Munch: Monday, March 5, 2007

                               


 

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

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


FAQ

What is Munch?
       Munch Archive

The Bullet Cluster 1E0657-558 evidence shows Modified Gravity in the absence of Dark Matter  astro-ph/0702146  (suggested by Miguel)

Authors: J. R. Brownstein, J. W. Moffat
Comments: 25 pages, 14 figures, 5 tables
A detailed analysis of the November 15, 2006 data release (Clowe et al., 2006) X-ray surface density Sigma-map and the strong and weak gravitational lensing convergence kappa-map for the Bullet Cluster 1E0657-558 is performed and the results are compared with the predictions of a modified gravity (MOG) and dark matter. Our surface density Sigma-model is computed using a King beta-model density, and a mass profile of the main cluster and an isothermal temperature profile are determined by the MOG. We find that the main cluster thermal profile is nearly isothermal. The MOG prediction of the isothermal temperature of the main cluster is T = 15.5 +- 3.9 keV, in good agreement with the experimental value T = 14.8{+2.0}{-1.7} keV. Excellent fits to the two-dimensional convergence kappa-map data are obtained without non-baryonic dark matter, accounting for the 8-sigma spatial offset between the Sigma-map and the kappa-map reported in Clowe et al. (2006). The MOG prediction for the kappa-map results in two baryonic components distributed across the Bullet Cluster 1E0657-558 with averaged mass-fraction of 83% intracluster medium (ICM) gas and 17% galaxies. Conversely, the Newtonian dark matter kappa-model has on average 76% dark matter (neglecting the indeterminant contribution due to the galaxies) and 24% ICM gas for a baryon to dark matter mass-fraction of 0.32, a statistically significant result when compared to the predicted Lambda-CDM cosmological baryon mass-fraction of 0.176{+0.019}{-0.012} (Spergel et al., 2006).

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The Sunyaev-Zeldovich Background  astro-ph/0702727

Authors: Gilbert Holder (McGill), Ian G. McCarthy (Durham), Arif Babul (Victoria)
Comments: 10 pages, 9 figures, submitted to MNRAS
The cosmic background due to the Sunyaev-Zeldovich (SZ) effect is expected to be the largest signal at mm and cm wavelengths at a resolution of a few arcminutes. We investigate some simple statistics of SZ maps and their scaling with the normalization of the matter power spectrum, sigma_8, as well as the effects of the unknown physics of the intracluster medium on these statistics. We show that the SZ background provides a significant background for SZ cluster searches, with the onset of confusion occurring around 10^{14} h^{-1} solar masses in a cosmology-dependent way, where confusion is defined as typical errors in recovered flux larger than 20%. The confusion limit, corresponds to the mass at which there are roughly ten clusters per square degree, with this number nearly independent of cosmology and cluster gas physics. Typical errors grow quickly as lower mass objects are included in the catalog.
We also point out that there is nothing in particular about the rms of the filtered map that makes it especially well-suited for capturing aspects of the SZ effect, and other indicators of the one-point SZ probability distribution function are at least as well suited for the task. For example, the full width at half maximum of the one point probability distribution has a field-to-field scatter that is about 60% that of the rms.
The simplest statistics of SZ maps are largely unaffected by cluster physics such aspreheating, although the impact of preheating is clear by eye in the maps.Studies aimed at learning about the physics of the intracluster medium will apparently require more specialized statistical indicators.

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Power Spectrum Shape from Peculiar Velocity Data   astro-ph/0702751

Authors: Richard Watkins (Willamette University), Hume A. Feldman (University of Kansas)
Comments: 17 pages, 2 figures, 3 tables
We put constraints on the velocity power spectrum shape parameter $\Gamma$ in linear theory using the nine bulk--flow and shear moments estimated from five recent peculiar velocity surveys. For each survey, a likelihood function for $\Gamma$ was found after marginalizing over the power spectrum amplitude $\Omega^{0.6}\sigma_8$ using constraints obtained from comparisons between redshift surveys and peculiar velocity data. In order to maximize the accuracy of our analyses, the velocity noise $\sigma_*$ was estimated directly for each survey. A statistical analysis of the differences between the values of the moments estimated from different surveys showed consistency with theoretical predictions, suggesting that all the surveys investigated reflect the same large scale flows. The peculiar velocity surveys were combined into a composite survey yielding the constraint $\Gamma=0.13^{+0.09}_{-0.05}$. This value is lower than, but consistent with, values obtained using redshift surveys and CMB data.

