The stellar distribution in ultra-faint dwarf galaxies suggests deviations from the collisionless cold dark matter paradigm

Unraveling the nature of dark matter (DM) stands as a primary objective in modern physics. I will present evidence suggesting deviations from the collisionless Cold DM (CDM) paradigm. In the standard cosmological model the dark matter (DM) particles are collisionless and, because of this very nature, they develop halos with the characteristic central cusp known as NFW profile. Real galaxies do not show NFW profiles but, rather, have a DM mass distribution with a central plateau or core, characteristic of self-gravitating systems in thermodynamic equilibrium. Within the standard model, the collisionless DM reaches equilibrium through baryon-driven processes able to transform the overall potential from cusp to core. Such mechanism becomes inoperative for galaxies with few stars (stellar mass <1e6 Msun), thus, finding cores in these low-mass galaxies would indicate that the DM is not collisionless, reflecting the much sought-after and currently unknown true nature of the DM (whether it is fuzzy, self-interacting, warm, or else). Measuring the DM distribution in tiny galaxies through traditional dynamical indicators is impossible, but we have developed a tool to constrain the distribution of DM from the stellar photometry alone. We have used the tool to analyze the stellar cores observed in 6 Ultra Faint Dwarf (UFD) galaxies with stellar masses between 1e3 and 1e4 Msun. They do not live in a NFW potential. Other conceivable explanations for the observed cores are disfavored by simulations and/or observations. Thus, the evidence suggests that collisions among DM particles or other alternatives to CDM are likely shaping these galaxies.