"Munch", May 15th, 2006

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Lyman Limit Systems in Cosmological Simulations Authors: Katharina Kohler, Nickolay Y. Gnedin Comments: 9 pages, 8 figures submitted to ApJ We used cosmological simulation with self-consistent radiative transfer to investigate the physical nature of Lyman Limit systems at z=4. In agreement with previous studies, we find that most of Lyman Limit systems are ionized by the cosmological background, while higher column density systems seem to be illuminated by the local sources of radiation. In addition, we find that most of Lyman limit systems in our simulations are located within the virial radii of galaxies with a wide range of masses, and are physically associated with them (``bits and pieces'' of galaxy formation). While the finite resolution of our simulations cannot exclude an existence of a second population of self-shielded, neutral gas clouds located in low mass dark matter halos (``minihalos''), our simulations are not consistent with ``minihalos'' dominating the total abundance of Lyman limit systems. Full-text: PostScript, PDF, or Other formats Towards a Phylogenetic Analysis of Galaxy Evolution : a Case Study with the Dwarf Galaxies of the Local Group Authors: Didier Fraix-Burnet (LAOG), Philippe Choler (LEA), Emmanuel J.P. Douzery (ISE) Comments: 13 pages 5 figures with 3 online only Context: The Hubble tuning fork diagram has always been the preferred scheme for classification of galaxies. It is based on morphology only. At the opposite, biologists have long taken into account the genealogical relatedness of living entities for classification purposes. Aims: Assuming branching evolution of galaxies as a 'descent with modification', we show here that the concepts and tools of phylogenetic systematics widely used in biology can be heuristically transposed to the case of galaxies. Methods: This approach that we call "astrocladistics" is applied to Dwarf Galaxies of the Local Group and provides the first evolutionary tree for real galaxies. Results: The trees that we present here are sufficiently solid to support the existence of a hierarchical organization in the diversity of dwarf galaxies of the Local Group. This also shows that these galaxies all derive from a common ancestral kind of objects. We find that some kinds of dIrrs are progenitors of both dSphs and other kinds of dIrrs.We also identify three evolutionary groups, each one having its own characteristics and own evolution. Conclusions: The present work opens a new way to analyze galaxy evolution and a path towards a new systematics of galaxies. Work on other galaxies in the Universe is in progress. Full-text: PostScript, PDF, or Other formats Direct Detection of Supersymmetric Particles in Neutrino Telescopes Authors: Ivone F. M. Albuquerque, Gustavo Burdman, Z. Chacko Comments: 11 pages, 9 figures In supersymmetric theories where the lightest supersymmetric particle is the gravitino the next to lightest supersymmetric particle is typically a long lived charged slepton. In this paper, following our earlier proposal, we perform a detailed study of the production of pairs of these particles induced by the interactions of high energy cosmic neutrinos with nucleons in the earth, their propagation through the earth and finally their detection in neutrino telescopes. We investigate the charged slepton energy loss in detail and establish that the relatively small cross-section for the production of supersymmetric particles is partially compensated for by the very long range of these heavy particles. The signal, consisting of two parallel charged tracks emerging from the earth, is characterized by a large track separation peaking at a few hundred meters. We perform a careful analysis of the main background, coming from direct di-muon production, and show that it can be separated from the signal due to its characteristically smaller track separation. We conclude that neutrino telescopes will complement collider searches in the determination of the supersymmetry breaking scale, and may even provide the first evidence for supersymmetry at the weak scale. Full-text: PostScript, PDF, or Other formats Constraints on Sterile Neutrino Dark Matter Authors: Kevork Abazajian, Savvas M. Koushiappas (Los Alamos National Laboratory) Comments: 8 pages, 3 figures; submitted to Phys. Rev. D Report-no: LA-UR 06-2765 We present a comprehensive analysis of constraints on the sterile neutrino as a dark matter candidate. The minimal production scenario with a standard thermal history and negligible cosmological lepton number is in conflict with conservative radiative decay constraints from the cosmic X-ray background in combination with stringent small-scale structure limits from the Lyman-alpha forest. We show that entropy release through massive particle decay after production does not alleviate these constraints. We further show that radiative decay constraints from local group dwarf galaxies are subject to large uncertainties in the dark matter density profile of these systems. Within the strongest set of constraints, resonant production of cold sterile neutrino dark matter in non-zero lepton number cosmologies remains allowed. Full-text: PostScript, PDF, or Other formats Searching for modified gravity with baryon oscillations: from SDSS to WFMOS Authors: Kazuhiro Yamamoto, Bruce A. Bassett, Robert C. Nichol, Yasushi Suto, Kazuhiro Yahata Comments: 16 pages, submitted to PRD We discuss how the baryon acoustic oscillation (BAO) signatures