Manoj Kaplinghat

Too big to fail? The puzzling darkness of massive Milky Way subhaloes

ABSTRACT We show that dissipationless CDM simulations predict that the majority of themost massive subhalos of the Milky Way are too dense to host any of its brightsatellites (L V > 10 5 L ). These dark subhalos have circular velocities at infall ofV infall = 30 1070kms 1 and infall masses of [0:2 4] 10 M . Unless the Milky Way isa statistical anomaly, this implies that galaxy formation becomes e ectively stochasticat these masses. This is in marked contrast to the well-established monotonic relationbetween galaxy luminosity and halo circular velocity (or halo mass) for more massivehalos. We show that at least two (and typically four) of these massive dark subhalosare expected to produce a larger dark matter annihilation ux than Draco. It maybe possible to circumvent these conclusions if baryonic feedback in dwarf satellites ordi erent dark matter physics can reduce the central densities of massive subhalos byorder unity on a scale of 0.3 { 1 kpc.Key words: Galaxy: halo { galaxies: abundances { dark matter { cosmology: theory

Accurate Masses for Dispersion-supported Galaxies

We derive an accurate mass estimator for dispersion-supported stellar systems and demonstrate its validity by analysing resolved line-of-sight velocity data for globular clusters, dwarf galaxies and elliptical galaxies. Specifically, by manipulating the spherical Jeans equation we show that the mass enclosed within the 3D deprojected half-light radius r 1/2 can be determined with only mild assumptions about the spatial variation of the stellar velocity dispersion anisotropy as long as the projected velocity dispersion profile is fairly flat near the half-light radius, as is typically observed. We find M 1/2 = 3G -1 (σ 2 los )r 1/2 ≃ 4G -1 (σ 2 los )R e , where (σ 2 los ) is the luminosity-weighted square of the line-of-sight velocity dispersion and R e is the 2D projected half-light radius. While deceptively familiar in form, this formula is not the virial theorem, which cannot be used to determine accurate masses unless the radial profile of the total mass is known a priori. We utilize this finding to show that all of the Milky Way dwarf spheroidal galaxies (MW dSphs) are consistent with having formed within a halo of a mass of approximately 3 x 10 9 M ⊙ . assuming a A cold dark matter cosmology. The faintest MW dSphs seem to have formed in dark matter haloes that are at least as massive as those of the brightest MW dSphs, despite the almost five orders of magnitude spread in luminosity between them. We expand our analysis to the full range of observed dispersion-supported stellar systems and examine their dynamical I-band mass-to-light ratios Υ I 1/2 . The Υ I 1/2 versus M 1/2 relation for dispersion-supported galaxies follows a U shape, with a broad minimum near Υ I 1/2 ≃ 3 that spans dwarf elliptical galaxies to normal ellipticals, a steep rise to Υ I 1/2 ≃ 3200 for ultra-faint dSphs and a more shallow rise to Υ I 1/2 ≃ 800 for galaxy cluster spheroids.

A common mass scale for satellite galaxies of the Milky Way.

The Milky Way has at least twenty-three known satellite galaxies that shine with luminosities ranging from about a thousand to a billion times that of the Sun. Half of these galaxies were discovered in the past few years in the Sloan Digital Sky Survey, and they are among the least luminous galaxies in the known Universe. A determination of the mass of these galaxies provides a test of galaxy formation at the smallest scales and probes the nature of the dark matter that dominates the mass density of the Universe. Here we use new measurements of the velocities of the stars in these galaxies to show that they are consistent with them having a common mass of about 107 within their central 300 parsecs. This result demonstrates that the faintest of the Milky Way satellites are the most dark-matter-dominated galaxies known, and could be a hint of a new scale in galaxy formation or a characteristic scale for the clustering of dark matter.

