Cosmological baryonic and matter densities from 600,000 SDSS Luminous Red Galaxies with photometric redshifts

TL;DR

This study uses MegaZ-LRG, a SDSS DR4-based photometric-redshift catalog of $\sim 6\times10^5$ LRGs in the range $0.4<z<0.7$, to perform cosmological parameter estimation from galaxy angular power spectra. Through a spherical-harmonic analysis across four photo-$z$ slices, incorporating a detailed window function, redshift-space distortions, and cross-slice covariances, the authors extract constraints on $\Omega_m h$ and $\Omega_b/\Omega_m$ that are consistent with CMB measurements and spectroscopic surveys. They validate their results with extensive systematic tests and also measure a direct spatial power spectrum from photo-$z$ data, finding good agreement with the angular analyses. The work demonstrates that photometric redshift surveys can yield competitive cosmological constraints in the vanilla $\Lambda$CDM framework, and highlights the potential for future deep imaging surveys to improve parameter precision while requiring tight control of systematics.

Abstract

We analyze MegaZ-LRG, a photometric-redshift catalogue of Luminous Red Galaxies (LRGs) based on the imaging data of the Sloan Digital Sky Survey (SDSS) 4th Data Release. MegaZ-LRG, presented in a companion paper, contains 10^6 photometric redshifts derived with ANNz, an Artificial Neural Network method, constrained by a spectroscopic sub-sample of 13,000 galaxies obtained by the 2dF-SDSS LRG and Quasar (2SLAQ) survey. The catalogue spans the redshift range 0.4 < z < 0.7 with an r.m.s. redshift error ~ 0.03(1+z), covering 5,914 deg^2 to map out a total cosmic volume 2.5 h^-3 Gpc^3. In this study we use the most reliable 600,000 photometric redshifts to present the first cosmological parameter fits to galaxy angular power spectra from a photometric redshift survey. Combining the redshift slices with appropriate covariances, we determine best-fitting values for the matter and baryon densities of Omega_m h = 0.195 +/- 0.023 and Omega_b/Omega_m = 0.16 +/- 0.036 (with the Hubble parameter h = 0.75 and scalar index of primordial fluctuations n = 1 held fixed). These results are in agreement with and independent of the latest studies of the Cosmic Microwave Background radiation, and their precision is comparable to analyses of contemporary spectroscopic-redshift surveys. We perform an extensive series of tests which conclude that our power spectrum measurements are robust against potential systematic photometric errors in the catalogue. We conclude that photometric-redshift surveys are competitive with spectroscopic surveys for measuring cosmological parameters in the simplest vanilla models. Future deep imaging surveys have great potential for further improvement, provided that systematic errors can be controlled.

Figures (23)

• Figure 1: The angular coverage map of the NGP region of the SDSS DR4, which was analyzed in this study. The right ascension and declination values are plotted in degrees and an equal-area Aitoff projection is used. The total area of this survey region is $5{,}914$ deg$^2$. The dashed lines indicate constant Galactic latitudes of $45^\circ$ (the innermost pair), $30^\circ$ and $15^\circ$. There are several small gaps within the observed regions due to faults or poor-quality data.
• Figure 2: Comparison between the photometric redshift $z_{\rm phot}$ and spectroscopic redshift $z_{\rm spec}$ of galaxies in our evaluation set (for the conservative version of the catalogue analyzed in this paper). The mean and standard deviation of the quantity $\delta z = z_{\rm phot} - z_{\rm spec}$ are indicated.
• Figure 3: Gaussian fits to the spectroscopic redshift distribution of Luminous Red Galaxies in each of four photometric redshift slices analyzed in this study. The best-fitting Gaussian parameters $\mu$ and $\sigma$ are indicated, where $p(z) \propto \exp{\{-[(z-\mu)^2/2\sigma^2]\}}$. $N$ is the number of galaxies in the evaluation set used to measure the distribution in each slice.
• Figure 4: The locations of the 2000 most luminous galaxies in each of a series of photometric redshift slices of width $\Delta z = 0.03$ in a narrow survey stripe. The characteristic patterns of large-scale structure may be observed. The axes are right ascension and declination in decimal degrees.
• Figure 5: The mixing matrix $R_{\ell,\ell'}$ for multipole $\ell = 200$ as a function of $\ell'$, indicating the range of multipoles correlated by the survey window function effects (see equations \ref{['eqclmix']} and \ref{['eqrll']} for the definition of $R_{\ell,\ell'}$).