Warm dark matter (WDM) might more easily account for small scale clustering measurements than the heavier particles typically invoked in Λ cold dark matter (ΛCDM) cosmologies. In this paper, we consider a ΛWDM cosmology in which sterile neutrinos νs, with a mass ms of roughly 1-100 keV, are the dark matter. We use the diffuse X-ray spectrum (total minus resolved point source emission) of the Andromeda galaxy to constrain the rate of sterile neutrino radiative decay: νs → νe,µ,τ + γ. Our findings demand that ms < 3.5 keV (95% C.L.) which is a significant improvement over the previous (95% C.L.) limits inferred from the X-ray emission of nearby clusters, ms < 8.2 keV (Virgo A) and ms < 6.3 keV (Virgo A + Coma).

We use the Chandra unresolved X-ray emission spectrum from a 12 ′ − 28 ′ (2.8-6.4 kpc) annular region of the Andromeda galaxy to constrain the radiative decay of sterile neutrino warm dark matter. By excising the most baryon-dominated, central 2.8 kpc of the galaxy, we reduce the uncertainties in our estimate of the dark matter mass within the field of view and improve the signal-to-noise ratio of prospective sterile neutrino decay signatures relative to hot gas and unresolved stellar emission. Our findings impose the most stringent limit on the sterile neutrino mass to date in the context of the Dodelson-Widrow model, ms < 2.2 keV (95% C.L.). Our results also constrain alternative sterile neutrino production scenarios at very small active-sterile neutrino mixing angles.

Sterile neutrinos are attractive dark matter candidates. Their parameter space of mass and mixing angle has not yet been fully tested despite intensive efforts that exploit their gravitational clustering properties and radiative decays. We use the limits on gamma-ray line emission from the Galactic center region obtained with the SPI spectrometer on the INTEGRAL satellite to set new constraints, which improve on the earlier bounds on mixing by more than 2 orders of magnitude, and thus strongly restrict a wide and interesting range of models.

Quintessence models based on a scalar field, φ, with an inverse power law potential display simple tracking behavior at early times, when the quintessence energy density, ρ φ , is sub-dominant. At late times, when ρ φ becomes comparable to the matter density, ρm, the evolution of φ diverges from its scaling behavior. We calculate the first order departure of φ from its tracker solution at low redshift. Our results for the evolution of φ, ρ φ , Ω φ , and w are surprisingly accurate even down to z = 0. We find that w and Ω φ are related linearly to first order, and derive a semi-analytic expression for w(z) which is accurate to within a few percent. Our analytic techniques are potentially applicable to any quintessence model in which the quintessence component comes to dominate at late times.PACS numbers: 98.80.Cq

We investigate the evolution of the hard X-ray luminosity of the red galaxy population using a large sample of 3316 red galaxies selected over a wide range in redshift (0.3