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Looking for signs of Anisotropic Cosmological Expansion in the High-z Supernova data   astro-ph/0702730

Authors: Brett Bochner
Comments: 6 pages, 8 figures. (Adapted from Poster presented at the 22nd Texas Symposium. Includes minor fixes from the originally published Proceedings paper.) This work is an early analysis of Supernova data available during 2003-2004. To be followed by a fuller analysis of the more complete Supernova data sets becoming available in subsequent years
Journal-ref: In Proceedings of the 22nd Texas Symposium on Relativistic Astrophysics at Stanford University, Dec. 13-17, 2004; paper #1301, pp. 1-6
Several problematical epochs in cosmology, including the recent period of structure formation (and acceleration), require us to understand cosmic evolution during times when the basis of FRW expansion, the cosmological principle, does not completely hold true. We consider that the breakdown of isotropy and homogeneity at such times may be an important key towards understanding cosmic evolution. To study this, we examine fluctuations in the high-z supernova data to search for signs of large-scale anisotropy in the Hubble expansion. Using a cosmological-model-independent statistical analysis, we find mild evidence of real anisotropy in various circumstances. We consider the significance of these results, and the importance of further searches for violations of the cosmological principle.

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Observing cosmic string loops with gravitational lensing surveys  astro-ph/0702648

Authors: Katherine J. Mack, Daniel H. Wesley, Lindsay J. King
Comments: 18 pages, 3 figures
We show that the existence of cosmic strings can be strongly constrained by the next generation of gravitational lensing surveys at radio frequencies. We focus on cosmic string loops, which we expect to be far more numerous than long (horizon-sized) strings, as suggested by simulations. Using simple models of the loop population and minimal assumptions about the lensing cross-section per loop, we estimate the optical depth to lensing and show that extant radio surveys such as CLASS have already ruled out a portion of the cosmic string model parameter space. Future radio interferometers, such as LOFAR and especially SKA, may constrain $G\mu/c^2 < 10^{-9}$ in some regions of parameter space, outperforming current constraints from pulsar timing and the CMB by up to two orders of magnitude. A fundamental advantage of this approach is that it is based on direct detections of cosmic strings; whereas lensing requires only that the loop have a mass, other constraints must rely on estimates of theoretical uncertainties such as the gravitational wave emission of loops or the population dynamics of string networks. This difference is essential since the properties of cosmic strings are essentially unknown.

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The sky distribution of 511 keV positron annihilation line emission as measured with INTEGRAL/SPI  astro-ph/0702621  (suggested by Jeter)

Authors: G. Weidenspointner, J. Knoedlseder, P. Jean, G.K. Skinner, J.-P. Roques, G. Vedrenne, P. Milne, B.J. Teegarden, R. Diehl, A. Strong, S. Schanne, B. Cordier, C. Winkler
Comments: to be published in the proceedings of the 6th INTEGRAL Workshop "The Obscured Universe" (3-7 July 2006, Moscow)
The imaging spectrometer SPI on board ESA's INTEGRAL observatory provides us with an unprecedented view of positron annihilation in our Galaxy. The first sky maps in the 511 keV annihilation line and in the positronium continuum from SPI showed a puzzling concentration of annihilation radiation in the Galactic bulge region. By now, more than twice as many INTEGRAL observations are available, offering new clues to the origin of Galactic positrons. We present the current status of our analyses of this augmented data set. We now detect significant emission from outside the Galactic bulge region. The 511 keV line is clearly detected from the Galactic disk; in addition, there is a tantalizing hint at possible halo-like emission. The available data do not yet permit to discern whether the emission around the bulge region originates from a halo-like component or from a disk component that is very extended in latitude.

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The large scale CMB cut-off and the tensor-to-scalar ratio  astro-ph/0701783

Authors: Gavin Nicholson, Carlo R. Contaldi
Comments: 4 pages, 3 figures
A kinetic dominated stage preceding the last N~60 e-foldings of inflation leads to a cut-off in both scalar and tensor primordial spectra on the largest observable scales. We discuss the overall probability of inflationary solutions with a limited number of e-foldings and point out an interesting feature. The tensor-to-scalar ratio in these models grows at large scales. This potentially observable signature could shed some light on the true origin of the low Cosmic Microwave Background (CMB) quadrupole power.

