P. L. Biermann

Relic keV sterile neutrinos and reionization.

A sterile neutrino with a mass of several keV can account for cosmological dark matter, as well as explain the observed velocities of pulsars. We show that x rays produced by the decays of these relic sterile neutrinos can boost the production of molecular hydrogen, which can speed up the cooling of gas and the early star formation, which can, in turn, lead to a reionization of the Universe at a high enough redshift to be consistent with the Wilkinson Microwave Anisotropy Probe results.

NEW LIMITS ON STERILE NEUTRINOS FROM SUZAKU OBSERVATIONS OF THE URSA MINOR DWARF SPHEROIDAL GALAXY

We present results of our search for X-ray line emission associated with the radiative decay of the sterile neutrino, a well motivated dark matter candidate, in Suzaku Observatory spectra of the Ursa Minor dwarf spheroidal galaxy. These data represent the first deep observation of one of these extreme mass-to-light systems and the first dedicated dark matter search using an X-ray telescope. No such emission line is positively detected, and we place new constraints on the combination of the sterile neutrino mass, m st, and the active-sterile neutrino oscillation mixing angle, θ. Line flux upper limits are derived using a maximum-likelihood-based approach that, along with the lack of intrinsic X-ray emission, enables us to minimize systematics and account for those that remain. The limits we derive match or approach the best previous results over the entire 1-20 keV mass range from a single Suzaku observation. These are used to place constraints on the existence of sterile neutrinos with given parameters in the general case and in the case where they are assumed to constitute all of the dark matter. The range allowed implies that sterile neutrinos remain a viable candidate to make up some—or all—of the dark matter and also explain pulsar kicks and various other astrophysical phenomena.

THERMAL EVOLUTION OF THE PRIMORDIAL CLOUDS IN WARM DARK MATTER MODELS WITH keV STERILE NEUTRINOS

We analyze the processes relevant for star formation in a model with dark matter in the form of sterile neutrinos. Sterile neutrino decays produce an X-ray background radiation that has a twofold effect on the collapsing clouds of hydrogen. First, the X-rays ionize the gas and cause an increase in the fraction of molecular hydrogen, which makes it easier for the gas to cool and to form stars. Second, the same X-rays deposit a certain amount of heat, which could, in principle, thwart the cooling of gas. We find that in all the cases we have examined the overall effect of sterile dark matter is to facilitate the cooling of gas. Hence, we conclude that dark matter in the form of sterile neutrinos can help the early collapse of gas clouds and the subsequent star formation.

Cosmic Ray Electrons and Positrons from Supernova Explosions of Massive Stars

We attribute the recently discovered cosmic ray electron and cosmic ray positron excess components and their cutoffs to the acceleration in the supernova shock in the polar cap of exploding Wolf-Rayet and red supergiant stars. Considering a spherical surface at some radius around such a star, the magnetic field is radial in the polar cap as opposed to most of 4pi (the full solid angle), where the magnetic field is nearly tangential. This difference yields a flatter spectrum, and also an enhanced positron injection for the cosmic rays accelerated in the polar cap. This reasoning naturally explains the observations. Precise spectral measurements will be the test, as this predicts a simple E;{-2} spectrum for the new components in the source, steepened to E;{-3} in observations with an E;{-4} cutoff.

Degenerate sterile neutrino dark matter in the cores of galaxies

Aims. We study the distribution of fermionic dark matter at the center of galaxies using NFW, Moore and isothermal density profiles and show that dark matter becomes degenerate for particle masses of axa0fewxa0keV and for distances less than axa0few parsec from the center of our galaxy. Methods. A compact degenerate core forms after galaxy merging and boosts the growth of supermassive black holes at the center of galaxies. Results. To explain the galactic center black hole of mass of ~3.5xa0

Cosmic ray measurements with LOPES: Status and recent results

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Dark Matter and Sterile Neutrinos

10^{6}~M_{odot}

ACTIVE GALACTIC NUCLEI: SOURCES FOR ULTRA HIGH ENERGY COSMIC RAYS

and axa0supermassive black hole of ~3xa0

Comparison of lopes measurements with coreas and reas 3.11 simulations

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