On the fairness of the main galaxy sample of SDSS

Flux-limited and volume-limited galaxy samples are constructed from the Sloan Digital Sky Survey（SDSS）data releases DR4,DR6 and DR7 for statistical analysis.The two-point correlation functionsξ（s）,monopole of three-point correlation functionsζ0,projected two-point correlation function wp and pairwise velocity dispersionσ12 are measured to test if galaxy samples are fair for these statistics.We find that with the increment of sky coverage of subsequent data releases in SDSS, ξ（s）of the flux-limited sample is extremely robust and insensitive to local structures at low redshift.However,for volume-limited samples fainter than L＊ at large scales s～10 h-1 Mpc,the deviation of ξ（s）from different SDSS data releases（DR7,DR6 and DR4）increases with the increment of absolute magnitude.The case of ζ0（s）is similar to that ofξ（s）.In the weakly nonlinear regime,there is no agreement between ζ0 of different data releases in all luminosity bins.Furthermore,wp of volume-limited samples of DR7 in luminosity bins fainter than -Mr,0.1=[18.5,19.5]are significantly larger and σ12 of the two faintest volume-limited samples of DR7 display a very different scale dependence than results from DR4 and DR6.Our findings call for caution in understanding clustering analysis results of SDSS faint galaxy samples and higher order statistics of SDSS volume-limited samples in the weakly nonlinear regime.The first zero-crossing points of ξ（s）from volume-limited samples are also investigated and discussed.

Clustering of K-band selected local galaxies

We present a detailed clustering analysis of a large K-band selected local galaxy sample, which is constructed from the 2MASS and the SDSS and consists of 82486 galaxies with 10 〈 K 〈 13.5 and 0.01 〈 z 〈 0.1. The two-point correlation function of the magnitude-limited sample in real space at small scales is well described by the power law ∈（r） = （r/6.44 ± 0.23）^-1.81±0.02. The pairwise velocity dispersion is derived from the anisotropic two-point correlation function and we find the dispersion δ12 = 685 ± 17 kms^-1 if scale invariance is assumed, which is larger than values measured in the optical bands of the selected galaxy samples. We further investigate the dependence of the two-point correlation function and the δ12 on the g- r color and the K- band luminosity, and obtain similar results to previous works in optical bands. Comparing a mock galaxy sample with our real data indicates that the semi-analytical model cannot mimic the δ12 in observation, although it can approximate the two-point correlation function within measurement uncertainties.