Day 1430

Tuesday.

1806.08395
Observations of the missing baryons in the warm-hot intergalactic medium
Nicastro, et al

It has been known for decades that the observed number of baryons in the local universe falls about 30-40% short of the total number of baryons predicted by BBN, as inferred from density fluctuations of the CMB and seen during the first 2-3 billion years of hate universe in the so called Lyman-alpha Forest.   A theoretical solution to this paradox locates the missing baryons in the hot and tenuous filamentary gas between galaxies, known as the warm-hot IGM.  However, it is difficult to detect them there because the largest by far constituent of this gas - hydrogen - is mostly ionized and therefore almost invisible in far-UV spectra with typical S/N ratios.  Despite the large observational efforts, only a few marginal claims of detection have been made so far.  Report observations of two absorbers of highly ionized oxygen (OVII) in the high S/N ratio X-ray spectrum of a quasar at z>0.4.  These absorbers show no variability over a 2yr timescale and have no associated cold absorption, making the assumption that they originate from the quasar's intrinsic outflow or the host galaxy's ISM implausible.  The OVII systems lie in regions characterized by large (x4 compared to average) galaxy over-densities and their number (Down to the sensitivity threshold of the data), agrees well with numerical simulations predictions for the long-sought warm-hot IGM (WHIM).  Conclude that the missing baryons have been found.


1806.09477
Comparing approximate methods for mock catalogues and covariance matrices I: correlation function
Lippich, et al

This paper is the first in a set that analyses the covariance matrices of clustering statistics obtained from several approximate methods for gravitational structure formation.  Focus here on the covariance matrices of anisotropic 2pt correlation function measurements.  The comparison includes 7 approximate methods, which can be divided into 3 categories: predictive methods that follow the evolution of the linear density field deterministically (ICE-COLA, Peak Patch, and Pinocchio), methods that require a calibration with N-body sims (Patchy and Halogen), and simpler recipes based on assumptions regarding the shape of the probability distribution function (PDF) of density fluctuations (log-normal and Gaussian density fields).  Analyse the impact of using covariance estimates obtained from these approximate methods on cosmological analyses of galaxy clustering measurements, using as a reference the covariances inferred from a set of full N-body sims.  Find that all approximate methods can accurately recover the mean parameter values inferred using the N-body covariances.  The obtained parameter uncertainties typically agree with the corresponding N-body results within 5% for the lower mass threshold, and 10% for the higher mass threshold.  Furthermore, find that the constraints for some methods can differ by up to 20% depending on whether the halo samples used to define the covariance matrices are defined by matching the mass, number density, or clustering amplitude of the parent N-body samples.  The results of the configuration-space analysis indicate that most approximate methods provide similar results, with no single method clearly outperforming the others.


1806.09497
Comparing approximate methods for mock catalogues and covariance matrices II: power spectrum multipoles
Blot, et al

[...] The variance of the multipoles is typically reproduced within 10%; overall, find that covariances built from ICE-COLA, Pinocchio, PeakPatch, Patchy and the Gaussian approximation yield errors on model parameters with 5% of those from the N-body based covariance, while for Halogen and lognormal this agreement degrades to ~10%.


1806.09499
Comparing approximate methods for mock catalogues and covariance matrices III: Bispectrum
Colavincenzo, et al

Employ a large set of 300 realizations of the same cosmology for each method, run with matching initial conditions in order to reduce the contribution of cosmic variance to the comparison.  In addition, compare how the error on cosmological parameters such as linear and nonlinear bias parameters depends on the approximate method used for the determination of the bispectrum variance.  As general result, most methods provide errors within 10% of the errors estimated from N-body simulations.  Exceptions are those methods requiring calibration of the clustering amplitude but restrict this to 2pt statistics.  Finally, test how the results are affected by being limited to a few hundreds measurements from N-body sims, and therefore to the bispectrum variance, by comparing with a larger set of several thousands realizations performed with one approximate method.