Observational consequences of Black Holes in the Universe: From dark matter candidates to quasars

Abstract

The existence of black holes that go beyond the mass given by their stellar origin has been known for some time. On the lower end, a subset of primordial black holes (PBHs) could form all of the dark matter with individually very low masses of up to 10-11 M⊙. On the opposite mass range Super Massive Black holes (SMBHs) have been found at very high redshifts. Using models and instrumental techniques, we study the observational consequences of both of these types of black holes. We start by studying current constraints on PBHs. We look at microlensing, which makes up the largest constraints on PBHs as dark matter, and conclude that despite its usefulness the method will not improve the constraints for lower mass ranges. We make similar analyses of gravitational waves and PBH evaporation, with the former having a lot of potential but requiring the next generation of experiments, and the later being a simple case with few further constraints. Finally, we zero in the last remaining window for PBHs to be all the dark matter around 10-12 M⊙. The PBHs in this mass range would leave little observational results, but a possible way to constrain them would be through their interaction with stars. As stars form in the very early universe, they will accrue a large dark matter density gravitationally bound to the star. If PBHs were the dark matter, they will orbit the star, following a flat eccentricity distribution. There is a chance some of the orbits of the PBHs naturally cross the star. The dynamical friction of a main sequence star on the PBH is enough to bring the PBH to the core of the star within the Hubble time, capturing it. There, the PBH will accrete the star resulting in a black hole the mass of the star. We compute the capture rate of such PBHs by a number of stars. We use stellar models from MENSA ranging from 0.3 to 1 M⊙ for the stars, compute the dynamical friction numerically using two different types and take into account the effect of perturbations coming from the rest of the galaxy. The result, which we call Ξ, is generic and can be used to compute the capture rate for any dark matter density and velocity dispersion. For the case z ∼ 20, and our models tells us that we expect stars represented on our stellar models that are very close of the center of their galaxies to end capturing a PBH and being accreted in turn. The capture rate lowers with distance, but it is still relevant at larger distances. This should result in a wealth of subsolar mass black holes that would survive to this day. We finally study the case of quasi stellar objects (QSOs), SMBHs with very high luminosity. The presence of Lymanα nebulae surrounding them is relevant, as we do not know whereas it is generic feature present in QSOs or if they are singular cases. A possible solution is through stacking. The technique allows the use of multiple images to reach depths otherwise impossible. For stacking QSOs, the upcoming survey J-PAS and its predecessor J-PLUS are the most promising. We use stacking in J-PLUS for a total of ∼ 1, 550 QSOs and more than three hundred thousand stars, the later to obtain an accurate recreation of the points spread. We find that J-PLUS cannot reach the magnitude needed, but we reach very high depths for the star stack in line with our projections. We expect to reach the magnitude needed to observe the diffuse Lymanα signal with J-PAS.

La tesi doctoral consisteix en un estudi de forats negres de massa extrema i les seves possibles conseqüències observacionals. Forats negres de molt baixa massa, forats negres primordials, podrien ser tota la matèria fosca però només en un rang de masses molt petit. Tot i aquestes limitacions queda un espai en un rang de masses properes a les d’asteroides i molt poques maneres d’aconseguir possibles observables de la seva existència. Una forma seria la captura d’aquests forats negres per part d’una estrella de seqüència principal. Això podria passar a alts redshifts, amb altes densitats i baixes velocitats. Analitzem aquest cas i proposem un model. A pesar de simplificacions, el nostre model millora sobre la literatura alhora de calcular numèricament la fricció dinàmica de dos formes diferents amb models estel·lars punters i amb un mecanisme que aproxima l’efecte de pertorbacions externes sobre el sistema. Utilitzem també un paràmetre que anomenem Xi que ens permet generalitzar els resultats. En el cas d’alts redshifts, concloem que la captura de forats negres primordials seria important, eliminant un percentatge notable d’estrelles i deixant molts forats negres de massa subsolar. L’altre extrem són els forats negres supermassius, entre ells els quàsars. Una forma d’estudiar-los és a través de nebuloses Lyman alfa. Són molt febles, pel que es dubta la seva existència per qualsevol quàsar. Amb la tècnica stacking, consistent en sumar imatges per reduir el soroll tèrmic, arribem a les majors profunditats necessàries, tot i que també magnifica la Point Spread Function (PSF). Utilitzem el cartografiatge J-PLUS pel stacking com a pas previ al futur cartografiatge J-PAS. J-PLUS no arriba a les magnituds necessàries per descobrir les nebuloses amb quàsars, però amb estrelles arribem a magnituds molt altes. Esperem que amb J-PAS hauria de ser fàcil veure les nebuloses.