Meghan E. Gray

The Shear Testing Programme ¿ I. Weak lensing analysis of simulated ground-based observations

The Shear Testing Programme (STEP) is a collaborative project to improve the accuracy and reliability of all weak lensing measurements in preparation for the next generation of wide-field surveys. In this first STEP paper, we present the results of a blind analysis of simulated ground-based observations of relatively simple galaxy morphologies. The most successful methods are shown to achieve percent level accuracy. From the cosmic shear pipelines that have been used to constrain cosmology, we find weak lensing shear measured to an accuracy that is within the statistical errors of current weak lensing analyses, with shear measurements accurate to better than 7 per cent. The dominant source of measurement error is shown to arise from calibration uncertainties where the measured shear is over or underestimated by a constant multiplicative factor. This is of concern as calibration errors cannot be detected through standard diagnostic tests. The measured calibration errors appear to result from stellar contamination, false object detection, the shear measurement method itself, selection bias and/or the use of biased weights. Additive systematics (false detections of shear) resulting from residual point-spread function anisotropy are, in most cases, reduced to below an equivalent shear of 0.001, an order of magnitude below cosmic shear distortions on the scales probed by current surveys. Our results provide a snapshot view of the accuracy of current ground-based weak lensing methods and a benchmark upon which we can improve. To this end we provide descriptions of each method tested and include details of the eight different implementations of the commonly used Kaiser, Squires & Broadhurst method (KSB+) to aid the improvement of future KSB+ analyses.

The STAGES view of red spirals and dusty red galaxies: mass-dependent quenching of star formation in cluster infall

We investigate the properties of optically passive spirals and dusty red galaxies in the A901/2 cluster complex at redshift ∼0.17 using rest-frame near-ultraviolet–optical spectral energy distributions, 24-μm infrared data and Hubble Space Telescope morphologies from the STAGES data set. The cluster sample is based on COMBO-17 redshifts with an rms precision of σcz ≈ 2000 km s −1 . We find that ‘dusty red galaxies’ and ‘optically passive spirals’ in A901/2 are largely the same phenomenon, and that they form stars at a substantial rate, which is only four times lower than that in blue spirals at fixed mass. This star formation is more obscured than in blue galaxies and its optical signatures are weak. They appear predominantly in the stellar mass range of log M∗/M� = [10, 11] where they constitute over half of the star-forming galaxies in the cluster; they are thus a vital ingredient for understanding the overall picture of star formation quenching in clusters. We find that the mean specific star formation rate (SFR) of star-forming galaxies in the cluster is clearly lower than in the field, in contrast to the specific SFR properties of blue galaxies alone, which appear similar in cluster and field. Such a rich red spiral population is best explained if quenching is a slow process and morphological transformation is delayed even more. At log M∗/M� < 10, such galaxies are rare, suggesting that their quenching is fast and accompanied by morphological change. We note that edge-on

Measures of galaxy environment – I. What is ‘environment’?

The influence of a galaxy’s environment on its evolution has been studied and compared extensively in the literature, although differing techniques are often used to define environment. Most methods fall into two broad groups: those that use nearest neighbours to probe the underlying density field and those that use fixed apertures. The differences between the two inhibit a clean comparison between analyses and leave open the possibility that, even with the same data, different properties are actually being measured. In this work we apply twenty published environment definitions to a common mock galaxy catalogue constrained to look like the local Universe. We find that nearest neighbour-based measures best probe the internal densities of high-mass haloes, while at low masses the inter-halo separation dominates and acts to smooth out local density variations. The resulting correlation also shows that nearest neighbour galaxy environment is largely independent of dark matter halo mass. Conversely, aperture-based methods that probe super-halo scales accurately identify high-density regions corresponding to high mass haloes. Both methods show how galaxies in dense environments tend to be redder, with the exception of the largest apertures, but these are the strongest at recovering the background dark matter environment. We also warn against using photometric redshifts to define environment in all but the densest regions. When considering environment there are two regimes: the ‘local environment’ internal to a halo best measured with nearest neighbour and ‘large-scale environment’ external to a halo best measured with apertures. This leads to the conclusion that there is no universal environment measure and the most suitable method depends on the scale being probed.

Less than 10 percent of star formation in z=0.6 massive galaxies is triggered by major interactions

Both observations and simulations show that major tidal interactions or mergers between gas-rich galaxies can lead to intense bursts of star formation. Yet, the average enhancement in star formation rate (SFR) in major mergers and the contribution of such events to the cosmic SFR are not well estimated. Here we use photometric redshifts, stellar masses, and UV SFRs from COMBO-17, 24 mu m SFRs from Spitzer, and morphologies from two deep Hubble Space Telescope (HST) cosmological survey fields (ECDFS/GEMS and A901/STAGES) to study the enhancement in SFR as a function of projected galaxy separation. We apply two-point projected correlation function techniques, which we augment with morphologically selected very close pairs (separation = 10(10) M(circle dot)) star-forming galaxies at 0.4 < z < 0.8, we find that the SFRs of galaxies undergoing a major interaction (mass ratios <= 1:4 and separations <= 40 kpc) are only 1.80 +/- 0.30 times higher than the SFRs of non-interacting galaxies when averaged over all interactions and all stages of the interaction, in good agreement with other observational works. Our results also agree with hydrodynamical simulations of galaxy interactions, which produce some mergers with large bursts of star formation on similar to 100 Myr timescales, but only a modest SFR enhancement when averaged over the entire merger timescale. We demonstrate that these results imply that only less than or similar to 10% of star formation at 0.4 <= z <= 0.8 is triggered directly by major mergers and interactions; these events are not important factors in the build-up of stellar mass since z = 1.

