Day 835

Tuesday.

1502.04166
The extragalactic background light, the Hubble constant, and anomalies: conclusions from 20 years of TeV gamma-ray observations
Biteau, Williams

Ground-based observatories have been collecting 0.2-20 TeV gamma rays from blazers for about twenty years.  These gamma rays can experience absorption along the line of sight due to interactions with the extragalactic background light (EBL).  In this paper, investigate the most extensive set of TeV spectra from blazers collected so far, twice as large as any other studied.  First show that the gamma-ray optical depth can be reduced to the convolution product of an EBL kernel with the EBL intensity.  Extract the EBL intensity from the gamma-ray spectra, show that it is preferred at 11 sigma to a null intensity, and unveil the broad-band spectrum of the EBL from mid-UV to far IR.  The measurement shows that the total radiative content of the universe between 0.1 and 1000 microns represents 6.5pm1.2% of the brightness of the CMB.  This is slightly above the accumulated emission of stars and galaxies and constrains the unresolved sources that could have recognized the universe.  Also propose a data-driven method to estimate the Hubble constant based on the comparison of local and gamma-ray measurements of the EBL, yielding H0=88pm8(stat)pm13(sys) km/s/Mpc.  After setting the most stringent upper-limits on the redshift of four TeV blazers, investigate the 106 intrinsic gamma-ray spectra in the sample and find no significant evidence for anomalies.  Do not find evidence for the so-called "pair-production anomaly" at large optical depths, which has been used previously to place lower limits on the coupling of TeV gamma rays with axion-like particles.  Finally, investigate the impact of a modification of the pair-creation threshold due to a Lorenz invariance violation.  A mild excess prevents the ruling out of an effect at the Planck energy; constrain for the first time the energy scale of the modification to values larger than 60% of the Planck energy.

1502.04198
Effective destruction of CO by cosmic rays: implications for tracing H$_2$ gas in the Universe
Bisbas, Papadopoulos, Viti

[CO is a H2 tracer.]  Conclude that the CR-induced destruction of CO in molecular clouds, unhindered by dust absorption, is perhaps the single most important factor controlling the CO-vilibility of molecular gas in vigorously SF galaxies.  The second-order effect of this CO destruction mechanism will be to make the H2 distribution in the gas-rich disks of such galaxies appear much clumpier in CO J=1-0, 2-1 line emission than it actually is.  Give an analytical approximation of the CO/H2 abundance ratio as a function of gas density and CR energy density for use in galaxy-size or cosmo hydro sims, and propose some key observational tests.


1502.04491
Constrained correlation functions from the Millennium Simulation
Wilking, Röseler, Schneider

In previous work, developed a quasi-Gaussian approximation for the likelihood of correlation functions, which, in contrast to the usual Gaussian approach, incorporates fundamental mathematical constraints on correlation functions.  The analytical computation of these constraints is only feasible in the case of correlation functions of 1-d random fields.  In this work, aim to obtain corresponding constraints in the case of higher-dimensional random fields and test them in a more realistic context.  Develop numerical methods to compute the constraints on correlation functions which are also applicable for 2- and 3-d fields.  In order to test the accuracy of the numerically obtained constraints, compare them to the analytical results for the 1-d case.  Finally, compute correlation functions from the halo catalog of the Millennium Simulation, check whether they obey the constraints, and examine the performance of the transformation used in the construction of the quasi-Gaussian likelihood.  Find that the numerical methods of computing the constraints are robust and that the correlation functions measured from the Millennium Simulation obey them.  Despite the fact that the measured correlation functions lie well inside the allowed region of parameter space, i.e., far away from the boundaries of the allowed volume defined by the constraints, find strong indications that the quasi-Gaussian likelihood yields a substantial more accurate description than the Gaussian one.


1502.04575
CFHTLenS: a gaussian likelihood is a sufficient approximation for a cosmological analysis of third-order cosmic shear statistics
Simon, Semboloni, van Waerbeke, Hoekstra, Erben, .. et al

Study the correlations of the shear signal between triplets of sources in the CFHTLenS to probe cosmo params via the matter bispectrum.  In contrast to previous studies, adopt a non-Gaussian model of the data likelihood which is supported by simulations of the survey.  Find that for state-of-the-art surveys, similar to CFHTLenS, a Gaussian likelihood analysis is a reasonable approximation, albeit small differences in the parameter constraints are already visible.  For future surveys, expect that a Gaussian model becomes inaccurate.  Algorithm for a refined non-Gaussian analysis and data compression is then of great utility especially because it is not much more elaborate if simulated data are available.  Applying this algorithm to the third-order correlations of shear alone in a blind analysis, find a good agreement with the standard cosmo model: Sigma_8=sigma8(Omega_m/0.27)^0.64=0.79+0.08-0.11 for a flat LCDM cosmology with h=0.7pm0.04 (68% CL).  Nevertheless, models provide only moderately good fits as indicated by chi2/dof=2.9, including a 20 rms uncertainty in the predicted signal amplitude.  The models cannot explain a signal drop on scales around 15 arcmin, which may be caused by systematics.  It is unclear whether the discrepancy can be fully explained by residual PSF systematics of which evidence is found at least on scales of a few arcmin.  Therefore need a better understanding of higher-order correlations of cosmic shear and their systematics to confidently apply them as cosmo probes.


1502.04584
On the definition of superclusters
Chon, Boehringer, Zaroubi

Propose to select superclusters with an overdenesity criterion that selects only those objects that will collapse in the future, including those that are at a turn-around in the present epoch.  Present numerical values for these criteria for range os cosmo models.  Express these criteria in terms of a density ratio, or as an infall velocity and show that these two criteria give almost identical results.  Applied the criteria to some prominent structures in the local universe: the Local supercluster, Shapley supercluster, and Laniakea supercluster.  Find that for the Local and Shapley superclusters, only the central regions will collapse in the future, while Laniakea does not constitute a significant over density and will disperse in the future.  Finally, suggest that those superclusters that will survive the accelerating cosmic expansion and collapse in the future be called "superstes-clusters", where "superstes" means surviver in Latin, to distinguish them from traditional superclusters.