in galaxy power spectrum can distinguish the two different models to explain the cosmic acceleration, the modified gravity and the cosmological constant. For this purpose, we consider a model characterized by a parameter n, which corresponds to the Dvali-Gabadadze-Porrati (DGP) model if n=2 and reduces to a spatially-flat cosmological model with a cosmological constant for n=\infty. We find that the different expansion history of the modified gravity model systematically shift the peak positions of BAO. A preliminary analysis using the current SDSS LRG sample indicates that the original DGP model is disfavored unless the matter density parameter exceeds 0.3. The constraints will be strongly tightened with future spectroscopic samples of galaxies at high redshifts. WFMOS, in collaboration with other surveys such as Planck, will powerfully constrain modified gravity alternatives to dark energy as the explanation of cosmic acceleration. Full-text: PostScript, PDF, or Other formats Moduli/Inflaton Mixing with Supersymmetry Breaking Field Authors: Motoi Endo, Koichi Hamaguchi, Fuminobu Takahashi Comments: 28 pages, no figure Report-no: DESY 06-035 A heavy scalar field such as moduli or an inflaton generally mixes with a field responsible for the supersymmetry breaking. We study the scalar decay into the standard model particles and their superpartners, gravitinos, and the supersymmetry breaking sector, particularly paying attention to decay modes that proceed via the mixing between the scalar and the supersymmetry breaking field. The impacts of the new decay processes on cosmological scenarios are also discussed; the modulus field generically produces too much gravitinos, and most of the inflation models tend to result in too high reheating temperature and/or gravitino overproduction. Full-text: PostScript, PDF, or Other formats Model selection forecasts for the spectral index from the Planck satellite Authors: Cédric Pahud, Andrew R Liddle, Pia Mukherjee, David Parkinson Comments: 4 pages RevTeX with one figure included The recent WMAP3 results have placed measurements of the spectral index n_S in an interesting position. While parameter estimation techniques indicate that the Harrison-Zel'dovich spectrum n_S=1 is strongly excluded (in the absence of tensor perturbations), Bayesian model selection techniques reveal that the case against n_S=1 is not yet conclusive. In this paper, we forecast the ability of the Planck satellite mission to use Bayesian model selection to convincingly exclude (or favour) the Harrison-Zel'dovich model. Full-text: PostScript, PDF, or Other formats Two knees and the Evasion of Greisen-Zatsepin-Kuz'min Cutoff in Cosmic Ray Spectrum -- Are Neutrinos the Tachyons? Authors: Guang-Jiong Ni, Zhi-Qiang Shi Comments: 14 pages, 9 figures The whole spectrum of high-energy cosmic ray (HECR) is, very likely, influenced by tachyonic neutrinos. Especially, the appearance of two knees can be fitted by the tachyon mass $m(\nu_e)=m(\nu_\mu)\simeq 0.51$ eV/$c^2$ as predicted by a minimal three - flavor model for tachyonic neutrino with one parameter $\delta\simeq 0.34$ eV only. Then the evasion of GZK cutoff could be ascribed to $Z^0(W^\pm)$-burst model together with the same mechanism for knees as well as a prediction of left-right polarization dependent lifetime asymmetry. A further conclusive experiment might be whether the protons of HECR detected on Earth are really right handed polarized? Full-text: PostScript, PDF, or Other formats Finite Temperature Effects and Axion Cosmology Authors: Namit Mahajan, Sukanta Panda Comments: 9 pages, 2 figures We investigate the impact of finite temperature effects on axions in the context of cosmology. The temperature dependence of the decay constant is modeled analogous to pions. For the two interesting cases considered here, we find that the temperature effects do lead to changes relevant for detailed and precise abundance and rate calculations. We also find that the axion decoupling temperature starts showing large deviations for larger values of the axion decay constant. Full-text: PostScript, PDF, or Other formats Inflation, dark matter and dark energy in the string landscape Authors: Andrew R Liddle, L Arturo Ureña-López Comments: 4 pages RevTex4 We consider the conditions needed to unify the description of dark matter, dark energy and inflation within the context of the string landscape. We find that incomplete decay of the inflaton field offers the possibility that a single field might be responsible for all of inflation, dark matter and dark energy. By contrast, unifying dark matter and dark energy into a single field which is separate from the inflaton appears rather difficult. Full-text: PostScript, PDF, or Other formats Why the cosmological constant is small and positive Authors: Paul J. Steinhardt, Neil Turok Comments: 15 pages, 1 figure Within conventional big bang cosmology, it has proven to be very difficult to understand why today's cosmological constant is so small. In this paper, we show that a cyclic model of the universe can naturally incorporate a dynamical mechanism that automatically relaxes the value of the cosmological constant, taking account of contributions to the vacuum density at all energy scales. Because the relaxation time grows exponentially as the vacuum density decreases, nearly every volume of space spends an overwhelming majority of the time at the stage when the cosmological constant is small and positive, as observed today. Full-text: PostScript, PDF, or Other formats