A direct probe of cosmological power spectra of the peculiar velocity field and the gravitational lensing magnification from photometric redshift surveys

Abstract

The cosmological peculiar velocity field (deviations from the pure Hubble flow) of matter carries significant information on dark energy,dark matter and the underlying theory of gravity on large scales. Peculiar motions of galaxies introduce systematic deviations betweenthe observed galaxy redshifts z and the corresponding cosmological redshifts zcos. A novel method for estimating the angularpower spectrum of the peculiar velocity field based on observations of galaxy redshifts and apparent magnitudes m (or equivalently fluxes)is presented. This method exploits the fact that a mean relation between zcos and m of galaxies can be derived from all galaxiesin a redshift-magnitude survey. Given a galaxy magnitude, it is shown that the zcos(m) relation yields its cosmological redshiftwith a 1σ error of σz ∼ 0.3 for a survey like Euclid ( ∼ 109 galaxies at z≲2), and can be used to constrainthe angular power spectrum of z−zcos(m) with a high signal-to-noise ratio. At large angular separations corresponding to l≲15,we obtain significant constraints on the power spectrum of the peculiar velocity field. At 15≲l≲60, magnitude shifts in thezcos(m) relation caused by gravitational lensing magnification dominate, allowing us to probe the line-of-sight integral of thegravitational potential. Effects relatedto the environmental dependence in the luminosity function can easily be computed and their contamination removed from the estimated power spectra.The amplitude of the combined velocity and lensing power spectra at z ∼ 1 can be measured with ≲5% accuracy.