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Crowded Field Spectroscopy and the Search for Intermediate-Mass Black

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Crowded Field Spectroscopy and the Search for Intermediate-Mass Black Crowded field spectroscopy and the search for intermediate-mass black holes in globular clusters Sebastian Kamann Leibniz-Institut fur¨ Astrophysik Potsdam (AIP) Dissertation zur Erlangung des akademischen Grades doctor rerum naturalium (Dr. rer. nat.) in der Wissenschaftsdisziplin Astrophysik Eingereicht an der Mathematisch-Naturwissenschaftlichen Fakultat¨ der Universitat¨ Potsdam Potsdam, den 06. Februar 2013 Published online at the Institutional Repository of the University of Potsdam: URL http://opus.kobv.de/ubp/volltexte/2013/6776/ URN urn:nbn:de:kobv:517-opus-67763 http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-67763 Abstract Globular clusters are dense and massive star clusters that are an integral part of any major galaxy. Careful studies of their stars, a single cluster may contain several millions of them, have revealed that the ages of many globular clusters are comparable to the age of the Universe. These remarkable ages make them valuable probes for the exploration of structure formation in the early universe or the assembly of our own galaxy, the Milky Way. A topic of current research relates to the question whether globular clusters harbour massive black holes in their centres. The attention that is drawn to this question results from the firm evidence that exists for even more massive black holes, so-called supermassive black holes, residing in the centres of galaxies. The black holes that are assumed to reside in globular clusters would therefore bridge the gap from stellar mass black holes, that represent the final stage in the evolution of massive stars, to supermassive ones. For this reason, they are referred to as intermediate-mass black holes. The most reliable method to detect and to weigh a black hole is to study the motion of objects inside its sphere of influence. The measurement of Doppler shifts via spectroscopy allows one to carry out such dynamical studies. However, spectroscopic observations in dense stellar fields such as Galactic globular clusters are challenging. In astronomical observations stars do not appear point-like, but with a finite width given by the point spread function (PSF). The PSF is the result of the diffraction processes that the starlight encounters on its way through the earth’ atmosphere and the telescope. In dense stellar fields, these processes cause the images of the individual stars to overlap in the observed data. As classical spectroscopy does not preserve any spatial information, it is impossible to separate the spectra of overlapping stars and to measure their velocities. Yet methods have been developed to perform imaging spectroscopy. The most widely used technique in this respect is integral field spectroscopy, where a continuous area on the sky is covered by a dense grid of spectra. This results in a three-dimensional data structure in which every spatial pixel contains an entire spectrum. As shown in this work, such data allows one to reconstruct the PSF and thus to deblend overlapping objects. In the course of this work, the first systematic study on the potential of integral field spectroscopy in the analysis of dense stellar fields is carried out. To this aim, a method is developed to reconstruct the PSF from the observed data and to use this information in the extraction of the stellar spectra. Based on dedicated simulations, predictions are made on the number of stellar spectra that can be extracted from a given data set and the quality of those spectra. Furthermore, the influence of uncertainties in the recovered PSF on the extracted spectra are quantified. The results clearly show that compared to traditional approaches, the new method makes a significantly larger number of spectra accessible to a spectroscopic analysis. This systematic study goes hand in hand with the development of a software package to automatize the individual steps of the data analysis: the reconstruction of the PSF, the determination of the positions of the stars and finally the extraction of the stellar spectra. In particular with regard to the latter step, the usage of advanced algebraic algorithms allows for a very efficient analysis of the data. The second part of this work centres around the application of the method to data of three Galactic globular clusters, M3, M13, and M92, observed with the PMAS integral field spectrograph at the Calar Alto observatory in Spain. The aim of these observations is to constrain the presence of intermediate-mass black holes in the centres of the clusters based on an analysis of the kinematics of the stars in the direct vicinities of the cluster centres. This is only possible if a sufficiently large sample of stars is probed kinematically. The application