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arXiv:0801.3485 [ps, pdf, other] :
Title: The Correlation Function of Optically Selected Galaxy Clusters in the Sloan Digital Sky Survey
Authors: Juan Estrada, Emiliano Sefusatti, Joshua A. Frieman
Comments: -
We measure the two-point spatial correlation function for clusters selected from the photometric MaxBCG galaxy cluster catalog for the Sloan Digital Sky Survey (SDSS). We evaluate the correlation function for several cluster samples using different cuts in cluster richness. Fitting the results to power-laws, $\xi_{cc}(r) = (r/R_0)^{-\gamma}$, the estimated correlation length $R_0$ as a function of richness is broadly consistent with previous cluster observations and with expectations from N-body simulations. Since these measurements extend to very large scales, we also compare them to models that include the baryon acoustic oscillation feature and that account for the smoothing effects induced by errors in the cluster photometric redshift estimates. For the largest cluster sample, corresponding to a richness threshold of $\Ng\ge 10$, we find weak evidence, of about $1.4-1.7\sigma$ significance, for the baryonic acoustic oscillation signature in the cluster correlation function. |
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arXiv:0801.3686 [ps, pdf, other] :
Title: A Natural Supersymmetric Model with MeV Dark Matter
Authors: Dan Hooper, Kathryn Zurek
Comments: 4 pages, 1 figure
It has previously been proposed that annihilating dark matter particles with MeV-scale masses could be responsible for the flux of 511 keV photons observed from the region of the Galactic Bulge. The conventional wisdom, however, is that it is very challenging to construct a viable particle physics model containing MeV dark matter. In this letter, we challenge this conclusion by describing a simple and natural supersymmetric model in which the lightest supersymmetric particle naturally has a MeV-scale mass and the other phenomenological properties required to generate the 511 keV emission. In particular, the small ($\sim$ $10^{-5}$) effective couplings between dark matter and the Standard Model fermions required in this scenario naturally lead to radiative corrections that generate MeV-scale masses for both the dark matter candidate and the mediator particle. |
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arXiv:0801.3440 [ps, pdf, other] :
Title: Heavy Dark Matter Through the Higgs Portal
Authors: John March-Russell, Stephen M. West, Daniel Cumberbatch, Dan Hooper
Comments: LaTex, 20 pages, 9 figures
Motivated by Higgs Portal and Hidden Valley models, heavy particle dark matter that communicates with the supersymmetric Standard Model via pure Higgs sector interactions is considered. We show that a thermal relic abundance consistent with the measured density of dark matter is possible for masses up to $\sim$ 30 TeV. For dark matter masses above $\sim$ 1 TeV, non-perturbative Sommerfeld corrections to the annihilation rate are large, and have the potential to greatly affect indirect detection signals. For large dark matter masses, the Higgs-dark matter sector couplings are large and we show how such models may be given a UV completion within the context of so-called "Fat-Higgs" models. Higgs Portal dark matter provides an example of an attractive alternative to conventional MSSM neutralino dark matter that may evade discovery at the LHC, while still being within the reach of current and upcoming indirect detection experiments. |
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arXiv:0712.3063 [ps, pdf, other] :
Title: A method to search for strong galaxy-galaxy lenses in optical imaging surveys
Authors: Jeffrey M. Kubo, Ian P. Dell'Antonio
Comments: Accepted for publication in MNRAS, 12 pages
We present a semi-automated method to search for strong galaxy-galaxy lenses in optical imaging surveys. Our search technique constrains the shape of strongly lensed galaxies (or arcs) in a multi-parameter space, which includes the third order (octopole) moments of objects. This method is applied to the Deep Lens Survey (DLS), a deep ground based weak lensing survey imaging to $R\sim26$. The parameter space of arcs in the DLS is simulated using real galaxies extracted from deep HST fields in order to more accurately reproduce the properties of arcs. Arcs are detected in the DLS using a pixel thresholding method and candidate arcs are selected within this multi-parameter space. Examples of strong galaxy-galaxy lens candidates discovered in the DLS F2 field (4 square degrees) are presented. |
