Munch: Monday, February 5, 2007

                               


 

 WHERE: 6TH FLOOR CONFERENCE ROOM
WHEN  : 12:30, MONDAY

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


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       Munch Archive

Intrinsic galaxy alignments from the 2SLAQ and SDSS surveys: luminosity and redshift scalings and implications for weak lensing surveys  astro-ph/0701671

Authors: Christopher M. Hirata, Rachel Mandelbaum, Mustapha Ishak, Uros Seljak, Robert Nichol, Kevin A. Pimbblet, Nicholas P. Ross, David Wake
Comments: 20 pages, 10 figures; to be submitted to MNRAS
Correlations between intrinsic shear and the density field on large scales, a potentially important contaminant for cosmic shear surveys, have been robustly detected at low redshifts with bright galaxies in SDSS data. Here we present a more detailed characterization of this effect, which can cause anti-correlations between gravitational lensing shear and intrinsic ellipticity (GI correlations). This measurement uses 36278 Luminous Red Galaxies (LRGs) from the SDSS spectroscopic sample with 0.15<z<0.35, split by redshift and luminosity; 7758 LRGs from the 2SLAQ Survey at 0.4<z<0.8; and a variety of other SDSS samples from previous, related work. We find >3sigma detections of the effect for all galaxy subsamples within the SDSS LRG sample; for the 2SLAQ sample, we find a 2sigma detection for a bright subsample, and no detection for a fainter subsample. Fitting formulae are provided for the scaling of the GI correlations with luminosity, transverse separation, and redshift. We estimate contamination in the measurement of sigma_8 for future cosmic shear surveys on the basis of the fitted dependence of GI correlations on galaxy properties. We find contamination to the power spectrum ranging from -1.5 (optimistic) to -33 per cent (pessimistic) for a toy cosmic shear survey using all galaxies to a depth of R=24 using scales l~500. This corresponds to a bias in sigma_8 of Delta sigma_8=-0.004 (optimistic), -0.02 (central), or -0.10 (pessimistic). We provide a prescription for inclusion of this error in cosmological parameter estimation codes. The principal uncertainty is in the treatment of the L<=L* blue galaxies. Characterization of the tidal alignments of these galaxies, especially at redshifts relevant for cosmic shear, should be a high priority for the cosmic shear community. (Abridged)

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Improved forecasts for the baryon acoustic oscillations and cosmological distance scale   astro-ph/0701079

Authors: Hee-Jong Seo, Daniel J. Eisenstein
Comments: Submitted to ApJ, 21 pages, LaTeX
We present the cosmological distance errors achievable using the baryon acoustic oscillations as a standard ruler. We begin from a Fisher matrix formalism that is upgraded from Seo & Eisenstein (2003). We isolate the information from the baryonic peaks by excluding distance information from other less robust sources. Meanwhile we accommodate the Lagrangian displacement distribution into the Fisher matrix calculation to reflect the gradual loss of information in scale and in time due to nonlinear growth, nonlinear bias, and nonlinear redshift distortions. We then show that we can contract the multi-dimensional Fisher matrix calculations into a 2-dimensional or even 1-dimensional formalism with physically motivated approximations. We present the resulting fitting formula for the cosmological distance errors from galaxy redshift surveys as a function of survey parameters and nonlinearity, which saves us going through the 12-dimensional Fisher matrix calculations. Finally, we show excellent agreement between the distance error estimates from the revised Fisher matrix and the precision on the distance scale recovered from N-body simulations.

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Information criteria for astrophysical model selection  astro-ph/0701113

Authors: Andrew R Liddle
Comments: 5 pages, no figures
Model selection is the problem of distinguishing competing models, perhaps featuring different numbers of parameters. The statistics literature contains two distinct sets of tools, those based on information theory such as the Akaike Information Criterion (AIC), and those on Bayesian inference such as the Bayesian evidence and Bayesian Information Criterion (BIC). The Deviance Information Criterion combines ideas from both heritages; it is readily computed from Monte Carlo posterior samples and, unlike the AIC and BIC, allows for parameter degeneracy. I describe the properties of the information criteria, and as an example compute them from WMAP3 data for several cosmological models. I find that at present the information theory and Bayesian approaches give significantly different conclusions from that data.

