arXiv subjects

Alma X. Gonzalez-Morales

Publications and source records attributed to Alma X. Gonzalez-Morales.

29 records · Page 2Linked to original sources

arXiv2018

Verlinde's emergent gravity versus MOND and the case of dwarf spheroidals

In a recent paper, Erik Verlinde has developed the interesting possibility that spacetime and gravity may emerge from the entangled structure of an underlying microscopic theory. In this picture, dark matter arises as a response to the standard model of particle physics from the delocalized degrees of freedom that build up the dark energy component of the Universe. Dark matter physics is then regulated by a characteristic acceleration scale $a_0$, identified with the radius of the (quasi)-de Sitter universe we inhabit. For a point particle matter source, or outside an extended spherically symmetric object, MOND's empirical fitting formula is recovered. However, Verlinde's theory critically departs from MOND when considering the inner structure of galaxies, differing by a factor of 2 at the centre of a regular massive body. For illustration, we use the eight classical dwarf spheroidal satellites of the Milky Way. These objects are perfect testbeds for the model given their approximate spherical symmetry, measured kinematics, and identified missing mass. We show that, without the assumption of a maximal deformation, Verlinde's theory can fit the velocity dispersion profile in dwarf spheroidals with no further need of an extra dark particle component. If a maximal deformation is considered, the theory leads to mass-to-light ratios that are marginally larger than expected from stellar population and formation history studies. We also compare our results with the recent phenomenological interpolating MOND function of McGaugh {\it et al}, and find a departure that, for these galaxies, is consistent with the scatter in current observations.

astro-ph.CO↗

arXiv2017

Prospects for indirect MeV Dark Matter detection with Gamma Rays in light of Cosmic Microwave Background Constraints

The self-annihilation of dark matter particles with mass in the MeV range can produce gamma rays via prompt or secondary radiation. The annihilation rate for such light dark matter particles is however tightly constrained by cosmic microwave background (CMB) data. Here we explore the possibility of discovering MeV dark matter annihilation with future MeV gamma-ray telescopes taking into account the latest and future CMB constraints. We study the optimal energy window as a function of the dominant annihilation final state. We consider both the (conservative) case of the dwarf spheroidal galaxy Draco and the (more optimistic) case of the Galactic center. We find that for certain channels, including those with one or two monochromatic photon(s) and one or two neutral pion(s), a detectable gamma-ray signal is possible for both targets under consideration, and compatible with CMB constraints. For other annihilation channels, however, including all leptonic annihilation channels and two charged pions, CMB data rule out any significant signal of dark matter annihilation at future MeV gamma-ray telescopes from dwarf galaxies, but possibly not for the Galactic center.

astro-ph.CO↗

arXiv2017

Unbiased constraints on ultralight axion mass from dwarf spheroidal galaxies

It has been suggested that the internal dynamics of dwarf spheroidal galaxies (dSphs) can be used to test whether or not ultralight axions with $m_a\sim 10^{-22}\text{eV}$ are a preferred dark matter candidate. However, comparisons to theoretical predictions tend to be inconclusive for the simple reason that while most cosmological models consider only dark matter, one observes only baryons. Here we use realistic kinematic mock data catalogs of Milky Way dSph's to show that the "mass-anisotropy degeneracy" in the Jeans equations leads to biased bounds on the axion mass in galaxies with unknown dark matter halo profiles. In galaxies with multiple chemodynamical components this bias can be partly removed by modelling the mass enclosed within each subpopulation. However, analysis of the mock data reveals that the least-biased constraints on the axion mass result from fitting the luminosity-averaged velocity dispersion of the individual chemodynamical components directly. Applying our analysis to two dSph's with reported stellar subcomponents, Fornax and Sculptor, and assuming that the halo profile has not been acted on by baryons, yields core radii $r_{c}>1.5$ kpc and $r_c> 1.2$ kpc respectively, and $m_a<0.4\times 10^{-22}\text{eV}$ at 97.5\% confidence. These bounds are in tension with the number of observed satellites derived from simple (but conservative) estimates of the subhalo mass function in Milky Way-like galaxies. We discuss how baryonic feedback might affect our results, and the impact of such a small axion mass on the growth of structures in the Universe.

astro-ph.CO↗

arXiv2017

Towards accurate cosmological predictions for rapidly oscillating scalar fields as dark matter

As we are entering the era of precision cosmology, it is necessary to count on accurate cosmological predictions from any proposed model of dark matter. In this paper we present a novel approach to the cosmological evolution of scalar fields that eases their analytic and numerical analysis at the background and at the linear order of perturbations. We apply the method to a scalar field endowed with a quadratic potential and revisit its properties as dark matter. Some of the results known in the literature are recovered, and a better understanding of the physical properties of the model is provided. It is shown that the Jeans wavenumber defined as $k_J = a \sqrt{mH}$ is directly related to the suppression of linear perturbations at wavenumbers $k>k_J$. We also discuss some semi-analytical results that are well satisfied by the full numerical solutions obtained from an amended version of the CMB code CLASS. Finally we draw some of the implications that this new treatment of the equations of motion may have in the prediction for cosmological observables.

astro-ph.CO↗

arXiv2014

Effect of Black Holes in Local Dwarf Spheroidal Galaxies on Gamma-Ray Constraints on Dark Matter Annihilation

