David Maurin

Dark matter annihilation and decay in dwarf spheroidal galaxies: the classical and ultrafaint dSphs

Dwarf spheroidal (dSph) galaxies are prime targets for present and future gamma-ray telescopes hunting for indirect signals of particle dark matter. The interpretation of the data requires careful assessment of their dark matter content in order to derive robust constraints on candidate relic particles. Here, we use an optimised spherical Jeans analysis to reconstruct the `astrophysical factor for both annihilating and decaying dark matter in 21 known dSphs. Improvements with respect to previous works are: (i) the use of more flexible luminosity and anisotropy profiles to minimise biases, (ii) the use of weak priors tailored on extensive sets of contamination-free mock data to improve the confidence intervals, (iii) systematic cross-checks of binned and unbinned analyses on mock and real data, and (iv) the use of mock data including stellar contamination to test the impact on reconstructed signals. Our analysis provides updated values for the dark matter content of 8 `classical and 13 `ultrafaint dSphs, with the quoted uncertainties directly linked to the sample size; the more flexible parametrisation we use results in changes compared to previous calculations. This translates into our ranking of potentially-brightest and most robust targets---viz., Ursa Minor, Draco, Sculptor---, and of the more promising, but uncertain targets---viz., Ursa Major 2, Coma---for annihilating dark matter. Our analysis of Segue 1 is extremely sensitive to whether we include or exclude a few marginal member stars, making this target one of the most uncertain. Our analysis illustrates challenges that will need to be addressed when inferring the dark matter content of new `ultrafaint satellites that are beginning to be discovered in southern sky surveys.

DARK MATTER ANNIHILATION and DECAY PROFILES for the RETICULUM II DWARF SPHEROIDAL GALAXY

The dwarf spheroidal galaxies (dSph) of the Milky Way are among the most attractive targets for indirect searches of dark matter. In this work, we reconstruct the dark matter annihilation (J-factor) and decay profiles for the newly discovered dSph Reticulum II. Using an optimized spherical Jeans analysis of kinematic data obtained from the Michigan/Magellan Fiber System (M2FS), we find Reticulum IIs J-factor to be among the largest of any Milky Way dSph. We have checked the robustness of this result against several ingredients of the analysis. Unless it suffers from tidal disruption or significant inflation of its velocity dispersion from binary stars, Reticulum II may provide a unique window on dark matter particle properties.

Spherical Jeans analysis for dark matter indirect detection in dwarf spheroidal galaxies – impact of physical parameters and triaxiality

Dwarf spheroidal (dSph) galaxies are among the most promising targets for the indirect detection of dark matter (DM) from annihilation and/or decay products. Empirical estimates of their DM content—and hence the magnitudes of expected signals—rely on inferences from stellar-kinematic data. However, various kinematic analyses can give different results and it is not obvious which are most reliable. Using extensive sets of mock data of various sizes (mimicking ‘ultra-faint’ and ‘classical’ dSphs) and an MCMC engine, here we investigate biases, uncertainties, and limitations of analyses based on parametric solutions to the spherical Jeans equation. For a variety of functional forms for the tracer and DM density profiles, as well as the orbital anisotropy profile, we examine reliability of estimates for the astrophysicalJ- and D-factors for annihilation and decay, respectively. For large (N & 1000) stellar-kinematic samples typical of ‘classical’ dSphs, errors tend to be dominated by systematics, which can be reduced through the use of sufficiently general and flexible functional forms. For small (N . 100) samples typical of ‘ultrafaints’, statistical uncertainties tend to dominate systematic errors and flexible models are less necessary. Finally, we find that the assumption of spherical symmetry can bias estimates of J (by up to a factor of a few) when the object is mildly triaxial (axis ratiosb=a = 0:8,c=a = 0:6). A concluding table summarises the typical error budget and biases for the different sample sizes considered. We also define an optimal strategy that would mitigate sensitivity to priors and other aspects of analyses based on the spherical Jeans equation.

