@phdthesis{Arentsen2020,
  author    = {Arentsen, Anke},
  title     = {Galactic archaeology with the oldest stars in the Milky Way},
  doi       = {10.25932/publishup-47602},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-476022},
  school      = {Universit{\"a}t Potsdam},
  year      = {2020},
  abstract  = {In einer dunklen Nacht kann man tausende Sterne sehen. All diese Sterne befinden sich innerhalb der Milchstraße, unsere Heimatgalaxie. Nicht alle Sterne sind gleich, sie k{\"o}nnen zum Beispiel unterschiedliche Gr{\"o}ßen, Massen, Temperaturen und Alter haben. Die schwereren Sterne leben (aus astronomischer Sicht) nicht lange, nur wenige Millionen Jahren, aber Sterne kleiner als die Sonne k{\"o}nnen mehr als zehn Milliarden Jahren alt werden. Kleine Sterne die ganz am Anfang des Universums entstanden sind leuchten immer noch. Diese uralten Sterne sind sehr hilfreich um mehr {\"u}ber das fr{\"u}he Universum, die erste Sterne und die Geschichte der Milchstraße zu erfahren. Aber wie erkennt man uralte Sterne? Anhand ihrer chemischen Fingerabdr{\"u}cke! Am Anfang des Universums gab es nur zwei chemische Elemente: Wasserstoff und Helium (und ein klein bisschen Lithium). Alle schwereren Elementen wie zum Beispiel Kohlenstoff, Kalzium und Eisen sind erst sp{\"a}ter innerhalb von Sternen und in Sternexplosionen entstanden. Je mehr Sternen geboren werden, sich entwickeln und explodieren, desto mehr chemische Elemente gibt es im Universum. Sterne die sp{\"a}ter entstehen werden mit einer gr{\"o}ßeren Menge an schweren Elementen, beziehungsweise einer gr{\"o}ßeren Metallizit{\"a}t, geboren. Im Bereich der Astronomie der sich „Galaktische Arch{\"a}ologie" nennt benutzt man Sterne mit unterschiedlichen Metallizit{\"a}ten um die Geschichte der Milchstraße zu erforschen. In dieser Doktorarbeit liegt der Fokus auf den metallarmen Sterne, da man erwartet dass diese Sterne am {\"a}ltesten sind und uns deswegen viel {\"u}ber die fr{\"u}he Geschichte erz{\"a}hlen k{\"o}nnen. Bis heute haben wir noch keinen metallfreien Stern entdeckt, aber die metall{\"a}rmsten Sterne geben uns wichtige Einblicke in das Leben und Sterben der ersten Sterne. Viele von diesen {\"a}ltesten, metall{\"a}rmsten Sternen haben unerwartet viel Kohlenstoff im Vergleich zu zum Beispiel Eisen. Diese kohlenstoffreichen, metallarmen Sterne (CEMP Sterne) erz{\"a}hlen uns etwas {\"u}ber die allerersten Sterne im Universum: sie haben relativ viel Kohlenstoff produziert. Wenn wir uns die genauen chemischen Fingerabdr{\"u}cke von CEMP Sterne angucken, erz{\"a}hlen sie uns noch viel mehr. Aber unsere Interpretation h{\"a}ngt von der Annahme ab, dass der chemische Fingerabdruck sich w{\"a}hrend des Lebens eines Sternes nicht ge{\"a}ndert hat. In dieser Dissertation werden neue Daten pr{\"a}sentiert die zeigen dass diese Annahme vielleicht zu einfach ist: viele extrem metallarme CEMP Sterne befinden sich in Doppelsternsystemen. Interaktion zwischen zwei Sternen in einem Doppelsternsystem k{\"o}nnte die Oberfl{\"a}che von CEMP Sternen verschmutzt haben. Zwar wurden die meisten CEMP Sterne h{\"o}chstwahrscheinlich nicht verschmutzt, aber wir sollten vorsichtig sein mit unserer Interpretation. Die CEMP Sterne und andere metallarme Sterne sind auch wichtig f{\"u}r unser Verst{\"a}ndnis der fr{\"u}hen Geschichte der Milchstraße. Die meisten Forscher, die metallarme Sterne studieren, suchen diese Sterne im Halo der Milchstraße: einer riesigen, diffuse Komponente die ungef{\"a}hr 1\% der Sterne in unserer Galaxie enth{\"a}lt. Modelle sagen aber vorher dass die {\"a}ltesten metallarmen Sterne sich im Zentrum der Milchstraße befinden (im „Bulge"). Das Zentrum ist leider, wegen großer Mengen Staub zwischen uns und dem Zentrum und einer {\"u}berw{\"a}ltigenden Mehrheit an metallreichen Sternen, schwierig zu beobachten. Diese Dissertation pr{\"a}sentiert Ergebnisse des „Pristine Inner Galaxy Survey" (PIGS), einer neuen Himmelsdurchmusterung, die die {\"a}ltesten Sterne im Bulge der Milchstraße sucht (und findet). PIGS benutzt Bilder mit einer Farbe, die f{\"u}r die Metallizit{\"a}t der Sterne empfindlich ist, und kann deswegen sehr effektiv die metallarmen Sterne aus Millionen anderer Sterne ausw{\"a}hlen. Von interessanten Kandidaten wurden Spektren aufgenommen und mit zwei unabh{\"a}ngigen Methoden analysiert. Mit dieser Strategie hat PIGS die bislang gr{\"o}ßte Anzahl an metallarmen Sternen in der inneren Galaxie entdeckt. Ein neues Ergebnis aus den PIGS Daten ist, dass die metall{\"a}rmeren Sterne langsamer um das Galaktische Zentrum drehen als die metallreichen Sterne, und dass sie mehr willk{\"u}rliche Bewegung zeigen. Eine zweite wichtige Leistung von PIGS ist die Entdeckung von dutzenden CEMP Sternen in der innere Galaxie, wo vorher nur zwei bekannt waren. Die neuen Ergebnisse aus dieser Dissertation helfen uns die ersten Sterne und die Geschichte der Milchstraße besser zu verstehen. Laufende und neue Himmelsdurchmusterungen in den n{\"a}chsten Jahren werden uns noch viel mehr Informationen geben: es ist eine aufregende Zeit f{\"u}r die Galaktische Arch{\"a}ologie.},
