arXiv subjects

R. Massey

Publications and source records attributed to R. Massey.

At least 145 records · Page 8Linked to original sources

arXiv2011

The impact of galaxy colour gradients on cosmic shear measurement

Cosmic shear has been identified as the method with the most potential to constrain dark energy. To capitalise on this potential it is necessary to measure galaxy shapes with great accuracy, which in turn requires a detailed model for the image blurring, the Point Spread Function (PSF). In general the PSF varies with wavelength and therefore the PSF integrated over an observing filter depends on the spectrum of the object. For a typical galaxy the spectrum varies across the galaxy image, thus the PSF depends on the position within the image. We estimate the bias on the shear due to such colour gradients by modelling galaxies using two co-centered, co-elliptical Sersic profiles, each with a different spectrum. We estimate the effect of ignoring colour gradients and find the shear bias from a single galaxy can be very large depending on the properties of the galaxy. We find that halving the filter width reduces the shear bias by a factor of about 5. We show that, to first order, tomographic cosmic shear two point statistics depend on the mean shear bias over the galaxy population at a given redshift. For a single broad filter, and averaging over a small galaxy catalogue, we find a mean shear bias which is subdominant to the predicted statistical errors for future cosmic shear surveys. However, the true mean shear bias may exceed the statistical errors, depending on how accurately the catalogue represents the observed distribution of galaxies in the cosmic shear survey. We then investigate the bias on the shear for two-filter imaging and find that the bias is reduced by at least an order of magnitude. Lastly, we find that it is possible to calibrate galaxies for which colour gradients were ignored using two-filter imaging of a fair sample of noisy galaxies, if the galaxy model is known. For a S/N of 25 the number of galaxies required in each tomographic redshift bin is of order 1e4.

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arXiv2010

Bars in early- and late-type disks in COSMOS

We investigate the (large-scale) bar fraction in a mass-complete sample of M > 10^10.5 Msun disk galaxies at 0.2 < z < 0.6 in the COSMOS field. The fraction of barred disks strongly depends on mass, disk morphology, and specific star formation rate (SSFR). At intermediate stellar mass (10^10.5 < M < 10^11 Msun) the bar fraction in early-type disks is much higher, at all redshifts, by a factor ~2, than that in late-type disks. This trend is reversed at higher stellar mass (M > 10^11 Msun), where the fraction of bars in early-type disks becomes significantly lower, at all redshifts, than that in late-type disks. The bar fractions for galaxies with low and high SSFRs closely follow those of the morphologically-selected early-type and late-type populations, respectively. This indicates a close correspondence between morphology and SSFR in disk galaxies at these earlier epochs. Interestingly, the total bar fraction in 10^10.5 < M < 10^11 Msun disks is built up by a factor of ~2 over the redshift interval explored, while for M > 10^11 Msun disks it remains roughly constant. This indicates that, already by z ~ 0.6, spectral and morphological transformations in the most massive disk galaxies have largely converged to the familiar Hubble sequence that we observe in the local Universe, while for intermediate mass disks this convergence is ongoing until at least z ~ 0.2. Moreover, these results highlight the importance of employing mass-limited samples for quantifying the evolution of barred galaxies. Finally, the evolution of the barred galaxy populations investigated does not depend on the large-scale environmental density (at least, on the scales which can be probed with the available photometric redshifts).

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arXiv2010

The Opacity of Galactic Disks at z~0.7

We compare the surface brightness-inclination relation for a sample of COSMOS pure disk galaxies at z~0.7 with an artificially redshifted sample of SDSS disks well matched to the COSMOS sample in terms of rest-frame photometry and morphology, as well as their selection and analysis. The offset between the average surface brightness of face-on and edge-on disks in the redshifted SDSS sample matches that predicted by measurements of the optical depth of galactic disks in the nearby universe. In contrast, large disks at z~0.7 have a virtually flat surface brightness-inclination relation, suggesting that they are more opaque than their local counterparts. This could be explained by either an increased amount of optically thick material in disks at higher redshift, or a different spatial distribution of the dust.

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arXiv2010

Radio galaxy feedback in X-ray selected groups from COSMOS: the effect on the ICM

We quantify the importance of the mechanical energy released by radio-galaxies inside galaxy groups. We use scaling relations to estimate the mechanical energy released by 16 radio-AGN located inside X-ray detected galaxy groups in the COSMOS field. By comparing this energy output to the host groups' gravitational binding energy, we find that radio galaxies produce sufficient energy to unbind a significant fraction of the intra-group medium. This unbinding effect is negligible in massive galaxy clusters with deeper potential wells. Our results correctly reproduce the breaking of self-similarity observed in the scaling relation between entropy and temperature for galaxy groups.

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arXiv2009

A Weak Lensing Study of X-ray Groups in the COSMOS survey: Form and Evolution of the Mass-Luminosity Relation

Measurements of X-ray scaling laws are critical for improving cosmological constraints derived with the halo mass function and for understanding the physical processes that govern the heating and cooling of the intracluster medium. In this paper, we use a sample of 206 X-ray selected galaxy groups to investigate the scaling relation between X-ray luminosity (Lx) and halo mass (M00) where M200 is derived via stacked weak gravitational lensing. This work draws upon a broad array of multi-wavelength COSMOS observations including 1.64 square degrees of contiguous imaging with the Advanced Camera for Surveys (ACS) and deep XMM-Newton/Chandra imaging. The combined depth of these two data-sets allows us to probe the lensing signals of X-ray detected structures at both higher redshifts and lower masses than previously explored. Weak lensing profiles and halo masses are derived for nine sub-samples, narrowly binned in luminosity and redshift. The COSMOS data alone are well fit by a power law, M200 ~ Lx^a, with a slope of a=0.66+-0.14. These results significantly extend the dynamic range for which the halo masses of X-ray selected structures have been measured with weak gravitational lensing. As a result, tight constraints are obtained for the slope of the M-Lx relation. The combination of our group data with previously published cluster data demonstrates that the M-Lx relation is well described by a single power law, a=0.64+-0.03, over two decades in mass, 10^13.5-10^15.5 h72^-1 Msun. These results are inconsistent at the 3.7 level with the self-similar prediction of a=0.75. We examine the redshift dependence of the M-Lx relation and find little evidence for evolution beyond the rate predicted by self-similarity from z ~ 0.25 to z ~ 0.8.