of the new analysis method yields samples of about 80 stars per cluster. These are by far the largest spectroscopic samples that have so far been obtained in the centre of any of the three clusters. In the course of the further analysis, Jeans models are calculated for each cluster that predict the velocity dispersion based on an assumed mass distribution inside the cluster. The comparison to the observed velocities of the stars shows that in none of the three clusters, a massive black hole is required to explain the observed kinematics. Instead, the observations rule out any black hole in M13 with a mass higher than 13 000 solar masses at the 99.7% level. For the other two clusters, this limit is at significantly lower masses, namely 2 500 solar masses in M3 and 2 000 solar masses in M92. In M92, it is possible to lower this limit even futher by a combined analysis of the extracted stars and the unresolved stellar component. This component consists of the numerous stars in the cluster that appear unresolved in our data. The final limit of 1 300 solar masses is the lowest limit obtained so far for any massive globular cluster. Zusammenfassung Kugelsternhaufen sind dichte, gravitativ gebundene Ansammlungen von teilweise mehreren Millionen Sternen, die ein fester Bestandteil jeder massiven Galaxie sind. Aus der Untersuchung der Kugelsternhaufen in unserer Milchstraße weiß man, dass das Alter von vielen dieser Objekte vergleichbar ist mit jenem des Universums. Dieses hohe Alter macht sie zu wertvollen Forschungsobjekten, beispielsweise um die Entstehung der Milchstraße und die Strukturbildung im fruhen¨ Universum zu verstehen. Eine aktuelle wissenschaftliche Fragestellung befasst sich damit, ob Kugelsternhaufen massive schwarze Locher¨ in ihren Zentren beherbergen. Die Bedeutung dieser Frage ergibt sich daraus, dass es eindeutige Hinweise darauf gibt, dass sich extrem massereiche, sogenannte supermas- sive schwarze Locher,¨ in den Zentren von Galaxien befinden. Jene schwarzen Locher,¨ die in den Zentren von Kugelsternhaufen vermutet werden, wurden¨ daher eine Brucke¨ von den stellaren, die durch den Kollaps massere- icher Sterne entstehen, zu den supermassiven schwarzen Lochern¨ schlagen. Man bezeichnet sie daher auch als mittelschwere schwarze Locher.¨ Die sicherste Diagnostik, um schwarze Locher¨ zu detektieren und ihre Masse zu bestimmen ist, die Bewegung der Objekte innerhalb ihrer gravitativen Einflusssphare¨ zu vermessen. Spektroskopische Untersuchungen vermogen¨ dies uber¨ die Dopplerverschiebung von Spektrallinien. Jedoch sind spektroskopische Untersuchungen in dichten stellaren Feldern wie Kugelsternhaufen schwierig. Aufgrund der Turbulenz in der Atmosphare¨ und dem endlichen Auflosungsverm¨ ogen¨ des Teleskops erscheinen die Sterne in den Beobachtungen nicht punktformig,¨ sondern mit einer durch die Punktspreizfunktion (PSF) gegebenen Breite. In dichten stellaren Feldern fuhrt¨ dies dazu, dass die Sterne uberlappen.¨ Da klassische spektroskopische Verfahren jedoch nicht bildgebend sind, lassen sich die Beitrage¨ der Einzelsterne zu einem beobachteten Spektrum nicht trennen und die Geschwindigkeiten der Sterne konnen¨ nicht vermessen werden. Bildgebende spektroskopische Verfahren bieten jedoch die Moglichkeit,¨ die PSF zu rekonstruieren und basierend darauf die Spektren uberlappender¨ Sterne zu trennen. Als Integralfeld-Spektroskopie bezeichnet man ein Verfahren, in dem ein Himmelsauschnitt mit einem dichten raumlichen¨ Raster von Spektren beobachtet wird. Dadurch entsteht eine dreidimensionale Datenstruktur, in welcher jeder Bildpunkt ein komplettes Spektrum enthalt.¨ Im Rahmen der vorgelegten Arbeit wird das Potential der Integralfeld-Spektroskopie in der Beobachtung dichter stellarer Felder zum ersten Mal systematisch analysiert. Hierzu wird eine Methodik entwickelt, die das Extrahieren von Einzelsternspektren uber¨ eine Rekonstruktion der PSF aus den vorhandenen Daten erlaubt. Anhand von Simulationen werden Voraussagen daruber¨ gemacht, wie viele Sternspektren aus einem gegebenen Datensatz ex- trahiert werden konnen,¨ welche Qualitat¨ diese Spektren haben und wie sich Ungenauigkeiten in der rekonstru- ierten PSF auf die Analyse auswirken. Es zeigt sich hierbei, dass die entwickelte Methodik die spektroskopische Analyse von deutlich mehr Sternen erlaubt als klassische Verfahren. Parallel zu dieser systematischen Studie er- folgt die Entwicklung einer dezidierten Analysesoftware, die folgende Schritte ausfuhrt:¨ die Bestimmung der PSF, die Messung der Positionen der einzelnen Sterne und schließlich die Extraktion aller zuganglichen¨ Sternspektren. Insbesondere im letzten Schritt gelingt es, den Prozess mittels ausgereifter algebraischer Verfahren sehr effizient zu gestalten. Im zweiten Teil der Arbeit wird die entwickelte Methodik auf Daten von drei Kugelsternhaufen angewendet, die mit dem PMAS Integralfeld-Spektrographen
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