Person:

Stalder, Brian

We have measured spatial and temporal variability in the y-band sky brightness over the course of four nights above Cerro Tololo near Cerro Pachon, Chile, the planned site for the Large Synoptic Survey Telescope (LSST). Our wide-angle camera lens provided a 41° field of view and a 145" pixel scale. We minimized potential system throughput differences by deploying a deep-depletion CCD and a filter that matches the proposed LSST (y_3) band (970 –1030 nm). Images of the sky exhibited coherent wave structure, attributable to atmospheric gravity waves at 90 km altitude, creating 3%–4% rms spatial sky flux variability on scales of about 2° and larger. Over the course of a full night, the (y_3) band additionally showed highly coherent temporal variability of up to a factor of 2 in flux. We estimate the mean absolute sky level to be approximately (y_3 = 17.8 mag(Vega)), or (y_3 = 18.3 mag(AB)). While our observations were made through a (y_3) filter, the relative sky brightness variability should hold for all proposed y bands, whereas the absolute levels should more strongly depend on spectral response. The spatial variability presents a challenge to wide-field cameras that require illumination correction strategies that make use of stacked sky flats. The temporal variability may warrant an adaptive y band imaging strategy for LSST, to take advantage of times when the sky is darkest.

We present Stellar Locus Regression (SLR), a method of directly adjusting the instrumental broadband optical colors of stars to bring them into accord with a universal stellar color-color locus, producing accurately calibrated colors for both stars and galaxies. This is achieved without first establishing individual zeropoints for each passband, and can be performed in real-time at the telescope. We demonstrate how SLR naturally makes one wholesale correction for differences in instrumental response, for atmospheric transparency, for atmospheric extinction, and for Galactic extinction. We perform an example SLR treatment of SDSS data over a wide range of Galactic dust values and independently recover the direction and magnitude of the canonical Galactic reddening vector with 14--18 mmag RMS uncertainties. We then isolate the effect of atmospheric extinction, showing that SLR accounts for this and returns precise colors over a wide of airmass, with 5--14 mmag RMS residuals. We demonstrate that SLR-corrected colors are sufficiently accurate to allow photometric redshift estimates for galaxy clusters (using red sequence galaxies) with an uncertainty (\sigma(z)/(1+z)) = 0.6% per cluster for redshifts 0.09< (z) <0.25. Finally, we identify our objects in the 2MASS all-sky catalog, and produce i-band zeropoints typically accurate to 18 mmag using only SLR. We offer open-source access to our IDL routines, validated and verified for the implementation of this technique, at http://stellar-locus-regression.googlecode.com

We present results of X-ray observations of a sample of 15 clusters selected via their imprint on the cosmic microwave background from the thermal Sunyaev-Zel'dovich (SZ) effect. These clusters are a subset of the first SZ-selected cluster catalog, obtained from observations of 178 deg2 of sky surveyed by the South Pole Telescope (SPT). Using X-ray observations with Chandra and XMM-Newton, we estimate the temperature, TX , and mass, Mg , of the intracluster medium within r 500 for each cluster. From these, we calculate YX = MgTX and estimate the total cluster mass using an M 500-YX scaling relation measured from previous X-ray studies. The integrated Comptonization, Y SZ, is derived from the SZ measurements, using additional information from the X-ray-measured gas density profiles and a universal temperature profile. We calculate scaling relations between the X-ray and SZ observables and find results generally consistent with other measurements and the expectations from simple self-similar behavior. Specifically, we fit a Y SZ-YX relation and find a normalization of 0.82 ± 0.07, marginally consistent with the predicted ratio of Y SZ/YX = 0.91 ± 0.01 that would be expected from the density and temperature models used in this work. Using the YX -derived mass estimates, we fit a Y SZ-M 500 relation and find a slope consistent with the self-similar expectation of Y SZvpropM 5/3 with a normalization consistent with predictions from other X-ray studies. We find that the SZ mass estimates, derived from cosmological simulations of the SPT survey, are lower by a factor of 0.78 ± 0.06 relative to the X-ray mass estimates. This offset is at a level of 1.3σ when considering the ~15% systematic uncertainty for the simulation-based SZ masses. Overall, the X-ray measurements confirm that the scaling relations of the SZ-selected clusters are consistent with the properties of other X-ray-selected samples of massive clusters, even allowing for the broad redshift range (0.29 < z < 1.08) of the sample.

