The SDSS Galaxy Angular Two-Point Correlation Function

We present the galaxy two-point angular correlation function for galaxies selected from the seventh data release of the Sloan Digital Sky Survey. The galaxy sample was selected with $r$-band apparent magnitudes between 17 and 21; and we measure the correlation function for the full sample as well as for the four magnitude ranges: 17-18, 18-19, 19-20, and 20-21. We update the flag criteria to select a clean galaxy catalog and detail specific tests that we perform to characterize systematic effects, including the effects of seeing, Galactic extinction, and the overall survey uniformity. Notably, we find that optimally we can use observed regions with seeing < 1\farcs5, and $r$-band extinction < 0.13 magnitudes, smaller than previously published results. Furthermore, we confirm that the uniformity of the SDSS photometry is minimally affected by the stripe geometry. We find that, overall, the two-point angular correlation function can be described by a power law, $\omega(\theta) = A_\omega \theta^{(1-\gamma)}$ with $\gamma \simeq 1.72$, over the range 0\fdg005--10\degr. We also find similar relationships for the four magnitude subsamples, but the amplitude within the same angular interval for the four subsamples is found to decrease with fainter magnitudes, in agreement with previous results. We find that the systematic signals are well below the galaxy angular correlation function for angles less than approximately 5\degr, which limits the modeling of galaxy angular correlations on larger scales. Finally, we present our custom, highly parallelized two-point correlation code that we used in this analysis.Comment: 22 pages, 17 figures, accepted by MNRA

Galaxy clustering with photometric surveys using PDF redshift information

Photometric surveys produce large-area maps of the galaxy distribution, but with less accurate redshift information than is obtained from spectroscopic methods. Modern photometric redshift (photo-z) algorithms use galaxy magnitudes, or colors, that are obtained through multi-band imaging to produce a probability density function (PDF) for each galaxy in the map. We used simulated data to study the effect of using different photo-z estimators to assign galaxies to redshift bins in order to compare their effects on angular clustering and galaxy bias measurements. We found that if we use the entire PDF, rather than a single-point (mean or mode) estimate, the deviations are less biased, especially when using narrow redshift bins. When the redshift bin widths are $\Delta z=0.1$, the use of the entire PDF reduces the typical measurement bias from 5%, when using single point estimates, to 3%.Comment: Matches the MNRAS published version. 19 pages, 19 Figure

A Definitive Optical Detection of a Supercluster at z = 0.91

We present the results from a multi-band optical imaging program which has definitively confirmed the existence of a supercluster at z = 0.91. Two massive clusters of galaxies, CL1604+4304 at z = 0.897 and CL1604+4321 at z = 0.924, were originally observed in the high-redshift cluster survey of Oke, Postman & Lubin (1998). They are separated by 4300 km/s in radial velocity and 17 arcminutes on the plane of the sky. Their physical and redshift proximity suggested a promising supercluster candidate. Deep BRi imaging of the region between the two clusters indicates a large population of red galaxies. This population forms a tight, red sequence in the color--magnitude diagram at (R-i) = 1.4. The characteristic color is identical to that of the spectroscopically-confirmed early-type galaxies in the two member clusters. The red galaxies are spread throughout the 5 Mpc region between CL1604+4304 and CL1604+4321. Their spatial distribution delineates the entire large scale structure with high concentrations at the cluster centers. In addition, we detect a significant overdensity of red galaxies directly between CL1604+4304 and CL1604+4321 which is the signature of a third, rich cluster associated with this system. The strong sequence of red galaxies and their spatial distribution clearly indicate that we have discovered a supercluster at z = 0.91.Comment: Accepted for publication in Astrophysical Journal Letters. 13 pages, including 5 figure