Indirect dark matter detection for flattened dwarf galaxies

Abstract

Gamma-ray experiments seeking to detect evidence of dark matter annihilation in dwarf spheroidal galaxies require knowledge of the distribution of dark matter within these systems. We analyze the effects of flattening on the annihilation (J) and decay (D) factors of dwarf spheroidal galaxies with both analytic and numerical methods. Flattening has two consequences: first, there is a geometric effect as the squeezing (or stretching) of the dark matter distribution enhances (or diminishes) the J-factor; second, the line of sight velocity dispersion of stars must hold up the flattened baryonic component in the flattened dark matter halo. We provide analytic formulae and a simple numerical approach to estimate the correction to the J-and D-factors required over simple spherical modeling. The formulae are validated with a series of equilibrium models of flattened stellar distributions embedded in flattened dark-matter distributions. We compute corrections to the J-and D-factors for the Milky Way dwarf spheroidal galaxies under the assumption that they are all prolate or all oblate and find that the hierarchy of J-factors for the dwarf spheroidals is slightly altered (typical correction factors for an ellipticity of 0.4 are 0.75 for the oblate case and 1.6 for the prolate case). We demonstrate that spherical estimates of the D-factors are very insensitive to the flattening and introduce uncertainties significantly less than the uncertainties in the D-factors from the other observables for all the dwarf spheroidals (for example, +10 per cent −3 per cent for a typical ellipticity of 0.4). We conclude by investigating the spread in correction factors produced by triaxial figures and provide uncertainties in the J-factors for the dwarf spheroidals using different physically-motivated assumptions for their intrinsic shape and axis alignments. We find that the uncertainty in the J-factors due to triaxiality increases with the observed ellipticity and, in general, introduces uncertainties of a factor of 2 in the J-factors. We discuss our results in light of the reported gamma-ray signal from the highly-flattened ultrafaint Reticulum II. Tables of the J-and D-factors for the Milky Way dwarf spheroidal galaxies are provided (assuming an oblate or prolate structure) along with a table of the uncertainty on these factors arising from the unknown triaxiality. PACS numbers: 95.35.+d, 95.55.Ka, 12.60.-i, 98.52.Wz * Electronic address: jls@ast.cam.ac.uk † Electronic address: nwe@ast.cam.ac.uk ‡ Electronic address: alexgs@cmu.edu § Electronic address: wd11@leicester.ac.ukjor Cherenkov telescope collaborations continue to invest significant time on pointed observations of Milky Way dSphs. The resulting upper limits are two to three orders of magnitude from the relic cross section [6-10], but the situation bodes well for the future CTA project [e.g. 11].An exciting development in this field is the recent and ongoing discovery of large numbers of new Milky Way satellites made possible by wide-area photometric surveys [e.g. 12-15]. Since ...

Sommerfeld-enhanced J -factors for dwarf spheroidal galaxies

“…1 assume an NFW profile and isotropic orbits. These J-factors may differ from previous calculations in the literature [10][11][12][13][14], which allow for non-NFW profiles, anisotropic stellar velocity dispersions, and assume a Gaussian likelihood for the stellar velocities. In order to consistently determine the impact of the Sommerfeld-enhanced J-factors, we must compare them against the J-factors represented by the gray dotted curves in Fig.…”

“…With this assumption the J-factor is independent of the underlying particle physics such as the dark matter mass and cross section, and furthermore, it is independent of the particular phase space distribution of the dark matter. Many authors have determined the J-factors from the stellar kinematics of dSphs under the assumption of a velocity-independent annihilation cross section [10][11][12][13][14][15].…”

Simple J-factors and D-factors for indirect dark matter detection

“…In particular, as θ becomes large, the first term in eq (19) dominates and the Dfactor diverges logarithmically. However, as θ → 0, the D-factor tends to…”

“…The red and yellow bands are the estimates from equation (11) using γ = 1 and γ = 1.49 respectively. The blue band is the estimate from equation (19) using rs = 5R half . The median and ±1σ estimates of log 10 (D(θ)) from [39] are given by the black solid and dashed lines respectively.…”

“…This leaves the value of computationally intensive approaches based on sphericity open to question, as they may suf-fer from systematic effects when applied to flattened or triaxial systems. While there have already been some investigations of axisymmetry and triaxiality [11,18], the implications of these effects on the dark matter annihilation signal warrants a systematic study, which we provide elsewhere [19].…”