Oxford astro-ph Coffee

There are a number of `screening' mechanisms that are designed to suppress the effects of modified gravity below cluster scales. What are the characteristic radii at which this switch-off occurs in the different mechanisms?

Point sources from the WMAP 7 catalogue are stacked, and the results are found to be consistent with the WMAP beam models. The correctness of the beam models used had been questioned by Shanks and others, who found that inaccurate models could introduce a significant bias into the measured CMB power spectrum. The authors here also find evidence for spectral steepening above 61GHz. This changes the estimates for the spectrum of unresolved point sources. Accounting for this effect, the primordial power spectrum seems to be closer to scale invariant than first thought.

There's been quite a bit of discussion about this issue this summer, it came up at the Bologna dark matter meeting as well as the Aosta strong lensing workshop. Basically, in strong lens systems we infer *more* subhalos/satellites than CDM predicts for massive lens galaxies (the satellites cause "millilensing", where the quasar/radio source image fluxes are affected by the small amounts of additional magnification and demagnification). One suggested resolution to this problem is to include all the subhalos along the line of sight - Metcalf (2008) claimed this was the answer, and now Dan Dan Xu has tested this claim using the Millenium Simulation, and various assumptions for the satellite subhalo density profile.

Epic paper by Sluse et al, on high precision astrometry in lensed quasar systems. Attention optical astronomers! They deconvolve their images! And get very small error bars as a result. Interesting claim about being able to quantify the lens environment (the dreaded "external covergence"). This is the biggest systematic error in H0/w determination - great if we can reduce it further through improved lens modelling.

Our peculiar velocity with respect to the CMB rest frame can be measured through off-diagonal correlations in harmonic expansion coefficients due to aberration. The author shows that a new technique called "pre-deboosting" will work for detecting such correlations up to l~10000. If the measured residual peculiar velocity is different from the expected value, it would be a hint of some anomalies on superhorizon scales.

Combining galaxy distribution / clustering measurements with weak lensing shear measurements is a hot topic at the moment and this is the latest attempt to use both to reconstruct the COSMOS density field.

Purcell and Zentner on why the "Too Big To Fail" problem might be a storm in a teacup: "We propose that the large variation in subhalo populations among different host halos can explain the dearth of large, dense subhalos orbiting the Milky Way without any making any adjustments to the host halo mass or accounting for baryonic feedback processes."

You can fix the acoustic scale using low-z luminosity distance measurements instead of the high-z CMB measurement. This would be a useful consistency test for LCDM, and can be used to constrain N_eff.

The chameleon mechanism suppresses the effects of a modified gravity theory in high-density environments. The authors of this paper suggest that this could lead to effects on stellar structure that would impact distance measures inferred from cepheid variables and red giant branch stars. Whilst it's not clear that the astrophysics involved is sufficiently well understood for such tests to be useful, I think there is some good thinking-outside-the-box here!

The authors numerically showed that angular momentum of isolated galactic halos with mass less than 10^11 solar mass is systematically larger in the f(R) gravity model since they are less affected by shielding via chameleon mechanism.

They claim a ~100x speed increase when using GPUs over a single processor. Execution time on the GPUs is comparable with a 128-processor MPI implementation.

Deason et al have an improved measurement of density and velocities of blue horizontal branch stars in the MW halo: here they fit dynamical models an infer a halo of about 1e12 Msun (as before) and quite a high concentration (cvir~20). Since concentration reflects age, this is consistent with the picture in which the MW has been undisturbed for a long time.

Authors note a new phenomenon: linear portions of quasar lightcurves have gradients that appear vary systematically with quasar redshift (higher z quasars have steeper gradients). Interesting if real: it could sharpen up QSO redshift estimation in cadenced imaging surveys beyond what you can do with the QSO colours.

Interesting Nature letter suggested by Pat Roche

Daniel Mortlock talked me through this when I visited ICL recently, its a nice result, especially the deep GP trough. The survey paper (that preceded this one) was interesting too, for its treatment of detection probablity and completeness, worth a look for anyone interesting in finding (and not finding) things.

Measurement of the homogeneity scale. See Fig. 6. Fractal dimension D_2=3 for homogeneity.

Astrophysics > Cosmology and Extragalactic Astrophysics Title: Dipoles in the Sky (Submitted on 29 May 2012) Abstract: We perform observational tests of statistical isotropy using data from large-scale structure surveys spanning a wide range of wavelengths.

Hints of something interesting or data artefact? Also: http://arxiv.org/pdf/1207.4781v1.pdf

Another paper looking at constraining allowed models for the primordial power spectrum (in this case the potential).