Influence of Population III stars on cosmic chemical evolution

New observations from the Hubble Ultra Deep Field suggest that the star formation rate at z > 7 drops off faster than previously thought. Using a newly determined star formation rate for the normal mode of Population II/I (PopII/I) stars, including this new constraint, we compute the Thomson scattering optical depth and find a result that is marginally consistent with Wilkinson Microwave Anisotropy Probe 5 results. We also reconsider the role of Population III (PopIII) stars in light of cosmological and stellar evolution constraints. While this input may be needed for reionization, we show that it is essential in order to account for cosmic chemical evolution in the early universe. We investigate the consequences of PopIII stars on the local metallicity distribution function of the Galactic halo (from the recent Hamburg/European Southern Observatory (ESO) survey of metal-poor stars) and on the evolution of abundances with metallicity (based on the ESO large programme on very metal-poor stars), with special emphasis on carbon-enhanced metal-poor stars. The metallicity distribution function shape is well reproduced at low iron abundance ([Fe/H] ≥―4), in agreement with other studies. However, the Hamburg/ESO survey hints at a sharp decrease of the number of low-mass stars at very low iron abundance, which is not reproduced in models with only PopII/I stars. The presence of PopIII stars, of typical masses 30-40 M ⊙ , helps us to reproduce this feature, leading to a prompt initial enrichment before the onset of PopII/I stars. The metallicity at which this cut-off occurs is sensitive to the lowest mass of the massive PopIII stars, which makes the metallicity distribution function a promising tool to constrain this population. Our most important results show that the nucleosynthetic yields of PopIII stars lead to abundance patterns in agreement with those observed in extremely metal-poor stars. This can be demonstrated by the transition discriminant (a criterion for low-mass star formation taking into account the cooling due to C II and O I ). In this chemical approach to cosmic evolution, PopIII stars prove to be a compulsory ingredient, and extremely metal-poor stars are inevitably born at high redshift.

γ -rays from annihilating dark matter in galaxy clusters: stacking versus single source analysis

Clusters of galaxies are potentially important targets for indirect searches for dark matter annihilation. Here, we reassess the detection prospects for annihilation in massive halos, based on a statistical investigation of 1743 clusters from the recent MCXC meta-catalogue. We derive a new data-driven limit for the extra-galactic DM annihilation background Jextra-gal>JGal/5 and consider a source-stacking approach. The number of clusters scales with their brightness (boosted by DM substructures) to the power of -2 for an integration angle 0.1deg. It suggests that stacking may provide a significant improvement over a single target analysis for gamma-ray observations at high-energies where the angular resolution achievable is comparable to this angle. In our study the mean angle containing 80% of the dark-matter signal for the entire sample (assuming an NFW DM profile) is 0.15deg. It indicates that instruments with this angular resolution or better would be optimal for a cluster annihilation search based on stacking. A detailed study based on realistic Fermi-LAT performance and position-dependent background suggests, however, that stacking is likely to result in only modest (a factor >1.7) sensitivity improvement, in comparison to the analysis of the most promising single source in our study (Virgo). This is a consequence of (i) the relatively poor resolution of Fermi-LAT in the energy range where most photon statistics are available, and (ii) the larger angles subtended by bright, nearby objects such as Virgo. Based on the expected performance of CTA, we find no improvement with stacking, due to the requirement for pointed observations. We note that several potentially important targets, Ophiuchus, A2199, A3627 (Norma) or CIZAJ1324.7-5736 may be disfavoured due to a poor contrast with respect to the Galactic DM signal [abridged]

