88 research outputs found
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~I's J-factor should
therefore be regarded with a larger degree of caution when planning and
interpreting gamma-ray observations. Moreover, robust interpretations regarding
dark matter annihilation in dSph galaxies in general will require explicit
examination of how interlopers might affect the inferred dark matter density
profile.Comment: 12 pages, 8 figures. New appendix A (joint light/dark matter
likelihood), results unchanged. Match accepted MNRAS versio
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 astrophysical J- 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. We define an optimal strategy that would
mitigate sensitivity to priors and other aspects of analyses based on the
spherical Jeans equation. We also find that the assumption of spherical
symmetry can bias estimates of J (with the 95% credibility intervals not
encompassing the true J-factor) when the object is mildly triaxial (axis ratios
b/a = 0.8, c/a = 0.6). A concluding table summarises the typical error budget
and biases for the different sample sizes considered.Comment: 21 pages, 20 figures. Minor changes (several clarifications): match
the MNRAS accepted versio
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 II's 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.Comment: 5 pages, 4 figures. Match the ApJL accepted versio
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.Comment: 19 pages, 14 figures, submitted to MNRAS. Supplementary material
available on reques
Phase field approach to optimal packing problems and related Cheeger clusters
In a fixed domain of we study the asymptotic behaviour of optimal
clusters associated to -Cheeger constants and natural energies like the
sum or maximum: we prove that, as the parameter converges to the
"critical" value , optimal Cheeger clusters
converge to solutions of different packing problems for balls, depending on the
energy under consideration. As well, we propose an efficient phase field
approach based on a multiphase Gamma convergence result of Modica-Mortola type,
in order to compute -Cheeger constants, optimal clusters and, as a
consequence of the asymptotic result, optimal packings. Numerical experiments
are carried over in two and three space dimensions
Magellan/M2FS Spectroscopy of Tucana 2 and Grus 1
We present results from spectroscopic observations with the Michigan/Magellan
Fiber System (M2FS) of stellar targets along the line of sight to the
newly-discovered `ultrafaint' stellar systems Tucana 2 (Tuc 2) and Grus 1 (Gru
1). Based on simultaneous estimates of line-of-sight velocity and
stellar-atmospheric parameters, we identify 8 and 7 stars as probable members
of Tuc 2 and and Gru 1, respectively. Our sample for Tuc 2 is sufficient to
resolve an internal velocity dispersion of km s
about a mean of km s (solar rest frame), and to
estimate a mean metallicity of [Fe/H]= . These results
place Tuc 2 on chemodynamical scaling relations followed by dwarf galaxies,
suggesting a dominant dark matter component with dynamical mass
enclosed within the central
pc, and dynamical mass-to-light ratio
. For Gru 1 we estimate a mean velocity of
km s and a mean metallicity of
[Fe/H]=, but our sample does not resolve Gru 1's
velocity dispersion. The radial coordinates of Tuc 2 and Gru 1 in Galactic
phase space suggest that their orbits are among the most energetic within
distance kpc. Moreover, their proximity to each other in this space
arises naturally if both objects are trailing the Large Magellanic Cloud.Comment: replaced with ApJ-accepted version, all spectra and data products
(including samples from posterior PDFs) are available at
http://www.andrew.cmu.edu/user/mgwalker/tuc2gru1_dataproducts.tar.g
Precision Measurement of the Boron to Carbon Flux Ratio in Cosmic Rays from 1.9Â GV to 2.6Â TV with the Alpha Magnetic Spectrometer on the International Space Station
Knowledge of the rigidity dependence of the boron to carbon flux ratio (B/C) is important in understanding the propagation of cosmic rays. The precise measurement of the B/C ratio from 1.9 GV to 2.6 TV, based on 2.3 million boron and 8.3 million carbon nuclei collected by AMS during the first 5 years of operation, is presented. The detailed variation with rigidity of the B/C spectral index is reported for the first time. The B/C ratio does not show any significant structures in contrast to many cosmic ray models that require such structures at high rigidities. Remarkably, above 65 GV, the B/C ratio is well described by a single power law R[superscript Δ] with index Δ=-0.333±0.014(fit)±0.005(syst), in good agreement with the Kolmogorov theory of turbulence which predicts Δ=-1/3 asymptotically.National Science Foundation (U.S.) (Grants 1455202 and 1551980)Wyle Research (Firm) (Grant 2014/T72497)United States. National Aeronautics and Space Administration (NASA Earth and Space Science Fellowship Grant HELIO15F-0005
Electron and positron fluxes in primary cosmic rays measured with the alpha magnetic spectrometer on the international space station
Precision measurements by the Alpha Magnetic Spectrometer on the International Space Station of the primary cosmic-ray electron flux in the range 0.5 to 700 GeV and the positron flux in the range 0.5 to 500 GeV are presented. The electron flux and the positron flux each require a description beyond a single power-law spectrum. Both the electron flux and the positron flux change their behavior at ∼30GeV but the fluxes are significantly different in their magnitude and energy dependence. Between 20 and 200 GeV the positron spectral index is significantly harder than the electron spectral index. The determination of the differing behavior of the spectral indices versus energy is a new observation and provides important information on the origins of cosmic-ray electrons and positrons.</p
High statistics measurement of the positron fraction in primary cosmic rays of 0.5-500 GeV with the alpha magnetic spectrometer on the international space station
A precision measurement by AMS of the positron fraction in primary cosmic rays in the energy range from 0.5 to 500 GeV based on 10.9 million positron and electron events is presented. This measurement extends the energy range of our previous observation and increases its precision. The new results show, for the first time, that above ∼200GeV the positron fraction no longer exhibits an increase with energy.</p
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