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Dark energy must be taken into account to estimate more reliably the amount of dark matter and how it is distributed in the local universe. For systems several Mpc across like the Local Group, we introduce three self-consistent independent mass estimators. These account for the antigravity effect of dark energy treated as Einsteins cosmological constant Lambda. The first is a modified Kahn-Woltjer model which gives a value of the Local Group mass via the particular motions of the two largest members, the Milky Way and M31. Inclusion of dark energy in this model increases the minimum mass estimate by a factor of three compared to the classical estimate. The increase is less but still significant for different ways of using the timing argument. The second estimator is a modified virial theorem which also demonstrates how dark energy can hide from detection a part of the gravitating mass of the system. The third is a new zero-gravity method which gives an upper limit to the group mass which we calculate with high precision HST observations. In combination, the estimators lead to a robust and rather narrow range for a groups mass, M. For the Local Group, 3.2 < M < 3.7 x 10^{12} M_sun. Our result agrees well with the Millennium Simulation based on the LambdaCDM cosmology.
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We revise the mass estimate of the Local Group (LG) when Dark Energy (in the form of the Cosmological Constant) is incorporated into the Timing Argument (TA) mass estimator for the Local Group (LG). Assuming the age of the Universe and the Cosmological Constant according to the recent values from the Planck CMB experiment, we find the mass of the LG to be M_TAL = (4.73 +- 1.03) x 10^{12} M_sun, which is 13% higher than the classical TA mass estimate. This partly explains the discrepancy between earlier results from LCDM simulations and the classical TA. When a similar analysis is performed on 16 LG-like galaxy pairs from the CLUES simulations, we find that the scatter in the ratio of the virial to the TA estimated mass is given by M_vir/M_TAL = 1.04 +-0.16. Applying it to the LG mass estimation we find a calibrated M_vir = (4.92 +- 1.08 (obs) +- 0.79 (sys)) x 10^{12} M_sun.
We use recent proper motion measurements of the tangential velocity of M31, along with its radial velocity and distance, to derive the likelihood of the sum of halo masses of the Milky Way and M31. This is done using a sample halo pairs in the Bolshoi cosmological simulation of $Lambda$CDM cosmology selected to match properties and environment of the Local Group. The resulting likelihood gives estimate of the sum of masses of $M_{rm MW,200}+M_{rm M31,200}=$ $2.40_{-1.05}^{+1.95}times10^{12},M_{odot}$ ($90%$ confidence interval). This estimate is consistent with individual mass estimates for the Milky Way and M31 and is consistent, albeit somewhat on the low side, with the mass estimated using the timing argument. We show that although the timing argument is unbiased on average for all pairs, for pairs constrained to have radial and tangential velocities similar to that of the Local Group the argument overestimates the sum of masses by a factor of $1.6$. Using similar technique we estimate the total dark matter mass enclosed within $1$ Mpc from the Local Group barycenter to be $M_{rm LG}(r<1, {rm Mpc})=4.2_{-2.0}^{+3.4}times10^{12},M_{odot}$ ($90%$ confidence interval).
If dark matter (DM) is composed by particles which are non-gravitationally coupled to ordinary matter, their annihilations or decays in cosmic structures can result in detectable radiation. We show that the most powerful technique to detect a particle DM signal outside the Local Group is to study the angular cross-correlation of non-gravitational signals with low-redshift gravitational probes. This method allows to enhance signal-to-noise from the regions of the Universe where the DM-induced emission is preferentially generated. We demonstrate the power of this approach by focusing on GeV-TeV DM and on the recent cross-correlation analysis between the 2MASS galaxy catalogue and the Fermi-LAT gamma-ray maps. We show that this technique is more sensitive than other extragalactic gamma-ray probes, such as the energy spectrum and angular autocorrelation of the extragalactic background, and emission from clusters of galaxies. Intriguingly, we find that the measured cross-correlation can be well fitted by a DM component, with thermal annihilation cross section and mass between 10 and 100 GeV, depending on the small-scale DM properties and gamma-ray production mechanism. This solicits further data collection and dedicated analyses.
We show that the canonical oscillation-based (non-resonant) production of sterile neutrino dark matter is inconsistent at $>99$% confidence with observations of galaxies in the Local Group. We set lower limits on the non-resonant sterile neutrino mass of $2.5$ keV (equivalent to $0.7$ keV thermal mass) using phase-space densities derived for dwarf satellite galaxies of the Milky Way, as well as limits of $8.8$ keV (equivalent to $1.8$ keV thermal mass) based on subhalo counts of $N$-body simulations of M 31 analogues. Combined with improved upper mass limits derived from significantly deeper X-ray data of M 31 with full consideration for background variations, we show that there remains little room for non-resonant production if sterile neutrinos are to explain $100$% of the dark matter abundance. Resonant and non-oscillation sterile neutrino production remain viable mechanisms for generating sufficient dark matter sterile neutrinos.
We contend that a single power law halo mass distribution is appropriate for direct matching to the stellar masses of observed Local Group dwarf galaxies, allowing the determination of the slope of the stellar mass-halo mass relation for low mass galaxies. Errors in halo masses are well defined as the Poisson noise of simulated local group realisations, which we determine using constrained local universe simulations (CLUES). For the stellar mass range 10$^7$<M*<10$^8$M$_odot$, for which we likely have a complete census of observed galaxies, we find that the stellar mass-halo mass relation follows a power law with slope of 3.1, significantly steeper than most values in the literature. The steep relation between stellar and halo masses indicates that Local Group dwarf galaxies are hosted by dark matter halos with a small range of mass. Our methodology is robust down to the stellar mass to which the census of observed Local Group galaxies is complete, but the significant uncertainty in the currently measured slope of the stellar-to halo mass relation will decrease dramatically if the Local Group completeness limit was $10^{6.5}$M$odot$ or below, highlighting the importance of pushing such limit to lower masses and larger volumes.
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