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arXiv:0801.3822 [ps, pdf, other] :
Title: Estimating the Redshift Distribution of Faint Galaxy Samples
Authors: Marcos Lima, Hiroaki Oyaizu, Carlos E. Cunha, Joshua Frieman, Huan Lin, Erin S. Sheldon
Comments: 14 pages, 9 figures, submitted to MNRAS
We present an empirical method for estimating the underlying redshift distribution N(z) of galaxy photometric samples from photometric observables. The method does not rely on photometric redshift (photo-z) estimates for individual galaxies, which typically suffer from biases. Instead, it assigns weights to galaxies in a spectroscopic subsample such that the weighted distributions of photometric observables (e.g., multi-band magnitudes) match the corresponding distributions for the photometric sample. The weights are estimated using a nearest-neighbor technique that ensures stability in sparsely populated regions of color-magnitude space. The derived weights are then summed in redshift bins to create the redshift distribution. We apply this weighting technique to data from the Sloan Digital Sky Survey as well as to mock catalogs for the Dark Energy Survey, and compare the results to those from the estimation of photo-z's derived by a neural network algorithm. We find that the weighting method accurately recovers the underlying redshift distribution, typically better than the photo-z reconstruction, provided the spectroscopic subsample spans the range of photometric observables covered by the photometric sample. |
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arXiv:0801.3297 [ps, pdf, other] :
Title: A Measurement of the Rate of type-Ia Supernovae at Redshift $z\approx$ 0.1 from the First Season of the SDSS-II Supernova Survey
Authors: Benjamin Dilday et al.
Comments: Submitted to to ApJ (65 pages, 12 figures)
We present a measurement of the rate of type Ia supernovae (SNe Ia) from the first of three seasons of data from the SDSS-II Supernova Survey. For this measurement, we include 17 SNe Ia at redshift $z\le0.12$. Assuming a flat cosmology with $\Omega_m = 0.3=1-\Omega_\Lambda$, we find a volumetric SN Ia rate of $[2.93^{+0.17}_{-0.04}({\rm systematic})^{+0.90}_{-0.71}({\rm statistical})] \times 10^{-5} {\rm SNe} {\rm Mpc}^{-3} h_{70}^3 {\rm year}^{-1}$, at a volume-weighted mean redshift of 0.09. This result is consistent with previous measurements of the SN Ia rate in a similar redshift range. The systematic errors are well controlled, resulting in the most precise measurement of the SN Ia rate in this redshift range. We use a maximum likelihood method to fit SN rate models to the SDSS-II Supernova Survey data in combination with other rate measurements, thereby constraining models for the redshift-evolution of the SN Ia rate. Fitting the combined data to a simple power-law evolution of the volumetric SN Ia rate, $r_V \propto (1+z)^{\beta}$, we obtain a value of $\beta = 1.5 \pm 0.6$, i.e. the SN Ia rate is determined to be an increasing function of redshift at the $\sim 2.5 \sigma$ level. Fitting the results to a model in which the volumetric SN rate, $r_V=A\rho(t)+B\dot \rho(t)$, where $\rho(t)$ is the stellar mass density and $\dot \rho(t)$ is the star formation rate, we find $A = (2.8 \pm 1.2) \times 10^{-14} \mathrm{SNe} \mathrm{M}_{\sun}^{-1} \mathrm{year}^{-1}$, $B = (9.3^{+3.4}_{-3.1})\times 10^{-4} \mathrm{SNe} \mathrm{M}_{\sun}^{-1}$. |
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arXiv:0801.1315 [ps, pdf, other] :
Title: The Hagedorn Soup and an Emergent Cyclic Universe
Authors: Tirthabir Biswas
Comments: 15 pages, 1 figure
One of the problems of constructing a successful cyclic universe scenario is that it has to incorporate the second law of thermodynamics. This leads to Tolman's ever shrinking cycles which eventually vanish at a finite proper time in the past. In this paper we construct a theoretically consistent (ghost-free) non-singular toy model where as the cycles shrink in the past they also spend more and more time in the Hagedorn phase where all the string states are in thermal equilibrium and entropy is conserved. Thus in such a scenario the entropy asymptotes to a finite non-zero constant in the infinite past. The universe ``emerges'' from a small (string size) geodesically complete quasi-periodic space-time. This paradigm also naturally addresses some of the classic puzzles of Big Bang cosmology, such as the largeness, horizon and flatness problems. |
