Can light dark matter solve the core-cusp problem?

Abstract

Recently there has been much interest in light dark matter, especially ultra-light axions, as they may provide a solution to the core-cusp problem at the center of galaxies. Since very light bosons can have a de Broglie wavelength that is of astrophysical size, they can smooth out the centers of galaxies to produce a core, as opposed to vanilla dark matter models, and so it has been suggested that this solves the core-cusp problem. In this work, we critically examine this claim. While an ultra-light particle will indeed lead to a core, we examine whether the relationship between the density of the core and its radius matches the data over a range of galaxies. We first review data that shows the core density of a galaxy ρc varies as a function of the core radius Rc as ρc ∝ 1/R β c with β ≈ 1. We then compare this to theoretical models. We examine a large class of light scalar dark matter models, governed by some potential V . For simplicity, we take the scalar to be complex with a global U (1) symmetry in order to readily organize solutions by a conserved particle number. However, we expect our central conclusions to persist even for a real scalar, and furthermore, a complex scalar matches the behavior of a real scalar in the non-relativistic limit which is the standard regime of interest. For any potential V , we find the relationship between ρc and Rc for ground state solutions is always in one of the following regimes: (i) β 1, or (ii) β 1, or (iii) unstable, and so it never matches the data. We also find similar conclusions for virialized dark matter, more general scalar field theories, degenerate fermion dark matter, superfluid dark matter, and general polytropes. We conclude that the solution to the core-cusp problem is more likely due to either complicated baryonic effects or some other type of dark matter interactions.

Stability of condensed fuzzy dark matter halos

“…We represent Andromeda by the 6-pointed star in Figure 2. Had the black hole or soliton of M31 been only a factor of a few heavier, this point would be to the right of the dash-dotted line, and the stability of M31 would 4 Some have even claimed that velocity measurements in the central bulge of the Milky Way actually suggest the existence of a condensate [84], though other analyses suggest tension [74]. thorough study of currently-known supermassive black holes, to further test the FDM paradigm in this way.…”

Oscillons and dark matter

“…These possibilities are especially likely to occur in the mass range which has been recently investigated in the context of fuzzy DM models [4,9]. Here, we try to make contact with another peculiarity of this class of DM candidates, by proposing oscillons as early seeds for the well-known solitons of ULDM, which are however sustained by gravity (see [42,43,44,45] for more recent studies), in contrast with the objects which we focus on in this work. We thus describe constraints on different scenarios with dark matter made of two components, at least during a part of the cosmological history: field oscillations around the minimum of the potential and MACHO-like oscillons, with masses which increase as F increases and m decreases and span the range 10 −12 M to 10 8 M .…”

Looking for ultralight dark matter near supermassive black holes

“…Rotation curves of low-surface-brightness galaxies (LSBs) also disfavour m 10 −21 eV [35,44], if one accepts the soliton cores predicted by numerical simulations [22,23,64]. Independent evidence from rotation curve data against ULDM cores was reported in [65]. Dynamical heating of the MW disk [66] and a preliminary analysis of stellar streams [67] disfavour m 10 −22 eV.…”