There are several issues to do with dwarf galaxy predictions in the standard Λ cold dark matter (ΛCDM) cosmology that have suscitated much recent debate about the possible modification of the nature ...of dark matter as providing a solution. We explore a novel solution involving ultralight axions that can potentially resolve the missing satellites problem, the cusp-core problem and the 'too big to fail' problem. We discuss approximations to non-linear structure formation in dark matter models containing a component of ultralight axions across four orders of magnitude in mass, 10−24
m
a
10−20 eV, a range too heavy to be well constrained by linear cosmological probes such as the cosmic microwave background and matter power spectrum, and too light/non-interacting for other astrophysical or terrestrial axion searches. We find that an axion of mass m
a
10−21 eV contributing approximately 85 per cent of the total dark matter can introduce a significant kpc scale core in a typical Milky Way satellite galaxy in sharp contrast to a thermal relic with a transfer function cut off at the same scale, while still allowing such galaxies to form in significant number. Therefore, ultralight axions do not suffer from the Catch 22 that applies to using a warm dark matter as a solution to the small-scale problems of CDM. Our model simultaneously allows formation of enough high-redshift galaxies to allow reconciliation with observational constraints, and also reduces the maximum circular velocities of massive dwarfs so that baryonic feedback may more plausibly resolve the predicted overproduction of massive Milky Way Galaxy dwarf satellites.
Abstract
The cosmic microwave background (CMB) places stringent constraints on models of dark matter (DM), and on the initial conditions of the Universe. The full Planck data set is used to test the ...possibility that some fraction of the DM is composed of ultralight axions (ULAs). This represents the first use of CMB lensing to test the ULA model. We find no evidence for a ULA component in the mass range 10−33 ≤ ma ≤ 10−24 eV. We put percent-level constraints on the ULA contribution to the DM, improving by up to a factor of two compared using temperature anisotropies alone. Axion DM also provides a low-energy window on to the physics of inflation through isocurvature perturbations. We perform the first systematic investigation into the parameter space of ULA isocurvature, using an accurate isocurvature transfer function at all ma values. We precisely identify a ‘window of co-existence’ for 10−25 eV ≤ ma ≤ 10−24 eV where the data allow, simultaneously, a ${\sim }10\,\,\rm{per\,\,cent}$ contribution of ULAs to the DM, and ${\sim } 1\,\,\rm{per\,\,cent}$ contributions of isocurvature and tensor modes to the CMB power. ULAs in this window (and all lighter ULAs) are shown to be consistent with a large inflationary Hubble parameter, HI ∼ 1014 GeV. The window of co-existence will be fully probed by proposed CMB Stage-IV observations with increased accuracy in the high-ℓ lensing power and low-ℓ E- and B-mode polarizations. If ULAs in the window exist, this could allow for two independent measurements of HI in the CMB using isocurvature, and the tensor contribution to B modes.
Self-gravitating bosonic fields can support stable and localized (solitonic) field configurations. Such solitons should be ubiquitous in models of axion dark matter, with their characteristic mass ...and size depending on some inverse power of the axion mass, m
a. Using a scaling symmetry and the uncertainty principle, the soliton core size can be related to the central density and axion mass in a universal way. Solitons have a constant central density due to pressure support, unlike the cuspy profile of cold dark matter (CDM). Consequently, solitons composed of ultralight axions (ULAs) may resolve the ‘cusp–core’ problem of CDM. In dark matter (DM) haloes, thermodynamics will lead to a CDM-like Navarro–Frenk–White (NFW) profile at large radii, with a central soliton core at small radii. Using Monte Carlo techniques to explore the possible density profiles of this form, a fit to stellar kinematical data of dwarf spheroidal galaxies is performed. The data favour cores, and show no preference concerning the NFW part of the halo. In order for ULAs to resolve the cusp–core problem (without recourse to baryon feedback, or other astrophysical effects) the axion mass must satisfy m
a < 1.1 × 10−22 eV at 95 per cent C.L. However, ULAs with m
a ≲ 1 × 10−22 eV are in some tension with cosmological structure formation. An axion solution to the cusp–core problem thus makes novel predictions for future measurements of the epoch of reionization. On the other hand, improved measurements of structure formation could soon impose a Catch 22 on axion/scalar field DM, similar to the case of warm DM.
If the dark matter (DM) were composed of axions, then structure formation in the Universe would be suppressed below the axion Jeans scale. Using an analytic model for the halo mass function of a ...mixed DM model with axions and cold dark matter, combined with the abundance-matching technique, we construct the UV-luminosity function. Axions suppress high-z galaxy formation and the UV-luminosity function is truncated at a faintest limiting magnitude. From the UV-luminosity function, we predict the reionization history of the universe and find that axion DM causes reionization to occur at lower redshift. We search for evidence of axions using the Hubble Ultra Deep Field UV-luminosity function in the redshift range z = 6–10, and the optical depth to reionization, τ, as measured from cosmic microwave background polarization. All probes we consider consistently exclude m
a
≲ 10−23 eV from contributing more than half of the DM, with our strongest constraint ruling this model out at more than 8σ significance. In conservative models of reionization a dominant component of DM with m
a
= 10−22 eV is in 3σ tension with the measured value of τ, putting pressure on an axion solution to the cusp-core problem. Tension is reduced to 2σ for the axion contributing only half of the DM. A future measurement of the UV-luminosity function in the range z = 10–13 by JWST would provide further evidence for or against m
a
= 10−22 eV. Probing still higher masses of m
a
= 10−21 eV will be possible using future measurements of the kinetic Sunyaev–Zel'dovich effect by Advanced ACTPol to constrain the time and duration of reionization.
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