A Comparison of Galaxy Properties in Two Luminous Red Galaxy Samples at Extremely High and Low Density

Abstract

Using the local three-dimensional galaxy density in a comoving sphere with a radius of the distance to the fifth-nearest galaxy, we construct two samples at the two density extremes from the luminous red galaxy (LRG) sample of the Sixth Data Release of the Sloan Digital Sky Survey (SDSS DR6), which contains 53,453 LRGs with redshift z = 0.2-0.36. We compare the basic properties of LRGs in the lowest density regimes with those of LRGs in the highest density regimes. It is found that the sample at low density has a higher proportion of faint galaxies (Mg≥ − 21.92) and a lower proportion of luminous galaxies (Mg≤ − 22.2) than the one at high density, but the colors of the LRGs nearly are independent of the local density. We also measure the projected local density ∑5, which is computed from the distance to the fifth-nearest neighbor within a redshift slice ±1000 km s−1 of each galaxy. For the color distributions of the two samples, we get the same results as when using the three-dimensional local density, but the tendency for LRG luminosity to change with the local density is opposite to what is found using the three-dimensional local density. This shows that projection effects or redshift space distortions may seriously influence any final conclusions about the dependence of LRG luminosity on environment.