Tuesday.
1406.3622
The cold spot in the cosmic microwave background: the shadow of a supervoid
Szapudi, ... Silk, ... et al
Standard inflationary hot big bang cosmology predicts small fluctuations in the CMB with isotropic Gaussian statistics. All measurements support the standard theory, except for a few anomalies discovered in WMAP maps and confirmed recently by Planck. The Cold Spot is one of the most significant of such anomalies, and the leading explanation of it posits a large void that imprints this extremely cold area via the linear ISW effect due to the decay of gravitational potentials over cosmic time, or via the RS effect due to late-time NL evolution. Despite several observational campaigns targeting the Cold Spot region, to date no suitable large void was found at higher z>0.3. Report the detection of an R=192pm15 Mpc/h size super void of depth delta=-0.13pm0.03, and centered at z=0.22. This super void, possibly the largest ever found, is large enough to significantly affect the CMB via the NL RS effect, as shown in the Lemaitre-Tolman-Bondi framework. This discovery presents the first plausible explanation for any of the physical CMB anomalies, and raises the possibility that local LSS could be responsible for other anomalies as well.
1406.3709
Evolution of the gas mass fraction in galaxy clusters
Dvorkin, Rephaeli
The mass fraction of hot gas in clusters is a basic quantity whose level and dependence on the cluster mass and redshift are intimately linked to all cluster X-ray and SZ measures. Modeling the evolution of the gas fraction is clearly a necessary ingredient in the description of the hierarchical growth of clusters through mergers of sub clumps and mass accretion on the one hand, and the dispersal of gas from the cluster galaxies by tidal interaction, galactic winds, and ram pressure stripping on the other hand. A reasonably complete description of this evolution can only be given by very detailed Hydro sims, which are, however, resource-intensive, and difficult to implement in the mapping of parameter space. A much more practical approach is the use of SAM that can be easily implemented to explore a wide range of parameters. Present first results from a simple model that describes the build up of the gas mass fraction in clusters by following the overall impact of the above processes during the merger and accretion history of each cluster in the ensemble. Acceptable ranges for model parameters are deduced through comparison with results of X-ray observations. Basic implications of the work for modeling cluster statistical properties, and the use of these properties in joint cosmological data analyses, are discussed.
1406.3710
Extremely flat haloes and the shape of the galaxy
Evans, Bowden
Present a set of highly flattened galaxy models with asymptotically constant rotation curves. The mass density in the equatorial plane falls like (distance)^-1 at large radii. Although the inner equidensity contours may be spherical, oblate or prolate, the outer parts are always severely flattened. The elongated shape is supported by rotation or tangential velocity anisotropy. The models are thickened Mestel discs, and form a previously undiscovered parti f the Miyamoto & Nagai sequence of flattened galaxies. The properties of the models -- axis ratios, velocity dispersions, streaming velocities and distribution functions -- are all discussed in some detail. Pose the question: are extremely flattened or disk-like haloes possible for the MW galaxy? This has never been examined before, as very flattened halo models were not available. Fit the rotation curve and the vertical kinematics of disc stars in the solar neighborhood to constrain the overall shape of the Galaxy. Denoting the ratio of polar axis to major axis by q, show that models with z<0.57 cannot simultaneously reproduce the in-plane and out-of-plane constraints. The kinematics of the Sagittarius galaxy also strongly disfavor models with high flattening, as the orbital plane precession is too great and the height reached above the Galactic plane is too small. At least for out Galaxy, the dark halo cannot be flatter than E4 (q~0.57) at the Solar circle. Models in which the DM is accounted for by a massive baryonic disc or by decaying neutrinos are therefore ruled out by constraints form the rotation curve and the vertical kinematics.
1406.3730
A very simple cusped halo model
Evans, Willams
A close relative of Hernuqist's model, being generated by the same transformation but this time applied to the logarithmic potential rather than the point mass. The density is proportional to (distance)^-1 at small radii, whilst the rotation curve is flat at large radii. Isotropic and radially anisotropic distributions functions are readily found, and the intrinsic and line of sight kinematical quantities are available as simple formulae. Also provide an analytical approximation to the Hamiltonian as a function of the actions. As an application, study the kinematic properties of stellar haloes and tracers in elliptical galaxies. Show that the radial velocity dispersion of a power-law population in a galaxy with a flat rotation curve always tends to the constant values. This holds true irrespective of the anisotropy of the length scales of the dark or luminous matter. An analogous result holds for the line of sight or projected velocity dispersion of a power-law surface brightness profile. The radial velocity dispersion of Pop II stars in the MW is a strongly declining function of Galactocentric radius. So, if the rotation curve is flat, conclude that the stellar halo density cannot follow a power-law at large radii, but must decrease more sharply (like an Einasto profile) or be abruptly truncated at large radii. Both the star count and kinematic data of the MW stellar halo are all-represented by an Einasto profile with index m~2 and effective radius ~20 kpc.
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