  language  = {en}
}
@phdthesis{Fournier2016,
  author    = {Fournier, Yori},
  title     = {Dynamics of the rise of magnetic flux tubes in stellar interiors},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-394533},
  school      = {Universit{\"a}t Potsdam},
  pages     = {xii, 98},
  year      = {2016},
  abstract  = {In sonnen{\"a}hnlichen Sternen erh{\"a}lt ein Dynamo-Mechanismus die Magnetfelder. Der Babcock-Leighton-Dynamo beruht auf einem solchen Mechanismus und erfordert insbesondere die Existenz von magnetischen Flussr{\"o}hren. Man nimmt an, dass magnetische Flussr{\"o}hren am Boden der Konvetionszone entstehen und durch Auftrieb bis zur Oberfl{\"a}che steigen. Es wird ein spezielles Dynamomodell vorgeschlagen, in dem der Verz{\"o}gerungseffekt durch das Aufsteigen der Flussr{\"o}hren ber{\"u}cksichtigt wird. Die vorliegende Dissertation besch{\"a}ftigt sich mit der Anwendbarkeit des Babcock-Leighton-Dynamos auf andere Sterne. Zu diesem Zweck versuchen wir, die Aufstiegszeiten von magnetischen Flussr{\"o}hren mit Hilfe von kompressiblen MHD-Simulationen in sp{\"a}rischen Kugelschalen mit Dichteschichtung zu bestimmen und einzugrenzen. Derartige Simulationen sind allerdings nur in einem unrealistischen Parameterbereich m{\"o}glich. Deshalb ist eine Skalierungsrelation n{\"o}tig, die die Ergebnisse auf realistische physikalische Regimes {\"u}bertr{\"a}gt. Wir erweitern fr{\"u}here Arbeiten zu Skalierungsrelationen in 2D und leiten ein allgemeines Skalierungsgesetz ab, das f{\"u}r 2D- und 3D-Flussr{\"o}hren g{\"u}ltig ist. In einem umfangreichen Satz von numerischen Simulationen zeigen wir, dass die abgeleitete Skalierungsrelation auch im vollst{\"a}ndig nichtlinearen Fall gilt. Wir haben damit ein Gesetz f{\"u}r die Aufstiegszeit von magnetischen Flussr{\"o}hren gefunden, dass in jedem sonnen{\"a}hnlichen Stern G{\"u}ltigkeit hat. Schließlich implementieren wir dieses Gesetz in einem Dynamomodell mit Verz{\"o}gerungsterm. Die Simulationen eines solchen verz{\"o}gerten Flussr{\"o}hren/Babcock-Leighton-Dynamos auf der Basis der Meanfield-Formulierung f{\"u}hrten auf ein neues Dynamo-Regime, das nur bei Anwesenheit der Verz{\"o}gerung existiert. Die erforderlichen Verz{\"o}gerungen sind von der Gr{\"o}{\"y}enordnung der Zyklusl{\"a}nge, die resultierenden Magnetfelder sind schw{\"a}cher als die {\"A}quipartitions-Feldst{\"a}rke. Dieses neue Regime zeigt, dass auch bei sehr langen Aufstiegszeiten der Flussr{\"o}hren/Babcock-Leighton-Dynamo noch nichtzerfallende L{\"o}sungen liefern und daher auf ein breites Spektrum von Sternen anwendbar sein kann.},
  language  = {en}
}
@phdthesis{Herenz2016,
  author    = {Herenz, Edmund Christian},
  title     = {Detecting and understanding extragalactic Lyman α emission using 3D spectroscopy},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-102341},
  school      = {Universit{\"a}t Potsdam},
  pages     = {175},
  year      = {2016},
  abstract  = {In this thesis we use integral-field spectroscopy to detect and understand of Lyman α (Lyα) emission from high-redshift galaxies. Intrinsically the Lyα emission at λ = 1216 {\AA} is the strongest recombination line from galaxies. It arises from the 2p → 1s transition in hydrogen. In star-forming galaxies the line is powered by ionisation of the interstellar gas by hot O- and B- stars. Galaxies with star-formation rates of 1 - 10 Msol/year are expected to have Lyα luminosities of 42 dex - 43 dex (erg/s), corresponding to fluxes ~ -17 dex - -18 dex (erg/s/cm²) at redshifts z~3, where Lyα is easily accessible with ground-based telescopes. However, star-forming galaxies do not show these expected Lyα fluxes. Primarily this is a consequence of the high-absorption cross-section of neutral hydrogen for Lyα photons σ ~ -14 dex (cm²). Therefore, in typical interstellar environments Lyα photons have to undergo a complex radiative transfer. The exact conditions under which Lyα photons can escape a galaxy are poorly understood. Here we present results from three observational projects. In Chapter 2, we show integral field spectroscopic observations of 14 nearby star-forming galaxies in Balmer α radiation (Hα, λ = 6562.8 {\AA}). These observations were obtained with the Potsdam Multi Aperture Spectrophotometer at the Calar-Alto 3.5m Telescope}. Hα directly traces the intrinsic Lyα radiation field. We present Hα velocity fields and velocity dispersion maps spatially registered onto Hubble Space Telescope Lyα and Hα images. From our observations, we conjecture a causal connection between spatially resolved Hα kinematics and Lyα photometry for individual galaxies. Statistically, we find that dispersion-dominated galaxies are more likely to emit Lyα photons than galaxies where ordered gas-motions dominate. This result indicates that turbulence in actively star-forming systems favours an escape of Lyα radiation. Not only massive stars can power Lyα radiation, but also non-thermal emission from an accreting super-massive black hole in the galaxy centre. If a galaxy harbours such an active galactic nucleus, the rate of hydrogen-ionising photons can be more than 1000 times higher than that of a typical star-forming galaxy. This radiation can potentially ionise large regions well outside the main stellar body of galaxies. Therefore, it is expected that the neutral hydrogen from these circum-galactic regions shines fluorescently in Lyα. Circum-galactic gas plays a crucial role in galaxy formation. It may act as a reservoir for fuelling star formation, and it is also subject to feedback processes that expel galactic material. If Lyα emission from this circum-galactic medium (CGM) was detected, these important processes could be studied in-situ around high-z galaxies. In Chapter 3, we show observations of five radio-quiet quasars with PMAS to search for possible extended CGM emission in the Lyα line. However, in four of the five objects, we find no significant traces of this emission. In the fifth object, there is evidence for a weak and spatially quite compact Lyα excess at several kpc outside the nucleus. The faintness of these structures is consistent with the idea that radio-quiet quasars typically reside in dark matter haloes of modest masses. While we were not able to detect Lyα CGM emission, our upper limits provide constraints for the new generation of IFS instruments at 8--10m class telescopes. The Multi Unit Spectroscopic Explorer (MUSE) at ESOs Very Large Telescopeis such an unique instrument. One of the main motivating drivers in its construction was the use as a survey instrument for Lyα emitting galaxies at high-z. Currently, we are conducting such a survey that will cover a total area of ~100 square arcminutes with 1 hour exposures for each 1 square arcminute MUSE pointing. As a first result from this survey we present in Chapter 5 a catalogue of 831 emission-line selected galaxies from a 22.2 square arcminute region in the Chandra Deep Field South. In order to construct the catalogue, we developed and implemented a novel source detection algorithm -- LSDCat -- based on matched filtering for line emission in 3D spectroscopic datasets (Chapter 4). Our catalogue contains 237 Lyα emitting galaxies in the redshift range 3 ≲ z ≲ 6. Only four of those previously had spectroscopic redshifts in the literature. We conclude this thesis with an outlook on the construction of a Lyα luminosity function based on this unique sample (Chapter 6).},
  language  = {en}
}
@phdthesis{Streich2015,
  author    = {Streich, David},
  title     = {Understanding massive disk galaxy formation through resolved stellar populations},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-81027},
  school      = {Universit{\"a}t Potsdam},
  pages     = {ix, 140},
  year      = {2015},
