Beecroft Institute for Particle Astrophysics and Cosmology

Abstract:

Future cosmic microwave background (CMB) polarisation experiments aim to measure an unprecedentedly small signal - the primordial gravity wave component of the polarisation field B-mode. To achieve this, they will analyse huge datasets, involving years worth of time-ordered data (TOD) from massively multi-detector focal planes. This creates the need for fast and precise methods to complement the M-L approach in analysis pipelines. In this paper, we investigate fast map-making methods as applied to long duration, massively multi-detector, ground-based experiments, in the context of the search for B-modes. We focus on two alternative map-making approaches: destriping and TOD filtering, comparing their performance on simulated multi-detector polarisation data. We have written an optimised, parallel destriping code, the DEStriping CARTographer DESCART, that is generalised for massive focal planes, including the potential effect of cross-correlated TOD 1/f noise. We also determine the scaling of computing time for destriping as applied to a simulated full-season data-set for a realistic experiment. We find that destriping can out-perform filtering in estimating both the large-scale E and B-mode angular power spectra. In particular, filtering can produce significant spurious B-mode power via EB mixing. Whilst this can be removed, it contributes to the variance of B-mode bandpower estimates at scales near the primordial B-mode peak. For the experimental configuration we simulate, this has an effect on the possible detection significance for primordial B-modes. Destriping is a viable alternative fast method to the full M-L approach that does not cause the problems associated with filtering, and is flexible enough to fit into both M-L and Monte-Carlo pseudo-Cl pipelines.

Abstract:

The largest galaxies acquire their mass early on, when the Universe is still youthful. Cold streams violently feed these young galaxies a vast amount of fresh gas, resulting in very efficient star formation. Using a well resolved hydrodynamical simulation of galaxy formation, we demonstrate that these mammoth galaxies are already in place a couple of billion years after the Big Bang. Contrary to local starforming galaxies, where dust re-emits a large part of the stellar ultraviolet (UV) light at infrared and sub-millimetre wavelengths, our self-consistent modelling of dust extinction predicts that a substantial fraction of UV photons should escape from primordial galaxies. Such a model allows us to compute reliably the number of high redshift objects as a function of luminosity, and yields galaxies whose UV luminosities closely match those measured in the deepest observational surveys available. This agreement is remarkably good considering our admittedly still simple modelling of the interstellar medium (ISM) physics. The luminosity functions (LF) of virtual UV luminous galaxies coincide with the existing data over the whole redshift range from 4 to 7, provided cosmological parameters are set to their currently favoured values. Despite their considerable emission at short wavelengths, we anticipate that the counterparts of the brightest UV galaxies will be detected by future sub-millimetre facilities like ALMA

Abstract:

We report on two quantitative, morphological estimators of the filamentary structure of the Cosmic Web, the so-called global and local skeletons. The first, based on a global study of the matter density gradient flow, allows us to study the connectivity between a density peak and its surroundings, with direct relevance to the anisotropic accretion via cold flows on galactic halos. From the second, based on a local constraint equation involving the derivatives of the field, we can derive predictions for powerful statistics, such as the differential length and the relative saddle to extrema counts of the Cosmic web as a function of density threshold (with application to percolation of structures and connectivity), as well as a theoretical framework to study their cosmic evolution through the onset of gravity-induced non-linearities.