Cosmological simulations with self-interacting dark matter – I. Constant-density cores and substructure

We use cosmological simulations to study the effects of self-interacting dark matter (SIDM) on the density profiles and substructure counts of dark matte r halos from the scales of spiral galaxies to galaxy clusters, focusing explicitly on mod els with cross sections over dark matter particle mass σ/m = 1 and 0.1 cm 2 /g. Our simulations rely on a new SIDM N-body algorithm that is derived self-consistently from the Boltz mann equation and that reproduces analytic expectations in controlled numerical experiments. We find that well-resolved SIDM halos have constant-density cores, with significantly lowe r central densities than their CDM counterparts. In contrast, the subhalo content of SIDM halos is only modestly reduced compared to CDM, with the suppression greatest for large hosts and small halo-centric distances. Moreover, the large-scale clustering and halo circular vel ocity functions in SIDM are effectively identical to CDM, meaning that all of the large-scale successes of CDM are equally well matched by SIDM. From our largest cross section runs we are able to extract scaling relations for core sizes and central densities over a range o f halo sizes and find a strong correlation between the core radius of an SIDM halo and the NFW scale radius of its CDM counterpart. We construct a simple analytic model, based on CDM scaling relations, that captures all aspects of the scaling relations for SIDM halos. Our results show that halo core densities in σ/m = 1 cm 2 /g models are too low to match observations of galaxy clusters, low surface brightness spirals (LSBs), and dwarf spheroidal galaxies. However, SIDM with σ/m ≃ 0.1 cm 2 /g appears capable of reproducing reported core sizes and central densities of dwarfs, LSBs, and galaxy clusters without the need for velocity dependence. Higher resolution simulations over a wider range of masses will be required to confirm this expectation. We discuss constraints arising from the Bullet cluster observ ations, measurements of dark matter density on small-scales and subhalo survival requirements, and show that SIDM models with σ/m ≃ 0.1 cm 2 /g ≃ 0.2 barn/GeV are consistent with all observational constraints.

Detection of a Gamma-Ray Source in the Galactic Center Consistent with Extended Emission from Dark Matter Annihilation and Concentrated Astrophysical Emission

We show the existence of a statistically significant, robust detection of a gamma-ray source in the Milky Way Galactic Center that is consistent with a spatially extended signal using about 4 years of Fermi-LAT data. The gamma-ray flux is consistent with annihilation of dark matter particles with a thermal annihilation cross-section if the spatial distribution of dark matter particles is similar to the predictions of dark matter only simulations. We find statistically significant detections of an extended source with gamma-ray spectrum that is consistent with dark matter particle masses of approximately 10 GeV to 1 TeV annihilating to b/b-bar quarks, and masses approximately 10 GeV to 30 GeV annihilating to tau+ tau- leptons. However, a part of the allowed region in this interpretation is in conflict with constraints from Fermi observations of the Milky Way satellites. The biggest improvement over the fit including just the point sources is obtained for a 30 GeV dark matter particle annihilating to b/b-bar quarks. The gamma-ray intensity and spectrum are also well fit with emission from a millisecond pulsar (MSP) population following a density profile like that of low-mass X-ray binaries observed in M31. The greatest goodness-of-fit of the extended emission is with spectra consistent with known astrophysical sources like MSPs in globular clusters or cosmic ray bremsstrahlung on molecular gas. Therefore, we conclude that the bulk of the emission is likely from an unresolved or spatially extended astrophysical source. However, the interesting possibility of all or part of the extended emission being from dark matter annihilation cannot be excluded at present.