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First limits on WIMP nuclear recoil signals in ZEPLIN-II: a two phase xenon detector for dark matter detection  astro-ph/0701858  (suggested by Scott)

Authors: G. J. Alner, H. M. Araujo, A. Bewick, C. Bungau, B. Camanzi, M. J. Carson, R. J. Cashmore, H. Chagani, V. Chepel, D. Cline, D. Davidge, J. C. Davies, E. Daw, J. Dawson, T. Durkin, B. Edwards, T. Gamble, J. Gao, C. Ghag, A. S. Howard, W. G. Jones, M. Joshi, E. V. Korolkova, V. A. Kudryavtsev, T. Lawson, V. N. Lebedenko, J. D. Lewin, P. Lightfoot, A. Lindote, I. Liubarsky, M. I. Lopes, R. Luscher, P. Majewski, K Mavrokoridis, J. E. McMillan, B. Morgan, D. Muna, A. St.J. Murphy, F. Neves, G. G. Nicklin, W. Ooi, S. M. Paling, J. Pinto da Cunha, S. J. S. Plank, R. M. Preece, J. J. Quenby, M. Robinson, F. Sergiampietri, C. Silva, V. N. Solovov, N. J. T. Smith, P. F. Smith, N. J. C. Spooner, T. J. Sumner, C. Thorne, D. R. Tovey, E. Tziaferi, R. J. Walker, H. Wang, J. White, F. L. H. Wolfs
Comments: 38 Pages, 16 Figures, (v2: corrected table 2 description)
Results are presented from the first underground data run of ZEPLIN-II, a 31 kg two phase xenon detector developed to observe nuclear recoils from hypothetical weakly interacting massive dark matter particles. Discrimination between nuclear recoils and background electron recoils is afforded by recording both the scintillation and ionisation signals generated within the liquid xenon, with the ratio of these signals being different for the two classes of event. This ratio is calibrated for different incident species using an AmBe neutron source and Co-60 gamma-ray sources. From our first 31 live days of running ZEPLIN-II, the total exposure following the application of fiducial and stability cuts was 225 kgxdays. A background population of radon progeny events was observed in this run, arising from radon emission in the gas purification getters, due to radon daughter ion decays on the surfaces of the walls of the chamber. An acceptance window, defined by the neutron calibration data, of 50% nuclear recoil acceptance between 5 keVee and 20 keVee, had an observed count of 29 events, with a summed expectation of 28.6+/-4.3 gamma-ray and radon progeny induced background events. These figures provide a 90% c.l. upper limit to the number of nuclear recoils of 10.4 events in this acceptance window, which converts to a WIMP-nucleon spin-independent cross-section with a minimum of 6.6x10^-7 pb following the inclusion of an energy dependent, calibrated, efficiency. A second run is currently underway in which the radon progeny will be eliminated, thereby removing the background population, with a projected sensitivity of 2x10^-7 pb for similar exposures as the first run.

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Testing the reliability of weak lensing cluster detections  astro-ph/0702031

Authors: Francesco Pace (1), Matteo Maturi (1), Massimo Meneghetti (2), Matthias Bartelmann (1), Lauro Moscardini (3,4), Klaus Dolag (5) ((1) ITA - Heidelberg University (2)INAF - Osservatorio Astronomico di Bologna (3) Dipartimento di Astronomia - Universita' di Bologna (4) INFN - Bologna (5)MPA - Garching bei Muenchen)
Comments: 12 pages, 13 figures
We study the reliability of dark-matter halo detections with three different linear filters applied to weak-lensing data. We use ray-tracing in the multiple lens-plane approximation through a large cosmological simulation to construct realizations of cosmic lensing by large-scale structures between redshifts zero and two. We apply the filters mentioned above to detect peaks in the weak-lensing signal and compare them with the true population of dark matter halos present in the simulation. We confirm the stability and performance of a filter optimized for suppressing the contamination by large-scale structure. It allows the reliable detection of dark-matter halos with masses above a few times 1e13 M_sun/h with a fraction of spurious detections below ~10%. For sources at redshift two, 50% of the halos more massive than ~7e13 M_sun/h are detected, and completeness is reached at ~2e14 M_sun/h.