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arXiv:0801.3659 [ps, pdf, other] :
Title: Measuring Baryon Acoustic Oscillations with Millions of Supernovae
Authors: Hu Zhan, Lifan Wang, Philip Pinto, J. Anthony Tyson
Comments: 4 pages, 3 figures, ApJL accepted
Since type Ia Supernovae (SNe) explode in galaxies, they can, in principle, be used as the same tracer of the large-scale structure as their hosts to measure baryon acoustic oscillations (BAOs). To realize this, one must obtain a dense integrated sampling of SNe over a large fraction of the sky, which may only be achievable photometrically with future projects such as the Large Synoptic Survey Telescope. The advantage of SN BAOs is that SNe have more uniform luminosities and more accurate photometric redshifts than galaxies, but the disadvantage is that they are transitory and hard to obtain in large number at high redshift. We find that a half-sky photometric SN survey to redshift z = 0.8 is able to measure the baryon signature in the SN spatial power spectrum. Although dark energy constraints from SN BAOs are weak, they can significantly improve the results from SN luminosity distances of the same data, and the combination of the two is no longer sensitive to cosmic microwave background priors. |
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arXiv:0801.3198 [ps, pdf, other] :
Title: Formation of hard VHE gamma-ray spectra of blazars due to internal photon-photon absorption
Authors: F. Aharonian, D. Khangulyan, L. Costamante
Comments: 10 pages, 8 figures, submitted to MNRAS
The energy spectra of TeV gamma-rays from blazars, after being corrected for intergalatic absorption in the Extragalactic Background Light (EBL), appear unusually hard, a fact that poses challenges to the conventional models of particle acceleration in TeV blazars and/or to the EBL models. In this paper we show that the internal absorption of gamma-rays caused by interactions with dense narrow-band radiation fields in the vicinity of compact gamma-ray production regions can lead to the formation of gamma-ray spectra of an almost arbitrary hardness. This allows significant relaxation of the current tight constraints on particle acceleration and radiation models, although at the expense of enhanced requirements to the available nonthermal energy budget. The latter, however, is not a critical issue, as long as it can be largely compensated by the Doppler boosting, assuming very large ($\geq 30$) Doppler factors of the relativistically moving gamma-ray production regions. The suggested scenario of formation of hard gamma-ray spectra predicts detectable synchrotron radiation of secondary electron-positron pairs which might require a revision of the current ``standard paradigm'' of spectral energy distributions of gamma-ray blazars. If the primary gamma-rays are of hadronic origin related to $pp$ or $p \gamma$ interactions, the ``internal gamma-ray absorption'' model predicts neutrino fluxes close to the detection threshold of the next generation high energy neutrino detectors. |
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arXiv:0801.3463 [ps, pdf, other] :
Title: The trispectrum of 21-cm background anisotropies as a probe of primordial non-Gaussianity
Authors: Asantha Cooray, Chao Li, Alessandro Melchiorri
Comments: 12 pages, PRD submitted
The 21-cm anisotropies from the neutral hydrogen distribution prior to the era of reionization is a sensitive probe of primordial non-Gaussianity. Unlike the case with cosmic microwave background, 21-cm anisotropies provide multi-redshift information with frequency selection and is not damped at arcminute angular scales. We discuss the angular trispectrum of the 21-cm background anisotropies and discuss how the trispectrum signal generated by the primordial non-Gaussianity can be measured with the three-to-one correlator and the corresponding angular power spectrum. We also discuss the separation of primordial non-Gaussian information in the trispectrum with that generated by the subsequent non-linear gravitational evolution of the density field. While with the angular bispectrum of 21-cm anisotropies one can limit the second order corrections to the primordial fluctuations below f_NL< 1, using the trispectrum information we suggest that the third order coupling term, f_2 or g_NL, can be constrained to be arounde 10 with future 21-cm observations over the redshift interval of 50 to 100. |
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