  abstract  = {In this thesis we utilize resolved stellar populations to improve our understanding of galaxy formation and evolution. In the first part we improve a method for metallicity determination of faint old stellar systems, in the second and third part we analyze the individual history of six nearby disk galaxies outside the Local Group. A New Calibration of the Color Metallicity Relation of Red Giants for HST data: It is well known, that the color distribution of stars on the the Red Giant Branch (RGB) can be used to determine metallicities of old stellar populations that have only shallow photometry. Based on the largest sample of globular clusters ever used for such studies, we quantify the relation between metallicity and color in the widely used HST ACS filters F606W and F814W. We use a sample of globular clusters from the ACS Globular Cluster Survey and measure their RGB color at given absolute magnitudes to derive the color-metallicity relation. We find a clear relation between metallicity and RGB color; we investigate the scatter and the uncertainties in this relation and show its limitations. A comparison with isochrones shows reasonably good agreement with BaSTI models, a small offset to Dartmouth models, and a larger offset to Padua models. Even for the best globular cluster data available, the metallicity of a simple stellar population can be determined from the RGB alone only with an accuracy of 0.3 dex for [M/H]<-1, and 0.15 dex for [M/H]>-1. For mixed populations, as they are observed in external galaxies, the uncertainties will be even larger due to uncertainties in extinction, age, etc. Therefore caution is necessary when interpreting photometric metallicities. The Structural History of Nearby Low Mass Disk Galaxies: We study the individual evolution histories of three nearby, low-mass, edge-on galaxies (IC5052, NGC4244, NGC5023). Using the color magnitude diagrams of resolved stellar populations, we construct star count density maps for populations of different ages and analyze the change of structural parameters with stellar age within each galaxy. The three galaxies show low vertical heating rates, which are much lower than the heating rate of the Milky Way. This indicates that heating agents, as giant molecular clouds and spiral structure are weak in low mass galaxies. We do not detect a separate thick disk in any of the three galaxies, even though our observations cover a larger range in equivalent surface brightness than any integrated light study. While scaleheights increase with age, each population can be well described by a single disk. Only two of the galaxies contain a very weak additional component, which we identify as the faint halo. The mass of these faint halos is less than 1\% of the mass of the disk. All populations in the three galaxies exhibit no or only little flaring. While this finding is consistent with previous integrated light studies, it poses strong constraints on galaxy formation models, because most theoretical simulations often find strong flaring due to interactions or radial migration. Furthermore, we find breaks in the radial profiles of all three galaxies. The radii of these breaks are independent of age, and the break strength is decreasing with age in two of the galaxies (NGC4244 and NGC5023). This is consistent with break formation models, that combine a star formation cutoff with radial migration. The differing behavior of IC5052 can be explained by a recent interaction or minor merger. The Structural History of Massive Disk Galaxies: We extend the structural analysis of stellar populations with distinct ages to three massive galaxies, NGC891, NGC4565 and NGC7814. While confusion effects due to the high stellar number densities in their central region, and the prominent dust lanes inhibit an detailed analysis of the radial profiles, we can study their vertical structure. These massive galaxies also have a slower heating than the Milky Way, comparable to the low mass galaxies. This can be traced back to their already thick young populations and thick layers of their interstellar medium. We do not find a clear separate thick disk in any of these three galaxies; all populations can be described by a single disk plus a S\'ersic bulge/halo component. In contrast to the low mass galaxies, we cannot rule out the presence of thick disks in the massive galaxies, because of the strong influence of the halo, that might hide the possible contribution of the thick disk to the vertical star count profiles. However, the faintness of the possible thick disks still points to problems in the earlier ubiquitous findings of thick disks in external galaxies.},
  language  = {en}
}
@phdthesis{Schulze2011,
  author    = {Schulze, Andreas},
  title     = {Demographics of supermassive black holes},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-54464},
