Extensive Summary of my PhD Thesis
“The nature of DM: 
gamma-ray searches and the formation of CDM halos”
 


PhD thesis awarded by the SEA (Spanish Society of Astronomy) with the distinction of being one out of the three best PhD Spanish Theses in Astrophysics in the period 2008-2009.


In this Thesis, the main research activities are focused on the understanding of the nature of the Dark Matter (DM). In order to shed some light on this challenging topic, I used different both theoretical and observational approaches. Below I summarize my work contribution to this field.


Since the beginning of my Thesis, a large effort was devoted to understanding how Cold DM halos form and evolve within the cosmological standard model. Indeed, I focused my first work on the understanding and characterization of the outskirts of CDM halos, i.e. well beyond the virial radius. This work is presented in detail in Chapter 2, and is based in the spherical infall model (SIM) without shell crossing. In addition, I also describe in the same Chapter the framework that allows for a comparison of these predictions with the results obtained from N-body cosmological simulations. The main conclusion of this work is that SIM, despite its simplicity, is capable to provide detailed predictions that are in good agreement with simulations at least at those large radii.


An ambitious work was also started to study the formation and evolution of cold DM halos by means of an improved SIM with shell-crossing. In Chapter 3, I present a framework to tackle this effect properly, that does not involve to use any adiabatic invariant, and that is based on the numerical follow-up of an individual shell of matter with time. This work does not include, by the moment, neither angular momentum nor velocity dispersion.


Within this framework -which I named as the Spherical Shell Tracker (SST)- I studied in detail the evolution of a halo, e.g. obtaining the exact moment when the first shell-crossing occurs, the exact values of time and radius for a given value of the linear or actual density contrasts, delta_l and delta respectively, the value of both density contrasts when the collapse occurs according to the standard SIM, or the relation between the linear and actual density contrasts, i.e., the function delta_l(delta). I investigated the dependence of the evolution with the virial mass, with the fraction considered respect to this virial mass, and with the cosmology for the cases of Einstein-deSitter and Omega_m=0.3, Omega_Lambda=0.7 cosmologies. What I found is that the results are very sensible to a variation of the virial mass or the fraction of virial mass that we consider. However, I obtain a negligible dependence with the cosmology. Furthermore, I show that the effect of shell-crossing plays a crucial role in the way that the halo evolves and reaches the virial equilibrium and the stabilization in radius. Indeed, the values currently adopted in the literature for the actual density contrast at the moment of virialization may not be accurate enough. This fact has important implications e.g. in the definition of a virial mass and a virial radius for the halo.


In this Thesis, most of the work related to DM detectability, and in particular all the work done concerning DM annihilations, is focused only on gamma-ray searches. This means that neither antimatter nor neutrinos as other possible annihilation products were explored. But why gamma-rays and not other wavelenghts? The keypoint is that the energy scale of the annihilation products is determined by the mass of the DM particles, as they typically carry a relatively large fraction of the available annihilation energy. Since the preferred DM candidates like the neutralino are expected to have masses of the order of GeV-TeV, this explains that DM searches are specially performed in the gamma-ray energy band.

Furthermore, I centered most of my DM search efforts in a DM annihilation scenario where the neutralino is the long-searched for WIMP that exists in sufficient quantities to constitute the totality of the non-baryonic DM in the Universe. This work is presented in Chapters 4, 6, 8, 9. The exception is Chapter 5, in which the DM detection prospects for another plausible candidate (the axion) was studied in detail, also in gamma-rays. In this case, predicted photon/axion mixings rather than self-annihilations are the vehicle used in the search of the DM particle.


Whenever possible, I combined both theory and observations, the latter being possible thanks to my participation in the MAGIC Collaboration. More in detail, and first regarding the theoretical approach, I carefully calculated the DM annihilation flux for the most promising candidates. In particular, flux predictions as well as detection prospects for a typical IACT and for the Fermi satellite for the Draco dwarf spheroidal galaxy are presented in Chapter 4. The results helped to understand the real potential of Draco as a good DM candidate and the real capabilities of the current IACTs in the search for DM in this dwarf. In the same Chapter, I also stress the crucial role of the angular resolution of the instrument in a correct interpretation of the observational data in the context of DM searches.


In the observational side, and as a member of the MAGIC Collaboration and active member of the MAGIC DM Working Group, I have been involved in the observational campaigns carried out for two dwarf galaxies: Draco (Chapter 8) and Willman 1 (Chapter 9). No gamma signal was found in any of these observations, and the derived upper limits seem to be still far from a successful detection according to theoretical predictions. An exclusion of some portion of the allowed region in the parameter space is not possible either; however, these observations represented the first serious attempt of DM searches in dwarf galaxy satellites carried out by an IACT, and the upper limits excluded a large annihilation signal at least (as claimed by some works in the literature). Finally, needless to say that the uncertainties in the flux predictions are huge, so IACT observations are encouraged in any case.


In addition to MAGIC, I also invested a significant effort  in order to launch the GAW R\&D experiment. GAW is an array of 3 IACTs planned to be located at Calar Alto Observatory, that will operate above 700 GeV in the near future. The main objective of GAW is to test the feasibility of a new generation of IACTs, which combine high sensitivity with a large Field of View. I worked on the definition of the science objectives of the instrument as well as in the justification of such an experiment. Chapter 6 is devoted to GAW and to describe my main scientific contributions inside the GAW Collaboration.


In an attempt to find and explore other plausible DM scenarios where the DM particle could be different from the neutralino, I have also investigated the possible role of ultra-light axions as DM candidates. The results, presented in Chapter 5, could be crucial for current gamma-ray experiments and observations. If these particles exist and have masses 1e-10 eV, photon/axion oscillations might occur in the presence of magnetic fields, such as those expected to be present in AGNs or in the Intergalactic Medium. This would lead to a distortion in the spectra of gamma-ray sources significantly, depending on source distance and the involved magnetic fields. Therefore, I did explore the detection prospects and propose the most appropriate observational strategy. This strategy would require a joint effort of Fermi and IACTs looking at distant AGNs (z>0.1). Moreover, I show that axions might be critical in a correct interpretation and modeling of the Extragalactic Background Light as well.


Main conclusions of the work presented in this Thesis and future work is presented in Chapters 10 and 11 respectively.


To conclude, it would be natural to ask why this huge interest on indirect gamma-ray DM searches precisely now, at the beginning of the 21st Century (specially taking into account that these kind of searches were proposed at least 25 years ago). There are good reasons to be specially optimistic at present: current gamma-ray experiments like IACTs and Fermi are reaching for the first time sensitivities good enough to be able to test some of the allowed and preferred scenarios (i.e. obeying WMAP constraints, taking well-motivated DM density profiles and using SUSY). The situation is expected to be even better when new generation telescopes enter in operation in the near future (CTA, AGIS...). The gamma-ray energy window has just opened to our discoveries, and a revolution in the GeV-TeV sky is on the way. It is time for gamma-ray DM searches.


DOWNLOAD THESIS