Day 970

Friday.


Icarus, 2015 08 037
Enceladus's measured physical liberation requires a global subsurface ocean
Thomas, et al

Several planetary satellites apparently have subsurface seas that are of great interest for, among other reasons, their possible habitability.  The geologically diverse Saturnian satellite Enceladus vigorously vents liquid water and vapor from fractures within a south polar depression and thus must have a liquid reservoir or active melting.  However, the extent and location of any subsurface liquid region is not directly observable.  Use measurements of control points across the surface of Enceladus accumulated over seven years of spacecraft observations to determine the satellite's precise rotation state, finding a forced physical libration of 0.120±0.014 deg (2sigma).  This value is too large to be consistent with Enceladus's core being rigidly connected to its surface, and thus implies the presence of a global ocean rather than a localized polar sea.  The maintenance of a global ocean within Enceladus is problematic according to many thermal models and so may constrain satellite properties or require a surprisingly dissipative Saturn.


Nature 525, 351-353
Relativistic boost as the cause of periodicity in a massive black-hole binary candidate
D'Orazio, Haiman, Schiminovich

Because most large galaxies contain a central BH, and galaxies often merge, BH binaries are expected to be common in galactic nuclei.  Although they cannot be imaged, periodicities in the light curves of quasars have been interpreted as evidence for binaries, most recently in PG 1302-102, which has a short rest-frame optical period of four years.  If the orbital period of the BH binary matches this value, then for the range of estimated BH masses, the components would be separated by 0.007-0.017 parsecs, implying relativistic orbital speeds.  There has been much debate over whether BH orbits could be smaller than one parsec.  Report that the amplitude and the sinusoid-like shape of the variability of the light curve of PG 1302-102 can be fitted by relativistic Doppler boosting of emission from a compact, steadily accreting, unequal-mass binary.  Predict that brightness variations in the UV light curve track those in the optical, but with a two to three times larger amplitude.  This prediction is relatively insensitive to the details of the emission process, and is consistent with archival UV data.  Follow-up UV and optical observations in the next few years can further test this prediction and confirm the existence of a binary BH in the relativistic regime.


1509.05034
Testing deviations from $/Lambda$CDM with growth rate measurements form 6 Large Scale Surveys at $/mathbf{z=0.06}$ to 1
Alam, Ho, Silvestri

Use measurements from Planck and galaxy redshift surveys over the last decade to test 3 of the basic assumptions of the standard model of cosmology, LCDM: the spatial curvature of the universe, the nature of DE and the laws of gravity on large scales.  Obtain improved constraints on several scenarios that violate one or more of these assumptions.  Measure w0=-0.94±0.17 (18% measurement) and 1+wa = 1.16±0.36 (31% measurement) for models with a time-dependent EoS, which is an improvement over current best constraints.  In the context of modified gravity, consider popular scalar tensor models as well as a parameterization of the growth factor.  In the case of one-parameter f(R) gravity models with a LCDM background, constrain B_0<1.36e-5 (1 sigma CL), which is an improvement by a factor of 4 on the current best.  Provide the very first constrain on the coupling parameters of general scalar-tensor theory and stringent constraint on the only free coupling parameter of Chameleon models.  Also derive constraints on extended Chameleon models, improving the constraint on the coupling by a factor of 6 on the current best.  Also measure gamma=0.612±0.072 (11.7% measurement) for growth index parameterization which is an improvement over the current best measurement of gamma=0.699±0.110 (16%).  Improve all the current constraints by combining results from various galaxy redshift surveys in a coherent way, which include a careful treatment of scale-dependence introduced by modified gravity.


1509.05058
On weak lensing shape noise
Niemi, Kitching, Cropper

One of the most powerful techniques to study the dark sector of the universe is weak gravitation lensing.  In practice, to infer the reduced shear, WL measures galaxy shapes, which are the consequence of both the intrinsic ellipticity of the sources and of the integrated gravitational lensing effect along the LoS.  Hence, a very large number of galaxies is required in order to average over their individual properties and to isolate the WL cosmic shear signal.  If this 'shape noise' can be reduced, significant advances in the power of a WL surveys can be expected.  This paper describes a general method for extracting the probability distributions of parameters from catalogues of data using Voronoi cells, which has several applications, and has synergies with Bayesian hierarchical modeling approaches.  This allows us to construct a probability distribution for the variance of the intrinsic ellipticity as a function of galaxy property using only photometric data, allowing a reduction of shape noise.  As a proof of concept the method is applied to the CFHTLenS survey data.  Use this approach to investigate trends of galaxy properties in the data and apply this to the case of WL power spectra.