Sunday. Went to Hello Vietnam with Mom and Dad--an excellent restaurant. Dad caught a cold with a fever of 38 C, is in bed. Made excellent grilled chicken leg, as we did last night. Delicious, but it makes the apartment smell of chicken smoke!
1112.3713
An MCMC determination of the primordial helium abundance
Aver, Olive, Skillman
Estimating the primordial helium abundance: spectroscopic observations of chemical abundances in metal-poor H II regions. Physical parameters of H II region: electron density, electron temperature, reddening, and the ratio of He to H ("y"). Customary to estimate or self-consistently determine these parameters to calculate y. Frequentist analyses of the parameter space have been shown to be successful in these determinations, and MCMC techniques have proven to be very efficient in sampling this parameter space. But accurate determination of primordial He from observing H II regions is constrained by both systematic and statistical uncertaities. To reduce the latter and better characterize the former, apply MCMC methods to the large dataset recently compiled by Izotov, Thuan, Stasinska (07). Variety of cuts explored, for better quality data. First examine efficacy of the He I 4026 emission line as a constraint on the solutions, show systematic bias in its absence. As a clear measure of the quality of the physical solution, a chi2 analysis proves instrumental in the selection of data compatible with the theoretical model. In addition, the method also allows to exclude systems for which parameter estimations are statistical outliers. As a result, the final selected dataset gains in reliability and exhibits improved consistency. Regression to zero metallicity yields Y_p = 0.2534 pm 0.0083, in broad agreement with the WMAP result. The inclusion of more observations show promise, but require high quality spectra.
1112.3769
SZ clusters in Millenium gas simulations
Kay, Peel, ... et al
Study SZ cluster population at low and high redshift using large-volume Millenium Gas N body hydro sims, for 3 models with varying gas physics. Confirm: intrinsic (spherical) Y500-M500 relation has very little scatter (sigma_log10Y ~ 0.04), is insensitive to cluster gas physics and evolves to redshift one in accord with self-similar expectations. Pre-heating and feedback models predict scaling relations that are in excellent agreement with the combined Planck and XMM-Newton data. Agreement largely preserved when r500 and M500 are derived using hydrostatic mass proxy, Y_X,500, but with significantly reduced scatter--a result that is due to the tight correlation between Y500 and Y_X,500. This assumption also hides any bias in the relation due to dynamical activity. Assess the importance of projection effects from LSS along the LoS, by extracting cluster Y500 values from fifty simulated 5x5 square degree sky maps. Once the (model-dependent) mean signal is subtracted from the maps, find that the integrated SZ signal is unbiased with respect to the underlying clusters, although the scatter in the (cylindrical) Y500-M500 relation increases in the pre-heating case, where a significant amount of energy was injected into the IGM at high redshift. Study the hot gas pressure profiles to investigate the origin of the SZ signal and find that the largest contribution comes from radii close to r500 in all cases. The profiles themselves are well described by generalised NFW profiles but there is significant cluster-to-cluster scatter.
(*)1112.4040
Weak gravitational lensing effects on cosmological parameters and DE from gamma-ray bursts
Wang, Dai
GRBs can constrain cosmology with high-z information. But the apparent magnitude of distant GRBs can be distorted by lensing from LSS along LoS. Investigate lensing effect on cosmological parameters and DE EoS from GRBs. First calibrate the GRB luminosity relations without assuming any cosmological models. Luminosity distances of low-z GRBs were calibrated with the cosmography method using SNe Ia sample. Find that the gravitational lensing has non-negligible effects on the determination of DE and cosmo parameters. Because high-z GRBs are more likely to be reduced, the most probable value of the observed matter density Omega_m will shift from 0.30 to 0.26 after including the gravitational lensing effect; also shifts w to a more negative value. Constratin DE EoS out to z~8 using GRBs, and find that the EoS deviates from LCDM at 1 sigma CL, but agrees with w=-1 at the 2 sigma CL.
(*)1112.4229
What do DM properties tell us about their mass assembly histories?
