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arXiv:0711.1570 [ps, pdf, other] :
Title: Sterile neutrino dark matter in warped extra dimensions
Authors: Kenji Kadota
Comments: -
We consider a (long-lived) sterile neutrino dark matter scenario in a five dimensional (5D) warped extra dimension model where the fields can live in the bulk, which is partly motivated from the absence of the absolutely stable particles in a simple Randall-Sundrum model. The dominant production of the sterile neutrino can come from the decay of the radion (the scalar field representing the brane separation) around the electroweak scale. The suppressions of the 4D parameters due to the warp factor and the small wave function overlaps in the extra dimension help alleviate the exceeding fine-tunings typical for a sterile neutrino dark matter scenario in a 4D setup. |
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arXiv:0711.2302 [ps, pdf, other] :
Title: Consequences of dark matter self-annihilation for galaxy formation
Authors: Priyamvada Natarajan, Darren Croton, Gianfranco Bertone
Comments: 16 pages, 8 figures, submitted to MNRAS
Galaxy formation requires a process that continually heats gas and quenches star formation in order to reproduce the observed shape of the luminosity function of bright galaxies. To accomplish this, current models invoke heating from supernovae, and energy injection from active galactic nuclei. However, observations of radio-loud active galactic nuclei suggest that their feedback is likely to not be as efficient as required, signaling the need for additional heating processes. We propose the self-annihilation of weakly interacting massive particles that constitute dark matter as a steady source of heating. In this paper, we explore the circumstances under which this process may provide the required energy input. To do so, dark matter annihilations are incorporated into a galaxy formation model within the Millennium cosmological simulation. Energy input from self-annihilation can compensate for all the required gas cooling and reproduce the observed galaxy luminosity function only for what appear to be extreme values of the relevant key parameters. The key parameters are: the slope of the inner density profile of dark matter haloes and the outer spike radius. The inner density profile needs to be steepened to slopes of -1.5 or more and the outer spike radius needs to extend to a few tens of parsecs on galaxy scales and a kpc or so on cluster scales. If neutralinos or any thermal relic WIMP with s-wave annihilation constitute dark matter, their self-annihilation is inevitable and could provide enough power to modulate galaxy formation. Energy from self-annihilating WIMPs could be yet another piece of the feedback puzzle along with supernovae and active galactic nuclei. |
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arXiv:0711.2065 [ps, pdf, other] :
Title: On the Origin and Survival of UHE Cosmic-Ray Nuclei in GRBs and Hypernovae
Authors: Xiang-Yu Wang, Soebur Razzaque, Peter Meszaros
Comments: -
The chemical composition of the ultra-high-energy (UHE) cosmic rays serves as an important clue for their origin. Recent measurements of the elongation rates by the Pierre Auger Observatory hint at the possible presence of heavy or intermediate mass nuclei in the UHE cosmic rays. Gamma-ray bursts (GRBs) and hypernovae have been suggested as possible sources of the UHE cosmic rays. Here we derive the constraints on the physical conditions under which UHE heavy nuclei, if they are accelerated in these sources, can survive in their intense photon fields. We find that in the GRB external shock and in the hypernova scenarios, UHE nuclei can easily survive photo-disintegration. In the GRB internal shock scenario, UHE nuclei can also survive, provided the dissipation radius and/or the bulk Lorentz factor of the relativistic outflow are relatively large, or if the low-energy self-absorption break in the photon spectrum of the prompt emission occurs above several KeV. In internal shocks and in the other scenarios, intermediate-mass UHE nuclei have a higher probability of survival against photo-disintegration than UHE heavy nuclei such as Fe. |
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arXiv:0710.5073 [ps, pdf, other] :
Title: Modeling the cosmological co-evolution of supermassive black holes and galaxies: I. BH scaling relations and the AGN luminosity function
Authors: Federico Marulli, Silvia Bonoli, Enzo Branchini, Lauro Moscardini
Comments: 15 pages, 7 figures, MNRAS submitted
We model the cosmological co-evolution of galaxies and their central supermassive black holes (BHs) within a semi-analytical framework developed on the outputs of the Millennium Simulation. This model, described in detail in Croton et al. (2006) and De Lucia & Blaizot (2007), introduces a `radio mode' feedback from Active Galactic Nuclei (AGN) at the centre of X-ray emitting atmospheres in galaxy groups and clusters. Thanks to this mechanism, the model can simultaneously explain: (i) the low observed mass drop-out rate in cooling flows; (ii) the exponential cut-off in the bright end of the galaxy luminosity function; and (iii) the bulge-dominated morphologies and old stellar ages of the most massive galaxies in clusters. This paper is the first of a series in which we investigate how well this model can also reproduce the physical properties