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QSO Lensing Magnification: A Comparison of 2QZ and SDSS Results                   astro-ph/0701870  (suggested by Scott)

Authors: G. Mountrichas, T. Shanks
Comments: 15 pages, 20 figures, submitted to MNRAS
The lensing of background QSOs by foreground galaxies is a powerful probe of the mass density of the Universe and the power spectrum of mass clustering. However, the 2dF QSO survey suggested that a strong anticorrelation effect at g<21 was seen for both galaxies and clusters which implied that galaxies are anti-biased (b~0.1) on small scales at a higher level than predicted by the standard cosmology (Myers et al., 2003, 2005) whereas results from SDSS suggested that the effect was much smaller (b~0.6) and in line with standard expectations (Scranton et al., 2005). We first cross-correlate the SDSS photo-z, g<21, 1.0<z_p<2.2 QSOs with g<21 galaxies and clusters in the same areas. These results are somewhat less negative than the results based on the 2QZ QSOs found by Myers et al. But contamination of the QSOs by low-z NELGs and QSOs can cause underestimation of the anticorrelation lensing signal. When a correction is applied to the photo-z QSO sample of Scranton et al. the anticorrelation increases and the agreement with the results of Myers et al. is improved. When we also take into account the fainter r<21 galaxy limit of Scranton et al. as opposed to g<21 for Myers et al., the two observational results appear to be in very good agreement. This therefore leaves open the question of why the theoretical interpretations are so different for these analyses. We note that the results of Guimaraes, Myers & Shanks based on mock catalogues from the LCDM Hubble Volume strongly suggest that QSO lensing at the levels detected by both Myers et al. and now Scranton et al. is incompatible with a galaxy bias of b~1 in the standard cosmological model. If the QSO lensing results are correct then the consequences for cosmology may be significant (see Shanks 2006).

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First ground based measurement of atmospheric Cherenkov light from cosmic rays            astro-ph/0701766 (suggested by Jeter)

Authors: HESS Collaboration: F.A. Aharonian, et al
Comments: 14 pages, 13 figures, accepted for publication in Phys. Rev. D
A recently proposed novel technique for the detection of cosmic rays with arrays of Imaging Atmospheric Cherenkov Telescopes is applied to data from the High Energy Stereoscopic System (H.E.S.S.). The method relies on the ground based detection of Cherenkov light emitted from the primary particle prior to its first interaction in the atmosphere. The charge of the primary particle (Z) can be estimated from the intensity of this light, since it is proportional to Z$^2$. Using H.E.S.S. data, an energy spectrum for cosmic-ray iron nuclei in the energy range 13--200 TeV is derived. The reconstructed spectrum is consistent with previous direct measurements and is one of the most precise so far in this energy range.

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Imprints of a Primordial Preferred Direction on the Microwave Background                    astro-ph/0701357  (suggested by Dan)

Authors: Lotty Ackerman, Sean M. Carroll, Mark B. Wise
Comments: 7 pages, no figures
Report-no: CALT-68-2626
Rotational invariance is a well-established feature of low-energy physics. Violations of this symmetry must be extremely small today, but could have been larger in earlier epochs. In this paper we examine the consequences of a small breaking of rotational invariance during the inflationary era when the primordial density fluctuations were generated. Assuming that a fixed-norm vector picked out a preferred direction during the inflationary era, we explore the imprint it would leave on the cosmic microwave background anisotropy, and provide explicit formulas for the expected amplitudes $<a_{lm}a_{l'm'}^*>$ of the spherical-harmonic coefficients. We suggest that it is natural to expect that the imprint on the primordial power spectrum of a preferred spatial direction is approximately scale-invariant, and examine a simple model in which this is true.

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Dark matter from late decays and the small-scale structure problems                  hep-ph/0701007 (suggested by Dan)

Authors: Torsten Bringmann, Francesca Borzumati, Piero Ullio
Comments: 15 pages revtex4 style; 10 figures
The generation of dark matter in late decays of quasi-stable massive particles has been proposed as a viable framework to address the excess of power found in numerical N-body simulations for cold dark matter cosmologies. We identify a convenient set of variable to illustrate which requirements need to be satisfied in any generic particle physics model to address the small scale problems and fulfill other astrophysical constraints. We re-examine the role of gravitinos and Kaluza-Klein gravitons in this context and find them disfavoured as a solution to the small-scale problems in case they are DM candidates generated in the decay of thermally produced WIMPs. We propose right-handed sneutrinos and right-handed Kaluza-Klein neutrinos as alternatives. We find that they are viable dark matter candidates, but that they can contribute to a solution of the small scale problems only in case the associated Dirac neutrino mass term appears as a subdominant contribution in the neutrino mass matrix.

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