MEASURING BARYON ACOUSTIC OSCILLATIONS ALONG THE LINE OF SIGHT WITH PHOTOMETRIC REDSHIFTS: THE PAU SURVEY

Abstract

Baryon Acoustic Oscillations (BAO) provide a standard ruler of known physical\nlength, making it a promising probe of the nature of dark energy. The detection\nof BAO requires measuring galaxy positions and redshifts. "Transversal"\n(angular distance) BAO measure the angular size of this scale, while\n"line-of-sight" (or "radial") BAO require precise redshifts, but provide a\ndirect measurement of the Hubble parameter at different redshifts, a more\nsensitive probe of dark energy. The main goal of this paper is to show that a\nprecision of sigma_z ~0.003(1 + z) is sufficient to measure BAO in the radial\ndirection. This precision can be achieved for bright, red galaxies, by using a\nfilter system comprising about 40 filters, each with a width of ~100 A, from ~\n4000 A to ~ 8000 A, supplemented by two broad-band filters. We describe a\npractical implementation, a new galaxy survey, PAU, to be carried out with a\ntelescope/camera combination with an etendue of about 20 m^2deg^2, and covering\n8000 sq. deg. in the sky in four years. We expect to measure positions and\nredshifts for over 14 million red, early-type galaxies with L > L* and i_AB <\n22.5 in the interval 0.1 < z < 0.9, with sigma_z < 0.003(1 + z). This\npopulation has a number density n > 10^-3 Mpc^-3 h^3 within the 9 (Gpc/h)^3\nvolume of the survey, ensuring that the error in the determination of the BAO\nscale is not limited by shot-noise. By itself, such a survey will deliver\nprecisions of order 5% in the dark-energy equation of state parameter w, if\nassumed constant, and can determine its time derivative when combined with\nfuture CMB measurements. In addition, PAU will yield high-quality redshift and\nlow-resolution spectroscopy for hundreds of millions of other galaxies.\n