of BHs and AGN. Here we analyze the scaling relations, the fundamental plane and the mass function of BHs, and compare them with the most recent observational data. Moreover, we extend the semi-analytic model to follow the evolution of the BH mass accretion and its conversion into radiation, and compare the derived AGN bolometric luminosity function with the observed one. While we find for the most part a very good agreement between predicted and observed BH properties, the semi-analytic model underestimates the number density of luminous AGN at high redshifts, independently of the adopted Eddington factor and accretion efficiency. However, an agreement with the observations is possible within the framework of our model, provided it is assumed that the cold gas fraction accreted by BHs at high redshifts is larger than at low redshifts.. |
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arXiv:0711.0594 [ps, pdf, other] :
Title: Lines in the cosmic microwave background spectrum from the epoch of cosmological helium recombination
Authors: J.A. Rubino-Martin, J.Chluba, R.A. Sunyaev
Comments: 16 pages, 15 figures. Submitted to A&A
The main goal of this work is to calculate the contributions to the cosmological recombination spectrum due to bound-bound transitions of helium. We show that due to the presence of helium in the early Universe unique features appear in the total cosmological recombination spectrum. These may provide a unique observational possibility to determine the relative abundance of primordial helium, well before the formation of first stars. We include the effect of the tiny fraction of neutral hydrogen atoms on the dynamics of HeII -> HeI recombination at redshifts $z\sim 2500$. As discussed recently, this process significantly accelerates HeII -> HeI recombination, resulting in rather narrow and distinct features in the associated recombination spectrum. In addition this process induces some emission within the hydrogen Lyman-$\alpha$ line, before the actual epoch of hydrogen recombination round $z\sim 1100-1500$. We also show that some of the fine structure transitions of neutral helium appear in absorption, again leaving unique traces in the Cosmic Microwave Background blackbody spectrum, which may allow to confirm our understanding of the early Universe and detailed atomic physics. |
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arXiv:0711.1163 [ps, pdf, other] :
Title: Implications for the Origin of GRB 070201 from LIGO Observations
Authors: B. Abbott et al., for the LIGO Scientific Collaboration
Comments: 10 pages, 8 figures
We analyzed the available LIGO data coincident with GRB 070201, a short duration hard spectrum gamma-ray burst whose electromagnetically determined sky position is coincident with the spiral arms of the Andromeda galaxy (M31). Possible progenitors of such short hard GRBs include mergers of neutron stars or a neutron star and black hole, or soft gamma-ray repeater (SGR) flares. These events can be accompanied by gravitational-wave emission. No plausible gravitational wave candidates were found within a 180 s long window around the time of GRB 070201. This result implies that a compact binary progenitor of GRB 070201, with masses in the range 1 M_sun < m_1 < 3 M_sun and 1 M_sun < m_2 < 40 M_sun, located in M31 is excluded at >99% confidence. Indeed, if GRB 070201 were caused by a binary neutron star merger, we find that D < 3.5 Mpc is excluded, assuming random inclination, at 90% confidence. The result also implies that an unmodeled gravitational wave burst from GRB 070201 most probably emitted less than 4.4 x 10^(-4) M_sun c^2 (7.9 x 10^(50) ergs) in any 100 ms long period within the signal region if the source was in M31 and radiated isotropically at the same frequency as LIGO's peak sensitivity (f ~ 150 Hz). This upper limit does not exclude current models of SGRs at the M31 distance. |
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arXiv:0705.4680 [ps, pdf, other] :
Title: Measuring the dark matter velocity anisotropy in galaxy clusters
Authors: Steen H. Hansen, Rocco Piffaretti
Comments: 10 pages 5 figures 1 table
The Universe contains approximately 6 times more dark matter than normal baryonic matter, and a directly observed fundamental difference between dark matter and baryons would both be significant for our understanding of dark matter structures and provide us with information about the basic characteristics of the dark matter particle. We discuss one distinctive feature of dark matter structures in equilibrium, namely the property that a local dark matter temperature may depend on direction. This is in stark contrast to baryonic gases. We used X-ray observations of two nearby, relaxed galaxy clusters, under the assumptions of hydrostatic equilibrium and identical dark matter and gas temperatures in the outer cluster region, to measure this dark matter temperature anisotropy beta_dm, with non-parametric Monte Carlo methods. We find that beta_dm is greater than the value predicted for baryonic gases, beta_gas=0, at more than 3 sigma confidence. The observed value of the temperature anisotropy is in fair agreement with the results of cosmological N-body simulations and shows that the equilibration of the dark matter particles is not governed by local point-like interactions in contrast to baryonic gases. |
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