Recent discoveries of optical signatures of black holes in dwarf galaxies indicates that low-mass galaxies can indeed host intermediate massive black holes. This motivates the assessment of the resulting effect on the host dark matter density profile, and the consequences for the constraints on the plane of the dark matter annihilation cross section versus mass, stemming from the non-observation of gamma rays from local dwarf spheroidals with the Fermi Large Area Telescope. We compute the density profile using three different prescriptions for the black hole mass associated with a given spheroidal galaxy, and taking into account the cutoff to the density from dark matter pair-annihilation. We find that the limits on the dark matter annihilation rate from observations of individual dwarfs are enhanced by factors of a few up to $10^6$, depending on the specific galaxy, on the black hole mass prescription, and on the dark matter particle mass. We estimate limits from combined observations of a sample of 15 dwarfs, for a variety of assumptions on the dwarf black hole mass and on the dark matter density profile prior to adiabatic contraction. We find that if black holes are indeed present in local dwarf spheroidals, then, independent of assumptions, (i) the dark matter interpretation of the Galactic center gamma-ray excess would be conclusively ruled out, (ii) wino dark matter would be excluded up to masses of about 3 TeV, and (iii) vanilla thermal relic WIMPs must be heavier than 100 GeV.

astro-ph.HE↗

arXiv2014

Dwarf spheroidal galaxies and Bose-Einstein condensate dark matter

We constrain the parameters of a self-interacting massive dark matter scalar particle in a condensate using the kinematics of the eight brightest dwarf spheroidal satellites of the Milky Way. For the case of a repulsive self-interaction the condensate develops a mass density profile with a characteristic scale radius that is closely related to the fundamental parameters of the theory. We find that the velocity dispersion of dwarf spheroidal galaxies suggests a scale radius of the order of 1 kpc, in tension with previous results found using the rotational curve of low-surface-brightness and dwarf galaxies. The new value is however favored marginally by the constraints coming from the number of relativistic species at Big-Bang nucleosynthesis. We discuss the implications of our findings for the particle dark matter model and argue that while a single classical coherent state can correctly describe the dark matter in dwarf spheroidal galaxies, it cannot play, in general, a relevant role for the description of dark matter in bigger objects.

astro-ph.GA↗

arXiv2014

Excluding the Light Dark Matter Window of a 331 Model Using LHC and Direct Dark Matter Detection Data

We sift the impact of the recent Higgs precise measurements, and recent dark matter direct detection results, on the dark sector of an electroweak extension of the Standard Model that has a complex scalar as dark matter. We find that in this model the Higgs decays with a large branching ratio into dark matter particles, and charged scalars when these are kinematically available, for any coupling strength differently from the so called Higgs portal. Moreover, we compute the abundance and spin-independent WIMP-nucleon scattering cross section, which are driven by the Higgs and $Z^{\prime}$ boson processes. We decisively exclude the $1-500$~GeV dark matter window and find the most stringent lower bound in the literature on the scale of symmetry breaking of the model namely $10$~TeV, after applying the LUX-2013 limit. Interestingly, the projected XENON1T constraint will be able to rule out the entire $1$~GeV-$1000$~GeV dark matter mass range. Lastly, for completeness, we compute the charged scalar production cross section at the LHC and comment on the possibility of detection at current and future LHC runnings.

hep-ph↗

arXiv2013

No-go theorem for static scalar field dark matter halos with no Noether charges

Classical scalar fields have been considered as a possible effective description of dark matter. We show that, for any metric theory of gravity, no static, spherically symmetric, regular, spatially localized, attractive, stable spacetime configuration can be sourced by the coherent excitation of a scalar field with positive definite energy density and no Noether charges. In the weak-field regime the result also applies for configurations with a repulsive gravitational potential. This extends Derrick's theorem to the case of a general (non-canonical) scalar field, including the self-gravitational effects. Some possible ways out are briefly discussed.

gr-qc↗

arXiv2013

Constraining dark matter sub-structure with the dynamics of astrophysical systems

The accuracy of the measurements of some astrophysical dynamical systems allows to constrain the existence of incredibly small gravitational perturbations. In particular, the internal Solar System dynamics (planets, Earth-Moon) opens up the possibility, for the first time, to prove the abundance, mass and size, of dark sub-structures at the Earth vicinity. We find that adopting the standard dark matter density, its local distribution can be composed by sub-solar mass halos with no currently measurable dynamical consequences, regardless of the mini-halo fraction. On the other hand, it is possible to exclude the presence of dark streams with linear mass densities higher than $λ_{\rm st}> 10^{-10} \Msun/\AU$ (about the Earth mass spread along the diameter of the SS up to the Kuiper belt). In addition, we review the dynamics of wide binaries inside the dwarf spheroidal galaxies in the MW. The dynamics of such kind of binaries seem to be compatible with the presence of a huge fraction of dark sub-structure, thus their existence is not a sharp discriminant of the dark matter hypothesis as been claimed before. However, there are regimes where the constraints from different astrophysical systems may reveal the sub-structure mass function cut-off scale.

astro-ph.CO↗

arXiv2012

Hints on halo evolution in SFDM models with galaxy observations

A massive, self-interacting scalar field has been considered as a possible candidate for the dark matter in the universe. We present an observational constraint to the model arising from strong lensing observations in galaxies. The result points to a discrepancy in the properties of scalar field dark matter halos for dwarf and lens galaxies, mainly because halo parameters are directly related to physical quantities in the model. This is an important indication that it becomes necessary to have a better understanding of halo evolution in scalar field dark matter models, where the presence of baryons can play an important role.

astro-ph.CO↗

arXiv2011

Are priors responsible for cosmology favoring additional neutrino species?

It has been suggested that both recent cosmological data and the results of flavor oscillation experiments (MiniBooNE and LSND) lend support to the existence of low-mass sterile neutrinos. The cosmological data appear to weakly favor additional forms of radiation in the Universe, beyond photons and three standard neutrino families. We reconsider the cosmological evidence by making the resulting confidence intervals on the additional effective neutrino species as prior-independent as possible. We find that, once the prior-dependence is removed, the latest cosmological data show no evidence for deviations from the standard number of neutrino species.

astro-ph.CO↗