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The probability distribution function of the SZ power spectrum: an analytical approach  astro-ph/0701879

Authors: Pengjie Zhang (SHAO), Ravi K. Sheth (UPenn)
Comments: 11 pages, 7 figures. Submitted to MNRAS
The Sunyaev Zel'dovich (SZ) signal is highly non-Gaussian, so the SZ power spectrum (along with the mean $y$ parameter) does not provide a complete description of the SZ effect. Therefore, SZ-based constraints on cosmological parameters and on cluster gastrophysics which assume Gaussianity will be biased.
We derive an analytic expression for the $n$-point joint PDF of the SZ power spectrum. Our derivation, which is based on the halo model, has several advantages: it is expressed in an integral form which allows quick computation; it is applicable to any given survey and any given angular scale; it is straightforward to incorporate many of the complexities which arise when modeling the SZ signal. To illustrate, we use our expression to estimate $p(C_\ell)$, the one-point PDF of the SZ power spectrum. For small sky coverage (applicable to BIMA/CBI and the Sunyaev Zel'dovich Array experiments), our analysis shows that $p(C_\ell)$ on the several arc-minute scale is expected to be strongly skewed, peaking at a value well below the mean and with a long tail which extends to tail high $C_\ell$ values. In the limit of large sky coverage (applicable to the South Pole Telescope and Planck), $p(C_\ell)$ approaches a Gaussian form. However, even in this limit, the variance of the power spectrum is very different from the naive Gaussian-based estimate. This is because different $\ell$ models are strongly correlated, making the cosmic variance of the SZ band-power much larger than the naive estimate. Our analysis should also be useful for modeling the PDF of the power spectrum induced by gravitational lensing at large $\ell$.

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Probing Dark Matter Substructure with Pulsar Timing  astro-ph/0702546  (suggested by Josh)

Authors: E. R. Siegel, M. P. Hertzberg, J. N. Fry
Comments: 8 pages, 4 figures, submitted to MNRAS
Report-no: MIT-CTP/3814
We demonstrate that pulsar timing measurements may be able to detect the presence of dark matter substructure within our own galaxy. As dark matter substructure transits near the line-of-sight between a pulsar and an observer, the change in the gravitational field will result in a delay of the light-travel-time of photons. We calculate the effect of this delay due to transiting dark matter substructure and find that the effect on pulsar timing ought to be observable for a wide range of substructure masses and density profiles. We find that transiting dark matter substructure with masses above 0.01 solar masses ought to be detectable at present by these means. With small improvements, this method may be able to distinguish between baryonic, thermal non-baryonic, and non-thermal non-baryonic types of dark matter. Additionally, information about structure formation on small scales and the density profiles of galactic dark matter substructure can be extracted via this method.

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The detectability of baryonic acoustic oscillations in future galaxy surveys          astro-ph/0702543  (suggested by Josh)

Authors: R. Angulo (1), C. M. Baugh (1), C. S. Frenk (1), C. G. Lacey (1) ((1) ICC, Durham)
Comments: 23 pages, 18 figures, submitted to MNRAS
We use N-body simulations of the hierarchical clustering of dark matter and semi-analytical modelling of galaxy formation to assess the detectability of baryonic acoustic oscillations in the power spectrum of galaxies. Our primary simulation has a volume of $2.4 h^{-3} {\rm Gpc}^{3}$, comparable to forthcoming redshift surveys at $z \sim 1$, with sufficient mass resolution to see the galaxies expected in these surveys. We present a step-by-step illustration of the effects which change the form of the galaxy power spectrum on large scales from the simple predictions of linear theory. Nonlinear effects are evident on scales in excess of $100 h^{-1}$Mpc. Nonlinearities, galaxy bias and redshift-space distortions erase some of the acoustic oscillations. We present an improved, robust method to find the equation of state of the dark energy parameter $w$. Our galaxy formation model allows us to construct synthetic galaxy samples with the selection criteria proposed for future surveys. We find a weak systematic difference between the equation of state parameter recovered using galaxies and dark matter. Sampling variance is the dominant source of error despite the huge volume simulated. We use our simulation results to estimate the accuracy with which $w$ will be measured in the future. Pan-STARRS could potentially yield a measurement with an accuracy of $\Delta w = 4-7%$, which is competitive with the proposed WFMOS survey ($\Delta w = 5%$). This represents a factor of two improvement over current constraints on $w$. To achieve $\Delta w \sim 1%$ using acoustic oscillations would require a survey with at least 16 times the effective volume of Pan-STARRS. Thus, it is unlikely that this level of accuracy will be reached by the next generation of galaxy surveys.

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