Gravitational Microlensing due to Dark Matter Subhalos

Abstract

Our Galaxy, the Milky way, is surrounded by a dark matter spherical halo. This halo is not homogeneous, indeed it has substructure. Bound dark matter substructure, "subhalos", are dark matter dynamic clumps with a roughly spherical shape. A promising indirect way to detect dark matter subhalos in our Galaxy is the phenomenon of microlensing. When a distant star's light path goes through the proximity of a heavy astronomical object, the path "bends" and the star appears to be in a different position. As the substructure is constantly moving, the distant star's image will move as well, which in fact is a signature of astronomic microlensing by dark matter subhalos. This thesis studies how gravitational microlensing can contribute to search of dark matter.

The first chapter depicts the current picture of dark matter. Some of the most compelling pieces of evidence for the existence of dark matter are explained. We present the characteristics of dark matter candidates and focus on a specific candidate, the Weakly Interacting Massive Particle. We also describe the dark matter distribution and substructure in the Milky Way. Finally, we outline two current approaches to detect dark matter.

In the second chapter, we derive an equation for the deflection angle that a galactic subhalo induces in the light path of a distant source. This equation is used to make predictions for the microlensing signature of possible dark matter subhalos.

The third chapter is devoted to the analysis of the relationships between the deflection angle and the parameters involved in a microlensing event. As an input we use the subhalos created in the N-body simulation \texttt{Via Lactea}. From the analysis we deduce under which conditions microlensing events are more likely to be observed.

Chapter 4 introduces the topic of dark matter detection from its particles' annihilation. Dark matter subhalos can be detected from the standard model products of the annihilations, e.g., photons. Gravitational microlensing can be used to crosscheck whether the subhalo candidates found are indeed dark matter. This chapter reviews the theory of dark matter annihilation into photons. Also, we reconstructed a whole sky map with dark matter contribution to the electromagnetic background radiation.

In chapter 5 we make predictions for the microlensing signatures that actual subhalo candidates can induce. Subhalo candidates are found using two techniques, the Non-Poissonian Template Fitting and looking for spatially extended sources with emission only in the gamma-rays. For both techniques we explain the underlying physics behind the respective approach, describe the process of subhalo candidates selection, relate the subhalos' information obtained from those techniques with the subhalo properties needed to calculate the deflection angle and finally, make predictions for possible microlensing events induced by the subhalo candidates under consideration.

The last chapter summarizes the important points of this thesis and proposes a detection strategy to observe microlensing events. We also describe the Gaia mission as it has the instruments needed to measure these microlensing signatures.