Probing the Distribution of Dark Matter in the A901/902 Supercluster with Weak Lensing

We present a weak-shear analysis of the A901/902 supercluster, composed of three rich clusters at z = 0.16. Using a deep R-band image from the 05 × 05 MPG/ESO Wide Field Imager together with supplementary B-band observations, we build up a comprehensive picture of the light and mass distributions in this region. We find that, on average, the light from the early-type galaxies traces the dark matter fairly well, although one cluster is a notable exception to this rule. The clusters themselves exhibit a range of mass-to-light (M/L) ratios, X-ray properties, and galaxy populations. We attempt to model the relation between the total mass and the light from the early-type galaxies with a simple scale-independent linear biasing model. We find M/LB = 130h for the early-type galaxies with zero stochasticity, which, if taken at face value, would imply Ωm < 0.1. However, this linear relation breaks down on small scales and on scales equivalent to the average cluster separation (~1 Mpc), demonstrating that a single M/L ratio is not adequate to fully describe the mass-to-light relation in the supercluster. Rather, the scatter in M/L ratios observed for the clusters supports a model incorporating nonlinear biasing or stochastic processes. Finally, there is a clear detection of filamentary structure connecting two of the clusters, seen in both the galaxy and dark matter distributions, and we discuss the effects of cluster-cluster and cluster-filament interactions as a means to reconcile the disparate descriptions of the supercluster.

OBSCURED STAR FORMATION IN INTERMEDIATE-DENSITY ENVIRONMENTS: A SPITZER STUDY OF THE ABELL 901/902 SUPERCLUSTER

We explore the amount of obscured star formation as a function of environment in the Abell 901/902 (A901/902) supercluster at z = 0.165 in conjunction with a field sample drawn from the A901 and CDFS fields, imaged with the Hubble Space Telescope as part of the Space Telescope A901/902 Galaxy Evolution Survey and Galaxy Evolution from Morphology and Spectral Energy Distributions (SEDs) Survey. We combine the combo-17 near-UV/optical SED with Spitzer 24 mu m photometry to estimate both the unobscured and obscured star formation in galaxies with M(*) > 10(10) M(circle dot). We find that the star formation activity in massive galaxies is suppressed in dense environments, in agreement with previous studies. Yet, nearly 40% of the star-forming (SF) galaxies have red optical colors at intermediate and high densities. These red systems are not starbursting; they have star formation rates (SFRs) per unit stellar mass similar to or lower than blue SF galaxies. More than half of the red SF galaxies have low infrared-to-ultraviolet (IR-to-UV) luminosity ratios, relatively high Sersicindices, and they are equally abundant at all densities. They might be gradually quenching their star formation, possibly but not necessarily under the influence of gas-removing environmental processes. The other greater than or similar to 40% of the red SF galaxies have high IR-to-UV luminosity ratios, indicative of high dust obscuration. They have relatively high specific SFRs and are more abundant at intermediate densities. Our results indicate that while there is an overall suppression in the SF galaxy fraction with density, the small amount of star formation surviving the cluster environment is to a large extent obscured, suggesting that environmental interactions trigger a phase of obscured star formation, before complete quenching.

STAGES: the Space Telescope A901/2 Galaxy Evolution Survey

We present an overview of the Space Telescope A901/2 Galaxy Evolution Survey (STAGES). STAGES is a multiwavelength project designed to probe physical drivers of galaxy evolution across a wide range of environments and luminosity. A complex multicluster system at z similar to 0.165 has been the subject of an 80-orbit F606W Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) mosaic covering the full 0 degrees.5 x 0 degrees.5 (similar to 5 x 5 Mpc(2)) span of the supercluster. Extensive multiwavelength observations with XMM-Newton, GALEX, Spitzer, 2dF, Giant Metrewave Radio Telescope and the 17-band COMBO-17 photometric redshift survey complement the HST imaging. Our survey goals include simultaneously linking galaxy morphology with other observables such as age, star formation rate, nuclear activity and stellar mass. In addition, with the multiwavelength data set and new high-resolution mass maps from gravitational lensing, we are able to disentangle the large-scale structure of the system. By examining all aspects of an environment we will be able to evaluate the relative importance of the dark matter haloes, the local galaxy density and the hot X-ray gas in driving galaxy transformation. This paper describes the HST imaging, data reduction and creation of a master catalogue. We perform the Sersic fitting on the HST images and conduct associated simulations to quantify completeness. In addition, we present the COMBO-17 photometric redshift catalogue and estimates of stellar masses and star formation rates for this field. We define galaxy and cluster sample selection criteria, which will be the basis for forthcoming science analyses, and present a compilation of notable objects in the field. Finally, we describe the further multiwavelength observations and announce public access to the data and catalogues.

