Cosmological constraints on the hot gas fraction in galaxy clusters [CEA]

The evolution of the X-ray emitting gas mass fraction in massive galaxy clusters can be used as an independent cosmological tool to probe the expansion history of the Universe. Its use, however, depends upon a crucial quantity, i.e., the depletion factor $\gamma$, which corresponds to the ratio by which the X-ray emitting gas fraction in galaxy clusters is depleted with respect to the universal baryonic mean. Since this quantity is not directly observed, assumptions about the cosmology need to be made and usually hydrodynamical cosmological simulations are used to calibrate it. In this letter, we obtain for the first time self-consistent observational constraints on the gas depletion factor combining 40 X-ray emitting gas mass fraction measurements and 580 distance measurements from type Ia supernovae. Using non-parametric methods to reconstruct a possible redshift evolution of $\gamma$, we find no evidence for such evolution, which confirms the current results from hydrodynamical simulations.

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R. Holanda, V. Busti, J. Gonzalez, et. al.
Fri, 23 Jun 17

Comments: 5 pages, 1 figure, 1 table

Gravitational effects of condensed dark matter on strange stars [CL]

In the present work we study the gravitational effects of condensed dark matter on strange stars. We consider self-interacting dark matter particles with properties consistent with current observational constraints, and dark matter inside the star is modelled as a Bose-Einstein condensate. We integrate numerically the Tolman-Oppenheimer-Volkoff equations in the two-fluid formalism assuming that strange stars are made of up to 4 per cent of dark matter. It is shown that for a mass of the dark matter particles in the range $50 MeV-160 MeV$ strange stars are characterized by a maximum mass and radius similar to the ones found for neutron stars.

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G. Panotopoulos and I. Lopes
Fri, 23 Jun 17

Comments: Two-column REVTEX, 5 pages, 3 figures

Thermal Dark Matter Below an MeV [CL]

We consider a class of models in which thermal dark matter is lighter than an MeV. If dark matter thermalizes with the Standard Model below the temperature of neutrino-photon decoupling, equilibration and freeze-out cools and heats the Standard Model bath comparably, alleviating constraints from measurements of the effective number of neutrino species. We demonstrate this mechanism in a model consisting of fermionic dark matter coupled to a light scalar mediator. Thermal dark matter can be as light as a few keV, while remaining compatible with existing cosmological and astrophysical observations. This framework motivates new experiments in the direct search for sub-MeV thermal dark matter and light force carriers.

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A. Berlin and N. Blinov
Fri, 23 Jun 17

Comments: 5 pages, 4 figures

Wormholes and masses for Goldstone bosons [CL]

There exist non-trivial stationary points of the Euclidean action for an axion particle minimally coupled to Einstein gravity, dubbed wormholes. They explicitly break the continuos global shift symmetry of the axion in a non-perturbative way, and generate an effective potential that may compete with QCD depending on the value of the axion decay constant. In this paper, we explore both theoretical and phenomenological aspects of this issue. On the theory side, we address the problem of stability of the wormhole solutions, and we show that the spectrum of the quadratic action features only positive eigenvalues. On the phenomenological side, we discuss, beside the obvious application to the QCD axion, relevant consequences for models with ultralight dark matter, black hole superradiance, and the relaxation of the electroweak scale. We conclude discussing wormhole solutions for a generic coset and the potential they generate.

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R. Alonso and A. Urbano
Fri, 23 Jun 17

Comments: 50 pages, 15 figures

Imprints of the super-Eddington accretion on the quasar clustering [GA]

Super-Eddington mass accretion has been suggested as an efficient mechanism to grow supermassive black holes (SMBHs). We investigate the imprint left by the radiative efficiency of the super-Eddington accretion process on the clustering of quasars using a new semi-analytic model of galaxy and quasar formation based on large-volume cosmological $N$-body simulations. Our model includes a simple model for the radiative efficiency of a quasar, which imitates the effect of photon trapping for a high mass accretion rate. We find that the model of radiative efficiency affects the relation between the quasar luminosity and the quasar host halo mass. The quasar host halo mass has only weak dependence on quasar luminosity when there is no upper limit for quasar luminosity. On the other hand, it has significant dependence on quasar luminosity when the quasar luminosity is limited by its Eddington luminosity. In the latter case, the quasar bias also depends on the quasar luminosity, and the quasar bias of bright quasars is in agreement with observations. Our results suggest that the quasar clustering studies can provide a constraint on the accretion disc model.

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T. Oogi, M. Enoki, T. Ishiyama, et. al.
Fri, 23 Jun 17

Comments: 6 pages, 4 figures, to appear in MNRAS Letters

High-Energy Gamma Rays and Neutrinos from Nearby Radio Galaxies [HEAP]

Multi-messenger data suggest that radio galaxies (i.e. non-blazar active galaxies) are perhaps the most likely class of sources for the diffuse flux of high-energy neutrinos reported by the IceCube Collaboration. In this study, we consider the gamma-ray spectrum observed from four nearby radio galaxies (Centaurus A, PKS 0625-35, NGC 1275 and IC 310) and constrain the intensity and spectral shape of the emission injected from these sources, accounting for the effects of attenuation and contributions from electromagnetic cascades (initiated both within the radio galaxy itself and during extragalactic propagation). Assuming that this gamma-ray emission is generated primarily through the interactions of cosmic-ray protons with gas, we calculate the neutrino flux predicted from each of these sources. Although this scenario is consistent with the constraints published by the IceCube and ANTARES Collaborations, the predicted fluxes consistently fall within an order of magnitude of the current point source sensitivity. The prospects appear very encouraging for the future detection of neutrino emission from the nearest radio galaxies.

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C. Blanco and D. Hooper
Fri, 23 Jun 17

Comments: 28 pages, 17 figures

Apparent cosmic acceleration from type Ia supernovae [CEA]

Parameters that quantify the acceleration of cosmic expansion are conventionally determined within the standard Friedmann-Lema\^{\i}tre-Robertson-Walker (FLRW) model, which fixes spatial curvature to be homogeneous. Generic averages of Einstein’s equations in inhomogeneous cosmology lead to models with non-rigidly evolving average spatial curvature, and different parameterizations of apparent cosmic acceleration. The timescape cosmology is a viable example of such a model without dark energy. Using the largest available supernova data set, the JLA catalogue, we find that the timescape model fits the luminosity distance-redshift data with a likelihood that is statistically indistinguishable from the standard spatially flat $\Lambda$CDM cosmology by Bayesian comparison. In the timescape case cosmic acceleration is non-zero but has a marginal amplitude, with best fit apparent deceleration parameter, $q_0=-0.042^{+0.04}_{-0.01}$. Systematic issues regarding standardization of supernova light curves are analysed. Cuts of data at the statistical homogeneity scale affect light curve parameter fits independent of cosmology. A cosmological model dependency of empirical changes to the mean colour parameter is also found. Irrespective of which model ultimately fits better, we argue that as a competitive model with a non-FLRW expansion history, the timescape model may prove a useful diagnostic tool for disentangling selection effects and astrophysical systematics from the underlying expansion history.

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L. Dam, A. Heinesen and D. Wiltshire
Fri, 23 Jun 17

Comments: 17 pages, 6 figures