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Register a new userThe NOG Sample: Galaxy Systems and 3D Real-Space Galaxy Density
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We discuss the 3D real-space reconstruction of the optical galaxy density field in the local Universe as derived from the galaxies of the Nearby Optical Galaxy (NOG) sample. NOG is a distance-limited (cz_{LG} < 6000 km/s) and magnitude--limited (B<14 mag) sample of 7076 optical galaxies which covers 2/3 (8.29 sr) of the sky (|b|>20). We have replaced ``true distances measurements for all the objects in order to correct for redshif distortions. Using homogenized photometric information for the whole sample, NOG is meant to be an approximation to a homogeneous all-sky 3D optically selected and statistically well-controlled galaxy sample that probes in great detail volumes of cosmological interest. Our goal is to construct a reliable, robust and unbiased field of density contrasts covering interesting regions of galaxy and mass overdensities of the local universe.
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In order to map the galaxy density field in the local universe, we select the Nearby Optical Galaxy (NOG) sample, which is a distance-limited (cz < 6000 km/s) and magnitude--limited (B < 14 mag) sample of 7076 optical galaxies which covers 2/3 (8.29 sr) of the sky (|b|>20^{circ}) and has a good completeness in redshift (98%). In order to trace the galaxy density field on small scales, we identify the NOG galaxy systems by means of both the hierarchical and the percolation (friends of friends) methods. The NOG provides high resolution in both spatial sampling of the nearby universe and morphological galaxy classification. The NOG is meant to be the first step towards the construction of a statistically well-controlled galaxy sample with homogenized photometric data covering most of the celestial sphere.
In order to map the galaxy density field on small scales in the local universe, we use the Nearby Optical Galaxy (NOG) sample, which is currently one of the largest, nearly complete, magnitude-limited ($Bleq$ 14 mag), all-sky sample of nearby optical galaxies ($sim$ 6400 galaxies with cz< 5500 km/s). We have corrected the redshift-dependent distances of these galaxies for non-cosmological motions by means of peculiar velocity field models. Relying on group assignments and on total B magnitudes fully corrected for internal and Galactic extinctions, we determine the total and morphological-type specific luminosity functions for field and grouped galaxies using their locations in real distance space. The related determination of the selection function is meant to be an important step in recovering the galaxy density field on small scales from the NOG sample. Local galaxy density parameters will be used in statistical studies of environmental effects on galaxy properties.
It is well known gravitational lensing, mainly via magnification bias, modifies the observed galaxy/quasar clustering. Such discussions have largely focused on the 2D angular correlation. Here and in a companion paper (Paper II) we explore how magnification bias distorts the 3D correlation function and power spectrum, as first considered by Matsubara. The interesting point is: the distortion is anisotropic. Magnification bias preferentially enhances the observed correlation in the line-of-sight (LOS) orientation, especially on large scales. For example at LOS separation of ~100 Mpc/h, where the intrinsic galaxy-galaxy correlation is rather weak, the observed correlation can be enhanced by lensing by a factor of a few, even at a modest redshift of z ~ 0.35. The opportunity: this lensing anisotropy is distinctive, making it possible to separately measure the galaxy-galaxy, galaxy-magnification and magnification-magnification correlations, without measuring galaxy shapes. The anisotropy is distinguishable from the well known distortion due to peculiar motions, as will be discussed in Paper II. The challenge: the magnification distortion of the galaxy correlation must be accounted for in interpreting data as precision improves. For instance, the ~100 Mpc/h baryon acoustic oscillation scale in the correlation function is shifted by up to ~3% in the LOS orientation, and up to ~0.6% in the monopole, depending on the galaxy bias, redshift and number count slope. The corresponding shifts in the inferred Hubble parameter and angular diameter distance, if ignored, could significantly bias measurements of the dark energy equation of state. Lastly, magnification distortion offers a plausible explanation for the well known excess correlations seen in pencil beam surveys.
Galaxy formation inside dark matter halos, as well as the halo formation itself, can be affected by large-scale environments. Evaluating the imprints of environmental effects on galaxy clustering is crucial for precise cosmological constraints with data from galaxy redshift surveys. We investigate such an environmental impact on both real-space and redshift-space galaxy clustering statistics using a semi-analytic model derived from the Millennium Simulation. We compare clustering statistics from original SAM galaxy samples and shuffled ones with environmental influence on galaxy properties eliminated. Among the luminosity-threshold samples examined, the one with the lowest threshold luminosity (~0.2L_*) is affected by environmental effects the most, which has a ~10% decrease in the real-space two-point correlation function (2PCF) after shuffling. By decomposing the 2PCF into five different components based on the source of pairs, we show that the change in the 2PCF can be explained by the age and richness dependence of halo clustering. The 2PCFs in redshift space are found to change in a similar manner after shuffling. If the environmental effects are neglected, halo occupation distribution modeling of the real-space and redshift-space clustering may have a less than 6.5% systematic uncertainty in constraining beta from the most affected SAM sample and have substantially smaller uncertainties from the other, more luminous samples. We argue that the effect could be even smaller in reality. In the Appendix, we present a method to decompose the 2PCF, which can be applied to measure the two-point auto-correlation functions of galaxy sub-samples in a volume-limited galaxy sample and their two-point cross-correlation functions in a single run utilizing only one random catalog.
We develop a maximum likelihood based method of reconstructing band powers of the density and velocity power spectra at each wavenumber bins from the measured clustering features of galaxies in redshift space, including marginalization over uncertainties inherent in the Fingers-of-God (FoG) effect. The reconstruction can be done assuming that the density and velocity power spectra depend on the redshift-space power spectrum having different angular modulations of mu with mu^{2n} (n=0,1,2) and that the model FoG effect is given as a multiplicative function in the redshift-space spectrum. By using N-body simulations and the halo catalogs, we test our method by comparing the reconstructed power spectra with the simulations. For the spectrum of mu^0 or equivalently the density power spectrum P_dd(k), our method recovers the amplitudes to a few percent accuracies up to k=0.3 h/Mpc for both dark matter and halos. For the power spectrum of mu^2, which is equivalent to the density-velocity spectrum P_dv(k) in the linear regime, our method can recover the input power spectrum for dark matter up to k=0.2 h/Mpc and at both z=0 and 1, if using the adequate FoG model. However, for the halo spectrum, the reconstructed spectrum shows greater amplitudes than the simulation P_dv(k). We argue that the disagreement is ascribed to nonlinearity effect that arises from the cross-bispectra of density and velocity perturbations. Using the perturbation theory, we derive the nonlinear correction term, and find that the leading-order correction term is proportional to mu^2 and increases the mu^2-power spectrum amplitudes at larger k, at lower redshifts and for more massive halos. We find that adding the nonlinearity correction term to the simulation P_dv(k) can fairly well reproduce the reconstructed P_dv(k) for halos up to k~0.2 h/Mpc.
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