4098 galaxy clusters to z~0.6 in the Sloan Digital Sky Survey equatorial Stripe 82

We present a catalogue of 4098 photometrically selected galaxy clusters with a median redshift <z> = 0.32 in the 270 square degree Stripe 82 region of the Sloan Digital Sky Survey (SDSS), covering the celestial equator in the Southern Galactic Cap (-50 < RA < 59 deg, |Dec| < 1.25 deg). Owing to the multi-epoch SDSS coverage of this region, the ugriz photometry is ~2 magnitudes deeper than single scans within the main SDSS footprint. We exploit this to detect clusters of galaxies using an algorithm that searches for statistically significant overdensities of galaxies in a Voronoi tessellation of the projected sky. 32% of the clusters have at least one member with a spectroscopic redshift from existing public data (SDSS Data Release 7, 2SLAQ & WiggleZ), and the remainder have a robust photometric redshift (accurate to ~5-9% at the median redshift of the sample). The weighted average of the member galaxies redshifts provides a reasonably accurate estimate of the cluster redshift. The cluster catalogue is publicly available for exploitation by the community to pursue a range of science objectives. In addition to the cluster catalogue, we provide a linked catalogue of 18,295 V<21 mag quasar sight-lines with impact parameters within <3 Mpc of the cluster cores selected from the catalogue of Veron et al. (2010). The background quasars cover 0.25 < z < 2, where MgII absorption-line systems associated with the clusters are detectable in optical spectra.

Investigating Clumpy Galaxies in the Sloan Digital Sky Survey Stripe 82 using the Galaxy Zoo

Giant, star-forming clumps are a common feature prevalent amongst high-redshift star-forming galaxies and play a critical role in shaping their chaotic morphologies and yet, their nature and role in galaxy evolution remains to be fully understood. A majority of the effort to study clumps has been focused at high redshifts, and local clump studies have often suffered from small sample sizes. In this work, we present an analysis of clump properties in the local universe, and for the first time, performed with a statistically significant sample. With the help of the citizen science-powered Galaxy Zoo: Hubble project, we select a sample of 92 $z<0.06$ clumpy galaxies in Sloan Digital Sky Survey Stripe 82 galaxies. Within this sample, we identify 543 clumps using a contrast-based image analysis algorithm and perform photometry as well as estimate their stellar population properties. The overall properties of our $z<0.06$ clump sample are comparable to the high-redshift clumps. However, contrary to the high-redshift studies, we find no evidence of a gradient in clump ages or masses as a function of their galactocentric distances. Our results challenge the inward migration scenario for clump evolution for the local universe, potentially suggesting a larger contribution of ex-situ clumps and/or longer clump migration timescales.

Spectral variability of quasars from multi-epoch photometric data in the Sloan Digital Sky Survey Stripe 82

We present a new approach to analysing the dependence of quasar variability on rest-frame wavelengths. We exploited the spectral archive of the Sloan Digital Sky Survey (SDSS) to create a sample of more than 9000 quasars in the Stripe 82. The quasar catalogue was matched with the Light Motion Curve Catalogue for SDSS Stripe 82 and individual first-order structure functions were computed. The structure functions are used to create a variability indicator that is related to the same intrinsic timescales for all quasars (1 to 2 yr in the rest frame). We study the variability ratios for adjacent SDSS filter bands as a function of redshift. While variability is almost always stronger in the bluer passband compared to the redder, the variability ratio depends on whether strong emission lines contribute to either one band or the other. The variability ratio-redshift relations resemble the corresponding colour index-redshift relations. From the comparison with Monte Carlo simulations of variable quasar spectra we find that the observed variability ratio-redshift relations are closely fitted assuming that (a) the r.m.s. fluctuation of the quasar continuum follows a power law-dependence on the intrinsic wavelength with an exponent -2 (i.e., bluer when brighter) and (b) the variability of the emission line flux is only about 10% of that of the underlying continuum. These results, based upon the photometry of more than 8000 quasars, confirm the previous findings by Wilhite et al. (2005) from 315 quasars with repeated SDSS spectroscopy. Finally, we find that quasars with unusual spectra and weak emission lines tend to have less variability than conventional quasars. This trend is opposite to what is expected from the dilution effect of variability due to line emission and may be indicative of high Eddington ratios in these unconventinal quasars.

Robust Weak-lensing Mass Calibration of Planck Galaxy Clusters

In light of the tension in cosmological constraints reported by the Planck team between their SZ-selected cluster counts and Cosmic Microwave Background (CMB) temperature anisotropies, we compare the Planck cluster mass estimates with robust, weak-lensing mass measurements from the Weighing the Giants (WtG) project. For the 22 clusters in common between the Planck cosmology sample and WtG, we find an overall mass ratio of $left< M_{Planck}/M_{rm WtG} right> = 0.688 pm 0.072$. Extending the sample to clusters not used in the Planck cosmology analysis yields a consistent value of $left< M_{Planck}/M_{rm WtG} right> = 0.698 pm 0.062$ from 38 clusters in common. Identifying the weak-lensing masses as proxies for the true cluster mass (on average), these ratios are $sim 1.6sigma$ lower than the default mass bias of 0.8 assumed in the Planck cluster analysis. Adopting the WtG weak-lensing-based mass calibration would substantially reduce the tension found between the Planck cluster count cosmology results and those from CMB temperature anisotropies, thereby dispensing of the need for new physics such as uncomfortably large neutrino masses (in the context of the measured Planck temperature anisotropies and other data). We also find modest evidence (at 95 per cent confidence) for a mass dependence of the calibration ratio and discuss its potential origin in light of systematic uncertainties in the temperature calibration of the X-ray measurements used to calibrate the Planck cluster masses. Our results exemplify the critical role that robust absolute mass calibration plays in cluster cosmology, and the invaluable role of accurate weak-lensing mass measurements in this regard.

AMICO galaxy clusters in KiDS-DR3: weak-lensing mass calibration

We present the mass calibration for galaxy clusters detected with the AMICO code in KiDS DR3 data. The cluster sample comprises $sim$ 7000 objects and covers the redshift range 0.1 < $z$ < 0.6. We perform a weak lensing stacked analysis by binning the clusters according to redshift and two different mass proxies provided by AMICO, namely the amplitude $A$ (measure of galaxy abundance through an optimal filter) and the richness $lambda^*$ (sum of membership probabilities in a consistent radial and magnitude range across redshift). For each bin, we model the data as a truncated NFW profile plus a 2-halo term, taking into account uncertainties related to concentration and miscentring. From the retrieved estimates of the mean halo masses, we construct the $A$-$M_{200}$ and the $lambda^*$-$M_{200}$ relations. The relations extend over more than one order of magnitude in mass, down to $M_{200} sim 2 (5) times 10^{13} M_odot/h$ at $z$ = 0.2 (0.5), with small evolution in redshift. The logarithmic slope is $sim 2.0$ for the $A$-mass relation, and $sim 1.7$ for the $lambda^*$-mass relation, consistent with previous estimations on mock catalogues and coherent with the different nature of the two observables.