Beecroft Institute for Particle Astrophysics and Cosmology

Abstract:

Providing a theoretical basis for the baryonic Tully-Fisher Relation (BTFR; baryonic mass vs rotational velocity in spiral galaxies) in the LCDM paradigm has proved problematic. Simple calculations suggest too low a slope and too high a scatter, and recent semi-analytic models and numerical galaxy simulations typically fail to reproduce some aspects of the relation. Furthermore, the assumptions underlying one model are often inconsistent with those behind another. This paper aims to develop a rigorous prediction for the BTFR in the context of LCDM, using only a priori expected effects and relations, a minimum of theoretical assumptions, and no free parameters. The robustness of the relation to changes in key galactic parameters will be explored. I adopt a modular approach, taking each of the stand alone galaxy relations necessary for constructing the BTFR from up-to-date numerical simulations of dark halos. These relations -- and their expected scatter -- are used to describe model spirals with a range of masses, resulting in a band in the space of the BTFR that represents the current best guess for the LCDM prediction. Consistent treatment of expected LCDM effects goes a large way towards reconciling the naive slope-3 LCDM prediction with the data, especially in the range 10^9 M_sun < M_bar < 10^11 M_sun. The theoretical BTFR becomes significantly curved at M_bar > 10^11 M_sun, but this is difficult to test observationally due to the scarcity of extremely high mass spirals. Low mass gas-rich galaxies have systematically lower rotational velocity than the LCDM prediction, although the relation used to describe baryon mass fractions must be extrapolated in this regime. The fact that the BTFR slope derived here is significantly greater than in early predictions is a direct consequence of a corresponding increase in the expected sensitivity of baryon mass fraction to total halo mass.

Abstract:

The Universe is inhomogeneous, and yet it seems to be incredibly well-characterised by a homogeneous relativistic model. One of the current challenges is to accurately characterise the properties of such a model. In this paper we explore how inhomogeneities may affect the overall optical properties of the Universe by quantifying how they can bias the redshift-distance relation in a number of toy models that mimic the real Universe. The models that we explore are statistically homogeneous on large scales. We find that the effect of inhomogeneities is of order of a few percent, which can be quite important in precise estimation of cosmological parameters. We discuss what lessons can be learned to help us tackle a more realistic inhomogeneous universe.

Abstract:

Here we present the results of various approaches to measure accurate colours and photometric redshifts (photo-z) from wide-field imaging data. We use data from the Canada-France-Hawaii Telescope Legacy Survey which have been re-processed by the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS) team in order to carry out a number of weak gravitational lensing studies. An emphasis is put on the correction of systematic effects in the photo-z arising from the different point spread functions (PSFs) in the five optical bands. Different ways of correcting these effects are discussed and the resulting photo-z accuracies are quantified by comparing the photo-z to large spectroscopic redshift (spec-z) data sets. Careful homogenization of the PSF between bands leads to increased overall accuracy of photo-z. The gain is particularly pronounced at fainter magnitudes where galaxies are smaller and flux measurements are affected more by PSF effects. We discuss ways of defining more secure subsamples of galaxies as well as a shape- and colour-based star-galaxy separation method, and we present redshift distributions for different magnitude limits. We also study possible re-calibrations of the photometric zero-points (ZPs) with the help of galaxies with known spec-z. We find that if PSF effects are properly taken into account, a re-calibration of the ZPs becomes much less important suggesting that previous such re-calibrations described in the literature could in fact be mostly corrections for PSF effects rather than corrections for real inaccuracies in the ZPs. The implications of this finding for future surveys like the Kilo Degree Survey (KiDS), Dark Energy Survey (DES), Large Synoptic Survey Telescope or Euclid are mixed. On the one hand, ZP re-calibrations with spec-z values might not be as accurate as previously thought. On the other hand, careful PSF homogenization might provide a way out and yield accurate, homogeneous photometry without the need for full spectroscopic coverage. This is the first paper in a series describing the technical aspects of CFHTLenS. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.