Dr. Sankha Subhra Chakrabarty

Dipartimento di Fisica, Università di Torino

Caustic rings and the quantum effects in axion dark matter

Axions or axion-like particles are highly motivated candidates for dark matter. In the literature, they are often treated as classical fields. However, in the context of thermalization, quantum and classical field descriptions differ from one another. We explain how the caustic ring model of dark matter halo is the manifestation of the quantum phenomena like Bose-Einstein condensation of axions. We predict various astrophysical signatures of the caustic ring model. By simulating the dynamics of the stars in the gravitational fields of the caustic rings, we predict stellar overdensities of order 120, 45, 30 and 15 per cent near 2nd, 3rd, 4th and 5th caustic rings, respectively. Stellar overdensities from the 2nd and 3rd caustic rings have observational support. Evidence of such overdensities near the 4th and 5th caustic rings may be found by analyzing the Gaia data. We also predict the density profile of the interstellar gas near a caustic ring assuming the gas to be in thermal equilibrium in the gravitational potential well of the caustic and the gas itself. The density profile is found to have a triangular feature due to the tricusp cross section of the caustic ring. Sky maps from IRAS and Gaia contain evidence of such features due to the 5th caustic ring. We argue that the bumps in the rotation curves of the spiral galaxies may be caused by the caustic rings in the halo.