GHRS Monitoring of the Outflowing Material in NGC 4151

We present the results of a program to monitor the absorption lines in the spectrum of the Seyfert 1 galaxy NGC 4151 caused by outflowing gas from the nucleus. Spectral observations were taken in the UV with the GHRS and FOS of HST. GHRS G160M and G270M exposures were taken of the wavelength regions centered on C IV and Mg II, respectively, extending over a nearly 4 yr period. Fits to the observations at all epochs but one require at least eight distinct components spanning a range of outflow velocities from 0 to ~1600 km s-1 with respect to the nucleus. (NGC 4151 has a redshift of ~1000 km s-1). Although we see subtle changes in GHRS spectra of the broader of the absorption features, the wavelength constancy of all the features is remarkable. The limits on the secular acceleration suggest that either (1) The absorbing clouds are well beyond the broad emission line region; (2) The clouds are experiencing significant drag from an intercloud medium; or (3) We are observing an extremely complex flow pattern rather than the motions of individual clouds. The exception to this constancy occurred during one of the epochs of our monitoring when a broad shallow C IV trough appeared at an outflow velocity of 3750 km s-1 and then subsequently disappeared. An archival G160M exposure taken by Ulrich et al. was studied in order to measure the population of fine structure levels of Si II via the lines at 1526 and 1533 A. The ratio of the upper to lower state appears to be less than the ratio of statistical weights, though additional observations are needed to secure this. This ratio is used to set upper limits on the electron density and lower limits on the distance of one of the absorbing clouds. Simultaneous FOS observations, covering a wider range of lines, but at a lower resolution, show significant changes in the Si IV absorption profile. We discuss the NGC 4151 absorption phenomenon in light of other observations of intrinsic absorption systems in both Seyfert nuclei and quasars.

The environmental dependence of the stellar-mass-size relation in STAGES galaxies

We present the stellar-mass-size relations for elliptical, lenticular and spiral galaxies in the field and cluster environments using Hubble Space Telescope/Advanced Camera for Surveys imaging and data from the Space Telescope A901/2 Galaxy Evolution Survey. We use a large sample of ~1200 field and cluster galaxies and a sub-sample of cluster core galaxies, and quantify the significance of any putative environmental dependence on the stellar-mass-size relation. For elliptical, lenticular and high-mass (log M * /M ⊙ > 10) spiral galaxies we find no evidence to suggest any such environmental dependence, implying that internal drivers are governing their size evolution. For intermediate-/low-mass spirals (log M * /M⊙ < 10) we find evidence, significant at the 2σ level, for a possible environmental dependence on galaxy sizes: the mean effective radius ā e for lower mass spirals is ~15-20 per cent larger in the field than in the cluster. This is due to a population of low-mass large-a e field spirals that are largely absent from the cluster environments. These large-a e field spirals contain extended stellar discs not present in their cluster counterparts. This suggests that the fragile extended stellar discs of these spiral galaxies may not survive the environmental conditions in the cluster. Our results suggest that internal physical processes are the main drivers governing the size evolution of galaxies, with the environment possibly playing a role affecting only the discs of intermediate-/low-mass spirals.

Cosmological constraints from COMBO‐17 using 3D weak lensing

We present the first application of the {3D} cosmic shear method developed in Heavens, Kitching \& Taylor and the geometric shear-ratio analysis developed in Taylor et al., to the {COMBO-17} data set. {3D} cosmic shear has been used to analyse galaxies with redshift estimates from two random {COMBO-17} fields covering 0.52 deg2 in total, providing a conditional constraint in the (??8, ??m) plane as well as a conditional constraint on the equation of state of dark energy, parametrized by a constant w ??? pde/??dec2. The (??8, ??m) plane analysis constrained the relation between ??8 and ??m to be ??8(??m/0.3)0.57+/-0.19 = 1.06+0.17-0.16, in agreement with a {2D} cosmic shear analysis of {COMBO-17.} The {3D} cosmic shear conditional constraint on w using the two random fields is w = -1.27+0.64-0.70. The geometric shear-ratio analysis has been applied to the A901/2 field, which contains three small galaxy clusters. Combining the analysis from the A901/2 field, using the geometric shear-ratio analysis, and the two random fields, using {3D} cosmic shear, w is conditionally constrained to w = -1.08+0.63-0.58. The errors presented in this paper are shown to agree with Fisher matrix predictions made in Heavens, Kitching \& Taylor and Taylor et al. When these methods are applied to large data sets, as expected soon from surveys such as {Pan-STARRS} and {VST-KIDS}, the dark energy equation of state could be constrained to an unprecedented degree of accuracy.