  school      = {Universit{\"a}t Potsdam},
  year      = {2011},
  abstract  = {Supermassive black holes are a fundamental component of the universe in general and of galaxies in particular. Almost every massive galaxy harbours a supermassive black hole (SMBH) in its center. Furthermore, there is a close connection between the growth of the SMBH and the evolution of its host galaxy, manifested in the relationship between the mass of the black hole and various properties of the galaxy's spheroid component, like its stellar velocity dispersion, luminosity or mass. Understanding this relationship and the growth of SMBHs is essential for our picture of galaxy formation and evolution. In this thesis, I make several contributions to improve our knowledge on the census of SMBHs and on the coevolution of black holes and galaxies. The first route I follow on this road is to obtain a complete census of the black hole population and its properties. Here, I focus particularly on active black holes, observable as Active Galactic Nuclei (AGN) or quasars. These are found in large surveys of the sky. In this thesis, I use one of these surveys, the Hamburg/ESO survey (HES), to study the AGN population in the local volume (z~0). The demographics of AGN are traditionally represented by the AGN luminosity function, the distribution function of AGN at a given luminosity. I determined the local (z<0.3) optical luminosity function of so-called type 1 AGN, based on the broad band B_J magnitudes and AGN broad Halpha emission line luminosities, free of contamination from the host galaxy. I combined this result with fainter data from the Sloan Digital Sky Survey (SDSS) and constructed the best current optical AGN luminosity function at z~0. The comparison of the luminosity function with higher redshifts supports the current notion of 'AGN downsizing', i.e. the space density of the most luminous AGN peaks at higher redshifts and the space density of less luminous AGN peaks at lower redshifts. However, the AGN luminosity function does not reveal the full picture of active black hole demographics. This requires knowledge of the physical quantities, foremost the black hole mass and the accretion rate of the black hole, and the respective distribution functions, the active black hole mass function and the Eddington ratio distribution function. I developed a method for an unbiased estimate of these two distribution functions, employing a maximum likelihood technique and fully account for the selection function. I used this method to determine the active black hole mass function and the Eddington ratio distribution function for the local universe from the HES. I found a wide intrinsic distribution of black hole accretion rates and black hole masses. The comparison of the local active black hole mass function with the local total black hole mass function reveals evidence for 'AGN downsizing', in the sense that in the local universe the most massive black holes are in a less active stage then lower mass black holes. The second route I follow is a study of redshift evolution in the black hole-galaxy relations. While theoretical models can in general explain the existence of these relations, their redshift evolution puts strong constraints on these models. Observational studies on the black hole-galaxy relations naturally suffer from selection effects. These can potentially bias the conclusions inferred from the observations, if they are not taken into account. I investigated the issue of selection effects on type 1 AGN samples in detail and discuss various sources of bias, e.g. an AGN luminosity bias, an active fraction bias and an AGN evolution bias. If the selection function of the observational sample and the underlying distribution functions are known, it is possible to correct for this bias. I present a fitting method to obtain an unbiased estimate of the intrinsic black hole-galaxy relations from samples that are affected by selection effects. Third, I try to improve our census of dormant black holes and the determination of their masses. One of the most important techniques to determine the black hole mass in quiescent galaxies is via stellar dynamical modeling. This method employs photometric and kinematic observations of the galaxy and infers the gravitational potential from the stellar orbits. This method can reveal the presence of the black hole and give its mass, if the sphere of the black hole's gravitational influence is spatially resolved. However, usually the presence of a dark matter halo is ignored in the dynamical modeling, potentially causing a bias on the determined black hole mass. I ran dynamical models for a sample of 12 galaxies, including a dark matter halo. For galaxies for which the black hole's sphere of influence is not well resolved, I found that the black hole mass is systematically underestimated when the dark matter halo is ignored, while there is almost no effect for galaxies with well resolved sphere of influence.},
