RT info:eu-repo/semantics/masterThesis
T1 Abundance of clustered primordial black holes from quasar microlensing
A1 Heydenreich, Sven Carl
A2 Máster Universitario en Astrofísica
K1 Quasar, Astrofísica
AB Ever since its discovery, the nature of Dark Matter has been a subject of many studies, papers andprojects. Despite nearly a century of research we are still unable to explain what forms about 80% ofthe matter in our Universe. While many believe that an unknown massive, collisionless elementaryparticle that only interacts gravitationally with baryonic matter is responsible for this phenomenon,there are some that argue that Dark Matter consists of MAssive Compact Halo Objects (MACHOs).This hypothesis has received new attention following the results of the Laster InterferometerGravitational Wave Observatory (LIGO). Since 2015, this observatory has reported the detectionof 5 black-hole mergers (and one merger between neutron stars, containing an electromagneticcounterpart). The masses of those black holes exceeded the ones expected for stellar remnants,so that the discussion about primordial black holes received new input. These black holes formedshortly after the big bang out of quantum density fluctuations and are speculated to exist untiltoday. They might be responsible for seeding the supermassive black holes in the centers of galaxiesand are argued to be a reasonable candidate for dark matter: As they posess an enormous densitythey can be seen as nearly collisionless and, considering their mass, black holes are usually relativelydark. The masses and abundances of primordial black holes are constrained by Cosmic MicrowaveBackground analysis, but there is a quite large mass range still open for these kinds of black holes.One of the most useful tools to study Dark Matter is arguably gravitational lensing. This phenomenonis sensitive to the total matter, baryonic and dark, of the lensing object. In contrastto collisionless elementary particles, that form a smooth matter distribution and only significantlychange over galactic distances, MACHOs can engage in microlensing, which is a peculiar form ofgravitational lensing where the lensed object is not measurably displaced or distorted, but nonethelessimmensely magnified. For this kind of gravitational lensing it is necessary that the source isextremely small. Usually, sources for gravitational microlensing are either stars or quasars (QUAsiStellAr Radio Sources). The unique aspect of microlensing is that it happens on small scales bothspatially (light-day scales over cosmological distances) and timewise (ranging from several hours tomonths). This means that there are in general two kinds of study one can perform:The first possibility is to observe objects over a long period of time and examine changes in flux,which can then be attributed to microlensing. The other possibility is to observe a large number ofobjects at one time and inspect the number of objects that are affected by microlensing. In bothcases a statistical analysis needs to be performed afterwards. It seems evident that gravitationalmicrolensing is an extremely helpful method to verify whether dark matter can exist of MACHOs.In this thesis we will develop a method to probe the abundance of primordial black holes inlens galaxies using data of gravitationally lensed quasars. These extremely luminous objects areoften lensed by other galaxies such that several images of the same quasar are visible, which allowsus to determine the magnification due to microlensing of the images. We want to use data onseveral lensed quasars to determine this change in magnification and compare this with numericalsimulations. To do this we will first develop an algorithm that constructs magnification maps for anarbitrary collection of microlenses. Afterwards we will write a script to extract the magnificationhistograms out of those maps and perform a bayesian analysis, using data from lensed quasars.We will discuss several uncertainties, especially the difficulty in determining the magnification of alensed quasar.This thesis builds on the work of Mediavilla et al. (2017), who already performed this analysisand concluded that the effect of primordial black holes would have been visible in microlensing data.In addition to their work, we will consider an additional aspect: Due to the nature and formationof primordial black holes it is reasonable to assume that they primarily appear in clusters. We willtherefore perform one analysis for a uniformly distributed set of microlenses and another one for aclustered set, comparing the results.Apart from the fraction of mass in microlenses, which is the primary parameter in almost everygravitational microlensing experiment, we chose to parametrize the cluster radius and the numberof black holes per cluster. Other possible parameters like the clustering efficiency or the source sizehad to be neglected due to finite processing power. We will discuss which values to choose for theseperate parameters and which effects the left-out ones could induce.We find that, as expected, clustered black holes induce a slightly different magnification probabilitydistribution and are in general less effective to cause microlensing (the mean values of themagnification maps differ by up to a factor of 2). Although we can not yet compare our analysiswith real data due to limited processing power, we conclude that these differences might be enoughto explain the apparent contradiction with current microlensing data. The reduced efficiency in microlensingmight be the reason as to why we have not yet seen the effects of primordial black holes inquasar gravitational microlensing. However, determining the magnification of the real counterpartof the simulations is more difficult than in the case of a uniform distribution: The clusters of blackholes appear for distant light rays as a single, massive lens which introduces huge caustics on themagnification maps that have a physical size which is larger than the broad line emission regionand even spans a significant fraction of the narrow line emission region, both of which are usuallyused to determine a baseline for the magnification of the quasar as they are normally large enoughthat microlensing by single sources gets ‘washed out’.The code developed in this thesis will in the following weeks be used on a computer cluster toperform the analysis for a set of gravitationally lensed quasars and see whether the existence ofclustered primordial black holes is consistent with current microlensing data.
YR 2018
FD 2018
LK http://riull.ull.es/xmlui/handle/915/7294
UL http://riull.ull.es/xmlui/handle/915/7294
LA es
DS Repositorio institucional de la Universidad de La Laguna
RD 29-abr-2024