Is Sextans dwarf galaxy in a scalar field dark matter halo?

Sub-horizon perturbations under the extreme initial condition of the axion model are investigated, where initial axion angles start near the potential maximum. This work focuses on a few new features found in the extreme axion model but absent in the free-particle model. A particularly novel new feature is the spectral excess relative to the CDM model in some wave number range, where the excess may be so large that landscapes of high-redshift universe beyond z = 10 can be significantly altered. For axions of particle mass 10 −22 eV, this range of wave number corresponds to first galaxies of few times 10 9 − 10 10 M⊙. We demonstrate that sub-horizon perturbations are accurately described by Mathieu's equation and subject to parametric instability, which explains this novel feature. Actually the axion model is not a special one; perturbations in a wide range of scalar field models can share the similar characteristic.

Section: Introductionmentioning

confidence: 64%

Cosmological perturbations of extreme axion in the radiation era

Zhang

Chiueh

2017

“…For example, it produces finite density cores for Milky Way dwarf spheroid galaxies to reside in [2,6], an observational fact [7][8][9][10][11] that has been puzzling cold dark matter (CDM) proponents for some time. This core in the ψDM scenario results from the uncertainty principle of these extremely light particles, which yields quantum pressure and avoids the central density singularity predicted by CDM [12].…”

Section: Introductionmentioning

confidence: 99%

Evolution of linear wave dark matter perturbations in the radiation-dominated era

Zhang

Chiueh

2017

Linear perturbations of the wave dark matter, or ψ dark matter (ψDM), of particle mass ∼ 10 −22 eV in the radiation-dominant era are analyzed, and the matter power spectrum at the photonmatter equality is obtained. We identify four phases of evolution for ψDM perturbations, where the dynamics can be vastly different from the counterparts of cold dark matter (CDM). While in late stages after mass oscillation long-wave ψDM perturbations are almost identical to CDM perturbations, some subtle differences remain, let alone intermediate-to-short waves that bear no resemblance with those of CDM throughout the whole evolutionary history. The dissimilarity is due to quantum mechanical effects which lead to severe mode suppression. We also discuss the axion model with a cosine field potential. The power spectrum of axion models are generally almost identical to those of ψDM, but in the extreme case when the initial axion angle is near the field potential top, this axion model predict a power excess over a range of wave number and a higher spectral cutoff than ψDM as if ψDM had a higher particle mass.

“…(130) applies in the very nonlinear regime (when the DM halos are formed). The mass-radius relationships (129) and (130) are therefore valid in two extremely different regimes (begining and end of structure formation). It is therefore intriguing that they have the same scaling and that they differ only by a numerical factor 1.64 of order unity.…”

Section: The Jeans Scalesmentioning

confidence: 99%

Jeans-type instability of a complex self-interacting scalar field in general relativity

Suárez

Chavanis

2018

We study the gravitational instability of a general relativistic complex scalar field with a quartic self-interaction in an infinite homogeneous static background. This quantum relativistic Jeans problem provides a simplified framework to study the formation of the large-scale structures of the Universe in the case where dark matter is made of a scalar field. The scalar field may represent the wave function of a relativistic self-gravitating Bose-Einstein condensate. Exact analytical expressions for the dispersion relation and Jeans length λJ are obtained from a hydrodynamical representation of the Klein-Gordon-Einstein equations [Suárez and Chavanis, Phys. Rev. D 92, 023510 (2015)]. We compare our results with those previously obtained with a simplified relativistic model [Khlopov, Malomed and Y. B. Zeldovich, Mon. Not. R. astr. Soc. 215, 575 (1985)]. When relativistic effects are fully accounted for, we find that the perturbations are stabilized at very large scales of the order of the Hubble length λH . Numerical applications are made for ultralight bosons without self-interaction (fuzzy dark matter), for bosons with a repulsive self-interaction, and for bosons with an attractive self-interaction (QCD axions and ultralight axions). We show that the Jeans instability is inhibited in the ultrarelativistic regime (early Universe and radiationlike era) because the Jeans length is of the order of the Hubble length (λJ ∼ λH ), except when the self-interaction of the bosons is attractive. By contrast, structure formation can take place in the nonrelativistic regime (matterlike era) for λJ ≤ λ ≤ λH . Since the scalar field has a nonzero Jeans length due to its quantum nature (Heisenberg uncertainty principle or quantum pressure due to the self-interaction of the bosons), it appears that the wave properties of the scalar field can stabilize gravitational collapse at small scales, providing halo cores and suppressing small-scale linear power. This may solve the CDM crisis such as the cusp problem and the missing satellite problem. We also compare our results with those obtained in the case where dark matter is made of fermions instead of bosons. PACS numbers: 95.35.+d, 98.80.Jk, 95.30.Sf