Astrophysical and Dark Matter Interpretations of Extended Gamma-Ray Emission from the Galactic Center

We construct empirical models of the diffuse gamma-ray background toward the Galactic Center. Including all known point sources and a template of emission associated with interactions of cosmic rays with molecular gas, we show that the extended emission observed previously in the Fermi Large Area Telescope data toward the Galactic Center is detected at high significance for all permutations of the diffuse model components. However, we find that the fluxes and spectra of the sources in our model change significantly depending on the background model. In particular, the spectrum of the central Sgr A* source is less steep than in previous works and the recovered spectrum of the extended emission has large systematic uncertainties, especially at lower energies. If the extended emission is interpreted to be due to dark matter annihilation, we find annihilation into pure b-quark and τ-lepton channels to be statistically equivalent goodness of fits. In the case of the pure b-quark channel, we find a dark matter mass of 39.4(+3.7-2.9stat)(±7.9sys)GeV, while a pure τ+τ - channel case has an estimated dark matter mass of 9.43(+0.63-0.52stat)(±1.2sys)GeV. Alternatively, if the extended emission is interpreted to be astrophysical in origin such as due to unresolved millisecond pulsars, we obtain strong bounds on dark matter annihilation, although systematic uncertainties due to the dependence on the background models are significant.

Cosmological simulations with self-interacting dark matter – II. Halo shapes versus observations

If dark matter has a large self-interaction scattering cross section, then interactions among dark-matter particles will drive galaxy and cluster halos to become spherical in their centers. Work in the past has used this effect to rule out velocity-independent, elastic cross sections larger than sigma/m ~ 0.02 cm^2/g based on comparisons to the shapes of galaxy cluster lensing potentials and X-ray isophotes. In this paper, we use cosmological simulations to show that these constraints were off by more than an order of magnitude because (a) they did not properly account for the fact that the observed ellipticity gets contributions from the triaxial mass distribution outside the core set by scatterings, (b) the scatter in axis ratios is large and (c) the core region retains more of its triaxial nature than estimated before. Including these effects properly shows that the same observations now allow dark matter self-interaction cross sections at least as large as sigma/m = 0.1 cm^2/g. We show that constraints on self-interacting dark matter from strong-lensing clusters are likely to improve significantly in the near future, but possibly more via central densities and core sizes than halo shapes.

Halo-Shape and Relic-Density Exclusions of Sommerfeld-Enhanced Dark Matter Explanations of Cosmic Ray Excesses

Dark matter with Sommerfeld-enhanced annihilation has been proposed to explain observed cosmic ray positron excesses in the 10 GeV to TeV energy range. We show that the required enhancement implies thermal relic densities that are too small to be all of dark matter. We also show that the dark matter is sufficiently self-interacting that observations of elliptical galactic dark matter halos exclude large Sommerfeld enhancement for light force carriers. Resonant Sommerfeld enhancement does not modify these conclusions, and the astrophysical boosts required to resolve these discrepancies are disfavored, especially when significant self-interactions suppress halo substructure.

The Most Dark-Matter-dominated Galaxies: Predicted Gamma-Ray Signals from the Faintest Milky Way Dwarfs

We use kinematic data from three new nearby, extremely low luminosity Milky Way dwarf galaxies (Ursa Major II, Willman 1, and Coma Berenices) to constrain the properties of their dark matter halos, and from these we make predictions for the γ-ray flux from annihilation of dark matter particles in these halos. We show that these ~103 L☉ dwarfs are the most dark-matter-dominated galaxies known, with total masses within 100 pc that are in excess of 106 M☉. Coupled with their relative proximity, their large masses imply that they should have mean γ-ray fluxes that are comparable to or greater than those of any other known satellite galaxy of the Milky Way. Our results are robust to both variations of the inner slope of the density profile and the effect of tidal interactions. The fluxes could be boosted by up to 2 orders of magnitude if we include the density enhancements caused by surviving dark matter substructure.

Determining neutrino mass from the cosmic microwave background alone

Distortions of Cosmic Microwave Background (CMB) temperature and polarization maps caused by gravitational lensing, observable with high angular resolution and high sensitivity, can be used to measure the neutrino mass. Assuming two massless species and one with mass m_nu we forecast sigma(m_nu) = 0.15 eV from the Planck satellite and sigma(m_nu)=0.04 eV from observations with twice the angular resolution and about 20 times the sensitivity. A detection is likely at this higher sensitivity since the observation of atmospheric neutrino oscillations require mass-squared differences greater than about (0.04 eV)^2.