We present radio continuum mapping, optical imaging, and spectroscopy of the newly discovered double-peaked, broad-lined active galactic nucleus (AGN) WISE J233237.05–505643.5 at redshift z = 0.3447. This source exhibits an FR-I and FR-II hybrid morphology, characterized by a bright core, jet, and Doppler-boosted lobe structures in Australian Telescope Compact Array continuum maps at 1.5, 5.6, and 9 GHz. Unlike most FR-II objects, W2332–5056 is hosted by a disk-like galaxy. The core has a projected 5'' linear radio feature that is perpendicular to the curved primary jet, hinting at unusual and complex activity within the inner 25 kpc. The multi-epoch, optical-near-IR photometric measurements indicate significant variability over a 3-20 yr baseline from the AGN component. Gemini South optical data show unusual double-peaked emission-line features: the centroids of the broad-lined components of H-alpha and H-beta are blueshifted with respect to the narrow lines and host galaxy by ~3800 km s–1. We examine possible cases that involve single or double supermassive black holes in the system and discuss the required future investigations to disentangle the mysterious nature of this system.

We use measurements from the South Pole Telescope (SPT) Sunyaev-Zel'dovich (SZ) cluster survey in combination with X-ray measurements to constrain cosmological parameters. We present a statistical method that fits for the scaling relations of the SZ and X-ray cluster observables with mass while jointly fitting for cosmology. The method is generalizable to multiple cluster observables, and self-consistently accounts for the effects of the cluster selection and uncertainties in cluster mass calibration on the derived cosmological constraints. We apply this method to a data set consisting of an SZ-selected catalog of 18 galaxy clusters at z > 0.3 from the first 178 deg2 of the 2500 deg2 SPT-SZ survey, with 14 clusters having X-ray observations from either Chandra or XMM-Newton. Assuming a spatially flat ΛCDM cosmological model, we find the SPT cluster sample constrains σ8(Ω m /0.25)0.30 = 0.785 ± 0.037. In combination with measurements of the cosmic microwave background (CMB) power spectrum from the SPT and the seven-year Wilkinson Microwave Anisotropy Probe data, the SPT cluster sample constrains σ8 = 0.795 ± 0.016 and Ω m = 0.255 ± 0.016, a factor of 1.5 improvement on each parameter over the CMB data alone. We consider several extensions beyond the ΛCDM model by including the following as free parameters: the dark energy equation of state (w), the sum of the neutrino masses (Σm ν), the effective number of relativistic species (N eff), and a primordial non-Gaussianity (f NL). We find that adding the SPT cluster data significantly improves the constraints on w and Σm ν beyond those found when using measurements of the CMB, supernovae, baryon acoustic oscillations, and the Hubble constant. Considering each extension independently, we best constrain w = –0.973 ± 0.063 and the sum of neutrino masses Σm ν < 0.28 eV at 95% confidence, a factor of 1.25 and 1.4 improvement, respectively, over the constraints without clusters. Assuming a ΛCDM model with a free N eff and Σm ν, we measure N eff = 3.91 ± 0.42 and constrain Σm ν < 0.63 eV at 95% confidence. We also use the SPT cluster sample to constrain f NL = –220 ± 317, consistent with zero primordial non-Gaussianity. Finally, we discuss the current systematic limitations due to the cluster mass calibration, and future improvements for the recently completed 2500 deg2 SPT-SZ survey. The survey has detected ~500 clusters with a median redshift of ~0.5 and a median mass of ~2.3 × 1014 M ☉ h –1 and, when combined with an improved cluster mass calibration and existing external cosmological data sets will significantly improve constraints on w.

The galaxy cluster SPT-CL J0205–5829 currently has the highest spectroscopically confirmed redshift, z = 1.322, in the South Pole Telescope Sunyaev-Zel'dovich (SPT-SZ) survey. XMM-Newton observations measure a core-excluded temperature of TX = 8.7+1.0 –0.8 keV producing a mass estimate that is consistent with the Sunyaev-Zel'dovich-derived mass. The combined SZ and X-ray mass estimate of M 500 = (4.8 ± 0.8) × 1014 h –1 70 M ☉ makes it the most massive known SZ-selected galaxy cluster at z > 1.2 and the second most massive at z > 1. Using optical and infrared observations, we find that the brightest galaxies in SPT-CL J0205–5829 are already well evolved by the time the universe was <5 Gyr old, with stellar population ages >≈ Gyr, and low rates of star formation (<0.5 M ☉ yr–1). We find that, despite the high redshift and mass, the existence of SPT-CL J0205–5829 is not surprising given a flat ΛCDM cosmology with Gaussian initial perturbations. The a priori chance of finding a cluster of similar rarity (or rarer) in a survey the size of the 2500 deg2 SPT-SZ survey is 69%.

