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The impact of collisional dark matter in galaxy formation: Time for a paradigm shift?

Final Report Summary - GALFORM_SELFIDM (The impact of collisional dark matter in galaxy formation: Time for a paradigm shift?)


Discovering the nature of dark matter (DM) and understanding its role in the formation and evolution of galaxies is one of the most fundamental problems in modern physics. Formidable progress over the last few decades in theoretical, numerical, and observational studies has led to the establishment of the Cold Dark Matter (CDM) paradigm, whose success lies in its capacity to reproduce the observed properties of the the Universe at the largest cosmological scales, using only simple assumptions about the DM nature. On galactic scales however, the CDM model faces enduring discrepancies with observations, primarily with the smallest galaxies, by seemingly over-predicting their abundance and central DM densities. It is possible that the cause of this discrepancy lays in our incomplete understanding of processes that are key to understand how galaxies form and evolve: gas cooling, star formation and feedback from supernovae (SNe), commonly referred to as "baryonic physics". It is also possible that a change in the fundamental hypotheses of the CDM model might be required to explain observations. In particular, well-motivated models where DM can collide with itself (Self-Interacting Dark Matter, SIDM) offer one the most viable solutions to the CDM challenges. The predictions of SIDM models however, have yet to be developed within the context of a full galaxy formation model.

This situation motivated the current project that accomplished the following goals:

1. Our team was the first in the world to perform hydrodynamical simulations of galaxy formation and evolution, in the context of a SIDM cosmology, that include: gas cooling, star formation and SNe feedback.

Our results show that small galaxies formed in SIDM have similar global properties than in CDM (e.g. their total stellar mass), but they are significantly different at subgalactic scales. In particular, contrary to the CDM case, their stellar distribution is tied to their dark matter distribution, which is a promising way to look for signatures of dark matter collisions in future observations.

2. Put forward the first steps towards developing ETHOS, an effective theory that generalizes the standard model of galaxy formation and evolution, by including a broad range of new dark matter interactions (including among other possibilities self-scattering, i.e. SIDM).

Achieving this goal required the collaboration of particle physicists, cosmologists and astrophysicists, which highlights the importance of interdisciplinary studies on the nature of dark matter. With ETHOS, we have established a mapping between a broad class of mycrophysical models of dark matter to the effective parameters that are relevant for the formation and evolution of galaxies. We have put forward the conditions necessary to solve the problems faced by CDM at the scale of dwarf galaxies through new dark matter interactions.

As a result of this research programme, 4 articles have been published in international peer-reviewed journals, with nearly 100 citations to date, 1 MSc. Thesis project, and 1 BSc. Thesis project were concluded, and 11 lectures were given about the project in seminars/colloquia/conferences at different institutions in the world.