, Universität zu Köln. Open Access

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Abstract

Radio interferometry observations are a powerful tool to study the central regions of the Galactic centre (GC). High angular resolution observations at millimetre wavelengths can disentangle the emission of Sgr A* from the thermal emission of the circum nuclear disk (CND) and the mini-spiral surrounding it. In this thesis I present the results of radio interferometric observations of the GC region with local millimetre-telescope arrays such as CARMA and ATCA, supplemented by observations in the infrared (IR) with ESO's VLT telescopes. My goal is to analyse the emission mechanisms present in the central few parsecs of the GC region, in particular the extended thermal emission of the ionized gas and dust of the mini-spiral region and the non-thermal emission of Sagittarius A* (Sgr A*), the radio source associated with the supermassive black hole (SMBH) at the GC. Observations were carried out in March and April 2009 at 1.3 and 3 mm with the mm telescope array CARMA in California, in June 2006 with the mid-infrared (MIR) instrument VISIR at ESO's VLT, and in the NIR Br_gamma in August 2009 with VLT NACO. I present high angular resolution continuum maps of the GC at 3 and 1.3 mm and the highest resolution spectral index map obtained at these wavelengths. I obtain a spectral index of 0.5 for Sgr A*, indicating an inverted synchrotron spectrum and a mixture of negative and positive values in the extended emission of the minispiral, including the thermal free-free emission from the ionized gas, and a possible contribution of dust at 1.3 mm. I infer the physical properties of the dust and gas in the mini-spiral by comparing the radio continuum maps to the MIR continuum map, and the Br_gamma line emission map in the NIR. An extinction map at NIR wavelengths for the mini-spiral region, showing extinctions ranging from 1.8-3 magnitudes, was also produced. To study the flaring activity of Sgr A*, global coordinated multiwavelength campaigns were carried out in 2007 and 2008 using ESO's VLT and the mm telescope arrays CARMA, ATCA, and the 30 m IRAM telescope in Spain, which detected four flares in the NIR, of which three were covered later by the mm data. I develop a new method for obtaining concatenated light curves of the compact mm-source Sgr A* from single dish telescopes and interferometers in the presence of significant flux density contributions from an extended and only partially resolved source, and model the observed flares in the NIR and mm using an adiabatic expansion model involving synchrotron source components. I derive physical quantities such as expansion velocities ranging from 0.005c-0.017c, source sizes of about one Schwarzschild radius, turnover frequencies of a few THz, flux densities of a few Janskys, and spectral indices of 0.6 to 1.3. These parameters suggest either a bulk motion of the adiabatically expanding source components greater than the expansion velocity or a confinement of expanding material within a corona or disk in the immediate surroundings of Sgr A*.

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Thesis (PhD thesis)

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