We present the results of an X-ray analysis of 80 galaxy clusters selected in the 2500 deg2 South Pole Telescope survey and observed with the Chandra X-ray Observatory. We divide the full sample into subsamples of ∼20 clusters based on redshift and central density, performing a joint X-ray spectral fit to all clusters in a subsample simultaneously, assuming self-similarity of the temperature profile. This approach allows us to constrain the shape of the temperature profile over 0 < r < 1.5R500, which would be impossible on a per-cluster basis, since the observations of individual clusters have, on average, 2000 X-ray counts. The results presented here represent the first constraints on the evolution of the average temperature profile from z = 0 to z = 1.2. We find that high-z (0.6 < z < 1.2) clusters are slightly (∼30%) cooler both in the inner (r < 0.1R500) and outer (r > R500) regions than their low-z (0.3 < z < 0.6) counterparts. Combining the average temperature profile with measured gas density profiles from our earlier work, we infer the average pressure and entropy profiles for each subsample. Confirming earlier results from this data set, we find an absence of strong cool cores at high z, manifested in this analysis as a significantly lower observed pressure in the central 0.1R500 of the high-z cool-core subset of clusters compared to the low-z cool-core subset. Overall, our observed pressure profiles agree well with earlier lower-redshift measurements, suggesting minimal redshift evolution in the pressure profile outside of the core. We find no measurable redshift evolution in the entropy profile at r . 0.7R500 – this may reflect a long-standing balance between cooling and feedback over long timescales and large physical scales. We observe a slight flattening of the entropy profile at r & R500 in our high-z subsample. This flattening is consistent with a temperature bias due to the enhanced (∼3×) rate at which group-mass (∼2 keV) halos, which would go undetected at our survey depth, are accreting onto the cluster at z ∼ 1. This work demonstrates a powerful method for inferring spatially-resolved cluster properties in the case where individual cluster signal-to-noise is low, but the number of observed clusters is high.

We describe a method for measuring the integrated Comptonization (Y SZ) of clusters of galaxies from measurements of the Sunyaev-Zel'dovich (SZ) effect in multiple frequency bands and use this method to characterize a sample of galaxy clusters detected in the South Pole Telescope (SPT) data. We use a Markov Chain Monte Carlo method to fit a β-model source profile and integrate Y SZ within an angular aperture on the sky. In simulated observations of an SPT-like survey that include cosmic microwave background anisotropy, point sources, and atmospheric and instrumental noise at typical SPT-SZ survey levels, we show that we can accurately recover β-model parameters for inputted clusters. We measure Y SZ for simulated semi-analytic clusters and find that Y SZ is most accurately determined in an angular aperture comparable to the SPT beam size. We demonstrate the utility of this method to measure Y SZ and to constrain mass scaling relations using X-ray mass estimates for a sample of 18 galaxy clusters from the SPT-SZ survey. Measuring Y SZ within a 0farcm75 radius aperture, we find an intrinsic log-normal scatter of 21% ± 11% in Y SZ at a fixed mass. Measuring Y SZ within a 0.3 Mpc projected radius (equivalent to 0farcm75 at the survey median redshift z = 0.6), we find a scatter of 26% ± 9%. Prior to this study, the SPT observable found to have the lowest scatter with mass was cluster detection significance. We demonstrate, from both simulations and SPT observed clusters that Y SZ measured within an aperture comparable to the SPT beam size is equivalent, in terms of scatter with cluster mass, to SPT cluster detection significance.

We present the design and lab performance of the Parallel Imager for Southern Cosmology Observations (PISCO), a photometer for the 6.5 m diameter Magellan telescopes that produces gl, rl, il, and zl band images simulta- neously within a 9 arcminute field of view. This design provides efficient follow-up observations of faint sources, particularly galaxy clusters and supernovae. Simultaneous imaging speeds the observing cadence by at a factor of ~ 3 (including optical losses) compared to other photometric imagers. Also, the determination of color (flux ratio between bands) is relatively immune to time variations in gray opacity due to clouds, so observations can proceed in less than optimal conditions. First light is expected in September 2014.

We present a catalog of galaxy clusters selected via their Sunyaev-Zel'dovich (SZ) effect signature from 2500 deg2 of South Pole Telescope (SPT) data. This work represents the complete sample of clusters detected at high significance in the 2500 deg2 SPT-SZ survey, which was completed in 2011. A total of 677 (409) cluster candidates are identified above a signal-to-noise threshold of ξ = 4.5 (5.0). Ground- and space-based optical and near-infrared (NIR) imaging confirms overdensities of similarly colored galaxies in the direction of 516 (or 76%) of the ξ > 4.5 candidates and 387 (or 95%) of the ξ > 5 candidates; the measured purity is consistent with expectations from simulations. Of these confirmed clusters, 415 were first identified in SPT data, including 251 new discoveries reported in this work. We estimate photometric redshifts for all candidates with identified optical and/or NIR counterparts; we additionally report redshifts derived from spectroscopic observations for 141 of these systems. The mass threshold of the catalog is roughly independent of redshift above z ~ 0.25 leading to a sample of massive clusters that extends to high redshift. The median mass of the sample is M 500c(ρcrit) $\sim 3.5\times 10^{14},M_\odot ,h_{70}^{-1}$, the median redshift is z med = 0.55, and the highest-redshift systems are at z > 1.4. The combination of large redshift extent, clean selection, and high typical mass makes this cluster sample of particular interest for cosmological analyses and studies of cluster formation and evolution.