Optimizing the yield of Sunyaev–Zel'dovich cluster surveys

We consider the optimum depth of a cluster survey selected using the Sunyaev–Zel'dovich effect. By using simple models for the evolution of the cluster mass function and detailed modelling for a variety of observational techniques, we show that the optimum survey yield is achieved when the average size of the clusters selected is close to the size of the telescope beam. For a total power measurement, we compute the optimum noise threshold per beam as a function of the beam size and then discuss how our results can be used in more general situations. As a by-product we gain some insight into what is the most advantageous instrumental set-up. In the case of beam switching observations one is not severely limited if one manages to set the noise threshold close to the point which corresponds to the optimum yield. Considering a variety of alternative scenarios, we discuss how robust our conclusions are to modifications in the cluster model and cosmological parameters. The precise optimum is particularly sensitive to the amplitude of fluctuations and the profile of the gas in the cluster.     

Quasar–galaxy and galaxy–galaxy cross-correlations: model predictions with realistic galaxies

Several measurements of quasi-stellar object (QSO)–galaxy correlations have reported signals much larger than predictions of magnification by large-scale structure. We find that the expected signal depends strongly on the properties of the foreground galaxy population. On arcmin scales, it can be either larger or smaller by a factor of 2 for different galaxy types in comparison with a linearly biased version of the mass distribution. Thus the resolution of some of the excess measurements may lie in examining the halo occupation properties of the galaxy population sampled by a given survey; this is also the primary information such measurements will provide.
We use the halo model of clustering and simulations to predict the magnification-induced cross-correlations and errors for forthcoming surveys. With the full Sloan Digital Sky Survey, the statistical errors will be below 1 per cent for the galaxy–galaxy correlations and significantly larger for QSO–galaxy correlations. Thus accurate constraints on parameters of the galaxy halo occupation distribution can be obtained from small-scale measurements and on the bias parameter from large scales. Since the lensing-induced cross-correlation measures the first moment of the halo occupation number of galaxies, these measurements can provide the basis for interpreting galaxy clustering measurements that measure the second- and higher-order moments.     

Study of young open clusters as tracers of spiral features in our galaxy. Paper 4: Czernik 20 (OCl 427)

Photoelectric and photographic photometry of 72 stars was done in the field of the not-well-studied open cluster Czernik 20= OCl 427 in the direction of the Auriga constellation. Of these stars, a total of 43 have been found to be probable members down tom _v− 15.75 mag. There is apparently a variable extinction across the field of the cluster withE(B-V) ranging from 0.53 to 0.38 mag. The cluster stars show a range in their ages from 1.0 × 10⁷ to 7.1 × 10⁷ years, indicating that Czernik 20 is young enough to be considered as a spiral-arm tracer in the study of our Galaxy. The distance of this cluster is found to be 4.27 ±0.14 kpc and it is located inside the outer Perseus arm of the Milky Way     

CCD uvbyβ photometry of the young open cluster NGC 663

NGC 663 is a young, moderately rich open cluster, known to contain one of the largest fractions of Be stars among all galactic clusters. In this work we present CCD u v by β photometry for stars in its central area. We have used these data to obtain the main cluster physical parameters. We find that the reddening is highly variable, with values ranging from   E ( b − y ) = 0.639 ± 0.032  in the central part to   E ( b − y ) = 0.555 ± 0.038  in the south-east. The distance modulus is found to be  11.6 ± 0.1 mag (2.1 kpc)  and the age  log  t = 7.4 ± 0.1 yr (25⁺⁷₋₅ Myr)  . The age obtained is consistent with the interpretation of the Be phenomenon as an evolutionary effect.