Wong, Taylor
Individual DM haloes in simulations vary widely in: shape, rotation, substructure and degree of internal relaxation. PCA analyses suggest that a few PC explain a large fraction of the scatter in halo properties. The main principal component is closely linked with concentration, which in turn is known to be related to the mass accretion history of the halo. Examine here more generally the connection between mass accretion history and structural parameters. The space of mass accretion histories has principal components of its own. Find: the strongest two components can be interpreted as the overall age of the halo and the acceleration or deceleration of growth at late times. These 2 components account for ~70% of the scatter in mass accretion histories, however, due to the stochastic effect of major mergers. Relating structural parameters to formation history, find that concentration correlates strongly with the early history of the halo, while relaxation correlates with the late history. Examine the inferences about formation history that can be drawn by splitting haloes into subsamples, based on observable properties such as concentration and shape at some final time. This approach suggests interesting possibilities, such as the possibility of defining young and old samples of galaxy clusters in a rigorous, quantitative way, or testing the dynamical assumptions of galaxy formation models empirically.
1112.4329
Solar flares as harbinger of new physics
Zioutas, et al
Provide additional evidence on the involvement of exotic particles (axions and/or other WISPs) from measurements of the quietest Sun and flaring Sun. Solar chameleons remain a viable candidate, since they may preferentially convert to photons in outer space.
(*)1112.4347
A significant problem with using the Amati relation for cosmological purposes
Collazzi, Schaefer, Goldstein, Preece
Consider the distribution of many samples of GRBs when plotted in a diagram with their bolometric fluence (Sbolo) vs the observed photon energy of peak spectral flux (Epeak,obs). In this diagram, bursts that obey the Amati relation must lie above some limiting line, although observational scatter is expected to be substantial. Confirm that early bursts with spectroscopic redshifts are consistent with this limit. But find that the bursts from BATSE, Swift, Suzaku, and Konus are all greatly in violation of the limit. Find that every satellite has a greatly different distribution, requires that selection effects are dominating these distributions. For detector selections, the trigger threshold and the threshold to measure Epeak,obs combine to make a diagonal cutoff, with the position of this cutoff varying greatly detector to detector. For selection effects due to the intrinsic properties of the burst population, the distribution of Epeak,obs makes for bursts with low and high values to be rare, while the fluence distribution makes bright bursts uncommon. For a poor threshold, the combination of selection effects serves to allow only bursts within a region along the limit to be measured; these bursts then appear to follow an Amati relation. Therefore, the Amati relation is an artifact of selection effects within the burst population and the detector. Therefore, the Amati relation should not be used for cosmological tasks. This failure is in no way prejudicial against the other luminosity relations.
1112.4386
Comment on "The Real Problem with MOND" by Scott Dodelson, arXiv:1112.1320
Moffat, Toth
Point out that BAO, while predicted by theories that do not incorporate collisionless CDM, are strongly suppressed by the statistical window function that is used to process finite-sized galaxy samples. Assert that with present-day data sets, the slope of the matter power spectrum is a much stronger indicator of a theory's validity. Also agrue that MOND should to be used as a strawman theory as it is not in general representative of modified gravity theories; some theories, notably SVT MOG, offer much more successful predictions of cosmological observations.
(*)1112.4444
The 400d galaxy cluster survey WL programme: II: Weak lensing study of seven clusters with MMT/Megacam
Israel, Erben, Reiprich, Vikhlinin, Sarazin, Schneider
Evolution in the mass function of galaxy clusters sensitively traces both the expansion history of the Universe and cosmological structure formation. Robust cluster mass determinations are a key ingredient for a reliable measurement of this evolution, especially at high z. WL is a promising tool for, on average, unbiased mass estimates. The WL project aims at measuring reliable WL masses for a complete X-ray selected sample of 36 high z (0.35<z<0.9) clusters. The goal of this paper is to demonstrate the robustness of the methodology against commonly encountered problems, including pure instrumental effects, the presence of bright (8-9 mag) stars close to the cluster center, ground based measurements of high-z (z~0.8) clusters, and the presence of massive unrelated structures along the LoS. Select a subsample of 7 clusters observed with MMT/Megacam. Instrumental effects are checked in detail by cross-comparision with an archival CFHT/MegaCam observation. We derive mass estimates for 7 clusters by modelling the tangential shear with an NFW profile, in two cases with multiple components to account for projected structures in the LoS. Firmly detect lensing signals from all 7 clusters at more that 3.5 sigma and determine their masses, ranging from 1e14 Msun to 1e15 Msun, despite the presence of nearby bright stars. Retrieve the lensing signal of more than one cluster in the CL 1701+6414 field, while apparently observing this cluster through a massive foreground filament. Also find a multi-peaked shear signal in CL 1641+4001. Shear structures measured in the MMT and CFHT images of CL 1701+6414 are highly correlated.
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