clumpy: A code for γ-ray signals from dark matter structures

Abstract We present the first public code for semi-analytical calculation of the γ -ray flux astrophysical J -factor from dark matter annihilation/decay in the Galaxy, including dark matter substructures. The core of the code is the calculation of the line of sight integral of the dark matter density squared (for annihilations) or density (for decaying dark matter). The code can be used in three modes: i) to draw skymaps from the Galactic smooth component and/or the substructure contributions, ii) to calculate the flux from a specific halo (that is not the Galactic halo, e.g. dwarf spheroidal galaxies) or iii) to perform simple statistical operations from a list of allowed DM profiles for a given object. Extragalactic contributions and other tracers of DM annihilation (e.g. positrons, anti-protons) will be included in a second release. Program summary Program title: CLUMPY Catalogue identifier: AEKS_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEEF_v1_0.html Program obtainable from: CPC Program Library, Queenʼs University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 207u2009466 No. of bytes in distributed program, including test data, etc.: 6u2009342u2009889 Distribution format: tar.gz Programming language: C/C++ Computer: PC and Mac Operating system: UNIX(Linux), MacOS X RAM: Depends on the requested size of skymaps ( ∼ 40 Mb for a 500 × 500 map) Classification: 1.1, 1.9 External routines: CERN ROOT library ( http://root.cern.ch/drupal/ ), Doxygen ( http://www.doxygen.org ) (optional) Nature of problem: Calculation of γ -ray signal from dark matter annihilation (resp. decay). This involves a particle physics term and an astrophysical one. The focus here is on the latter. Solution method: Integration of the DM density squared (resp. density) along a line of sight. The code is optimised to deal with the DM density peaks encountered along the line of sight (DM substructures). A semi-analytical approach (calibrated on N-body simulations) is used for the spatial and mass distributions of the dark matter substructures in the Galaxy. Restrictions: Some generic dark matter annihilation spectra are provided but are not included in the calculation so far as it is assumed that the particle physics is independent of the astrophysics of the problem. Running time: This is highly dependent on the DM profiles considered, the requested precision e and integration angle α int : • about 60 mn for a 5 ° × 5 ° map towards the Galactic centre, with α int = 0.01 ° , NFW dark matter profiles and e = 10 − 2 ; • about 2 h for the same set-up towards the anti-centre; • 0.1 to 10 DM models per second, depending on integration angle and DM profile.

COSMIC RAY PRODUCTION OF BERYLLIUM AND BORON AT HIGH REDSHIFT

Recently, new observations of 6Li in Population II stars of the Galactic halo have shown a surprisingly high abundance of this isotope, about a thousand times higher than its predicted primordial value. In previous papers, a cosmological model for the cosmic-ray-induced production of this isotope in the intergalactic medium (IGM) has been developed to explain the observed abundance at low metallicity. In this paper, given this constraint on the 6Li, we calculate the nonthermal evolution with redshift of D, Be, and B in the IGM. In addition to cosmological cosmic ray interactions in the IGM, we include additional processes driven by supernova explosions: neutrino spallation and a low-energy component in the structures ejected by outflows to the IGM. We take into account CNO CRs impinging on the intergalactic gas. Although subdominant in the Galactic disk, this process is shown to produce the bulk of Be and B in the IGM, due to the differential metal enrichment between structures (where CRs originate) and the IGM. We also consider the resulting extragalactic gamma-ray background, which we find to be well below existing data. The computation is performed in the framework of hierarchical structure formation, considering several star formation histories, including Population III stars. We find that D production is negligible and that a potentially detectable Be and B plateau is produced by these processes at the time of the formation of the Galaxy (z ~ 3).

Contamination of stellar-kinematic samples and uncertainty about dark matter annihilation profiles in ultrafaint dwarf galaxies: the example of Segue I

The expected gamma-ray flux coming from dark matter annihilation in dwarf spheroidal (dSph) galaxies depends on the so-called J-factor, the integral of the squared dark matter density along the line-of-sight. We examine the degree to which estimates of J are sensitive to contamination (by foreground Milky Way stars and stellar streams) of the stellar-kinematic samples that are used to infer dark matter densities in ultrafaint dSphs. Applying standard kinematic analyses to hundreds of mock data sets that include varying levels of contamination, we find that mis-classified contaminants can cause J-factors to be overestimated by orders of magnitude. Stellar-kinematic data sets for which we obtain such biased estimates tend 1) to include relatively large fractions of stars with ambiguous membership status, and 2) to give estimates for J that are sensitive to specific choices about how to weight and/or to exclude stars with ambiguous status. Comparing publicly-available stellar-kinematic samples for the nearby dSphs Reticulum II and Segue I, we find that only the latter displays both of these characteristics. Estimates of Segue Is J-factor should therefore be regarded with a larger degree of caution when planning and interpreting gamma-ray observations.

Dark matter substructure modelling and sensitivity of the Cherenkov Telescope Array to Galactic dark halos

Hierarchical structure formation leads to a clumpy distribution of dark matter in the Milky Way. These clumps are possible targets to search for dark matter annihilation with present and future

CLUMPY: Jeans analysis, γ-ray and ν fluxes from dark matter (sub-)structures

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