  language  = {en}
}
@phdthesis{Jappsen2005,
  author    = {Jappsen, Anne-Katharina},
  title     = {Present and early star formation : a study on rotational and thermal properties},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-7591},
  school      = {Universit{\"a}t Potsdam},
  year      = {2005},
  abstract  = {We investigate the rotational and thermal properties of star-forming molecular clouds using hydrodynamic simulations. Stars form from molecular cloud cores by gravoturbulent fragmentation. Understanding the angular momentum and the thermal evolution of cloud cores thus plays a fundamental role in completing the theoretical picture of star formation. This is true not only for current star formation as observed in regions like the Orion nebula or the ρ-Ophiuchi molecular cloud but also for the formation of stars of the first or second generation in the universe. In this thesis we show how the angular momentum of prestellar and protostellar cores evolves and compare our results with observed quantities. The specific angular momentum of prestellar cores in our models agree remarkably well with observations of cloud cores. Some prestellar cores go into collapse to build up stars and stellar systems. The resulting protostellar objects have specific angular momenta that fall into the range of observed binaries. We find that collapse induced by gravoturbulent fragmentation is accompanied by a substantial loss of specific angular momentum. This eases the "angular momentum problem" in star formation even in the absence of magnetic fields. The distribution of stellar masses at birth (the initial mass function, IMF) is another aspect that any theory of star formation must explain. We focus on the influence of the thermodynamic properties of star-forming gas and address this issue by studying the effects of a piecewise polytropic equation of state on the formation of stellar clusters. We increase the polytropic exponent γ from a value below unity to a value above unity at a certain critical density. The change of the thermodynamic state at the critical density selects a characteristic mass scale for fragmentation, which we relate to the peak of the IMF observed in the solar neighborhood. Our investigation generally supports the idea that the distribution of stellar masses depends mainly on the thermodynamic state of the gas. A common assumption is that the chemical evolution of the star-forming gas can be decoupled from its dynamical evolution, with the former never affecting the latter. Although justified in some circumstances, this assumption is not true in every case. In particular, in low-metallicity gas the timescales for reaching the chemical equilibrium are comparable or larger than the dynamical timescales. In this thesis we take a first approach to combine a chemical network with a hydrodynamical code in order to study the influence of low levels of metal enrichment on the cooling and collapse of ionized gas in small protogalactic halos. Our initial conditions represent protogalaxies forming within a fossil HII region -- a previously ionized HII region which has not yet had time to cool and recombine. We show that in these regions, H2 is the dominant and most effective coolant, and that it is the amount of H2 formed that controls whether or not the gas can collapse and form stars. For metallicities Z <= 10<sup>-3 Zsun, metal line cooling alters the density and temperature evolution of the gas by less than 1\% compared to the metal-free case at densities below 1 cm<sup>-3 and temperatures above 2000 K. We also find that an external ultraviolet background delays or suppresses the cooling and collapse of the gas regardless of whether it is metal-enriched or not. Finally, we study the dependence of this process on redshift and mass of the dark matter halo.},
  subject      = {Sternentstehung},
  language  = {en}
}