The nature of dark matter from its distribution in galaxies

Abstract

Rotation curves of galaxies trace their gravitational potential and provide key information about the mass distribution within them. These were one of the first indicators about the presence of a new form of matter that is non-luminous, called dark matter. Over the years, the existence of dark matter has been proven unquestionably, but its nature still remains to be determined. Apart from giving information about the distribution of dark matter in galaxies, rotation curves can provide hints about its nature. The ΛCDM model, which is presently the Standard Model of cosmology, predicts the existence of dark matter halo cusps in the centres of galaxies. On the other hand, observations of galaxies of different Hubble types and luminosities indicate the presence of central cores. The Milky Way galaxy, which is a spiral galaxy, is usually modeled using the cuspy NFW profile. We instead attempt to model the Galaxy using an empirical cored profile known as the Burkert profile. We fit the rotation curve of the Milky Way constructed using kinematical data from various sources using the model of Burkert dark matter halo, thin exponential disk and point mass bulge by the 𝜒² minimization method. We obtain very good fits using the model, which shows that the Galaxy can be modeled using a cored dark matter profile. In addition, we obtain the values of the dark matter halo parameters, the central density ⍴₀ = 1.53+0.26-0.15 × 10⁻²⁴ g cm⁻³, the core radius R₀ = 12.4+1.2-0.2 kpc and the local dark matter density 0.65+0.05-0.06 × 10⁻²⁴ g cm⁻³. These are important quantities for dark matter detection experiments. Next, we investigate the dark matter haloes of massive high-luminosity spiral galaxies. We select a sample of 14 nearby massive galaxies from sources of rotation curves and surface photometry. We then model these galaxies using a Burkert halo, thin exponential disk and Sersic/point bulge. We find that massive galaxies also have cores in the inner regions of the DM halo. These cores are quite large, with radii greater than 10 kpc. These cannot be accounted for by ΛCDM models incorporating baryonic feedback. Hence, this observation provides a push towards other dark matter candidates like self-interacting dark matter, warm dark matter and fuzzy dark matter, which could directly lead to formation of cores.