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arXiv2009

The Build-Up of the Hubble Sequence in the COSMOS Field

We use ~8,600 >5e10 Msol COSMOS galaxies to study how the morphological mix of massive ellipticals, bulge-dominated disks, intermediate-bulge disks, bulge-less disks and irregular galaxies evolves from z=0.2 to z=1. The morphological evolution depends strongly on mass. At M>3e11 Msol, no evolution is detected in the morphological mix: ellipticals dominate since z=1, and the Hubble sequence has quantitatively settled down by this epoch. At the 1e11 Msol mass scale, little evolution is detected, which can be entirely explained with major mergers. Most of the morphological evolution from z=1 to z=0.2 takes place at masses 5e10 - 1e11 Msol, where: (i) The fraction of spirals substantially drops and the contribution of early-types increases. This increase is mostly produced by the growth of bulge-dominated disks, which vary their contribution from ~10% at z=1 to >30% at z=0.2 (cf. the elliptical fraction grows from ~15% to ~20%). Thus, at these masses, transformations from late- to early-types result in disk-less elliptical morphologies with a statistical frequency of only 30% - 40%. Otherwise, the processes which are responsible for the transformations either retain or produce a non-negligible disk component. (ii) The bulge-less disk galaxies, which contribute ~15% to the intermediate-mass galaxy population at z=1, virtually disappear by z=0.2. The merger rate since z=1 is too low to account for the disappearance of these massive bulge-less disks, which most likely grow a bulge via secular evolution.

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arXiv2009

Stellar and total baryon mass fractions in groups and clusters since redshift 1

We investigate if the discrepancy between estimates of the total baryon mass fraction obtained from observations of the cosmic microwave background (CMB) and of galaxy groups/clusters persists when a large sample of groups is considered. To this purpose, 91 candidate X-ray groups/poor clusters at redshift 0.1 < z < 1 are selected from the COSMOS 2 deg^2 survey, based only on their X-ray luminosity and extent. This sample is complemented by 27 nearby clusters with a robust, analogous determination of the total and stellar mass inside R_500. The total sample of 118 groups and clusters with z < 1 spans a range in M_500 of ~10^13--10^15 M_sun. We find that the stellar mass fraction associated with galaxies at R_500 decreases with increasing total mass as (M_500)^-0.37 \pm 0.04, independent of redshift. Estimating the total gas mass fraction from a recently derived, high quality scaling relation, the total baryon mass fraction (f_500^stars+gas=f_500^stars+f_500^gas) is found to increase by ~ 25% when M_500 increases from =5 X 10^13 M_sun to = 7 X 10^14 M_sun. After consideration of a plausible contribution due to intra--cluster light (11--22% of the total stellar mass), and gas depletion through the hierarchical assembly process (10% of the gas mass), the estimated values of the total baryon mass fraction are still lower than the latest CMB measure of the same quantity (WMAP5), at a significance level of 3.3σfor groups of =5 X 10^13~M_sun. The discrepancy decreases towards higher total masses, such that it is 1σat = 7 X 10^14~M_sun. We discuss this result in terms of non--gravitational processes such as feedback and filamentary heating.

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arXiv2009

On the contribution of large scale structure to strong gravitational lensing

We study the correlation between the locations of galaxy-galaxy strong lensing candidates and tracers of large-scale structure from both weak lensing or X-ray emission. The COSMOS survey is a unique data set, combining deep, high resolution and contiguous imaging in which strong lenses have been discovered, plus unparalleled multiwavelength coverage. To help interpret the COSMOS data, we have also produced mock COSMOS strong and weak lensing observations, based on ray-tracing through the Millenium simulation. In agreement with the simulations, we find that strongly lensed images with the largest angular separations are found in the densest regions of the COSMOS field. This is explained by a prevalence among the lens population in dense environments of elliptical galaxies with high total-to-stellar mass ratios, which can deflect light through larger angles. However, we also find that the overall fraction of elliptical galaxies with strong gravitational lensing is independent of the local mass density; this observation is not true of the simulations, which predict an increasing fraction of strong lenses in dense environments. The discrepancy may be a real effect, but could also be explained by various limitations of our analysis. For example, our visual search of strong lens systems could be incomplete and suffer from selection bias; the luminosity function of elliptical galaxies may differ between our real and simulated data; or the simplifying assumptions and approximations used in our lensing simulations may be inadequate. Work is therefore ongoing. Automated searches for strong lens systems will be particularly important in better constraining the selection function.

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arXiv2008

AGN Host Galaxy Morphologies in COSMOS

We use HST/ACS images and a photometric catalog of the COSMOS field to analyze morphologies of the host galaxies of approximately 400 AGN candidates at redshifts 0.3 < z < 1.0. We compare the AGN hosts with a sample of non-active galaxies drawn from the COSMOS field to match the magnitude and redshift distribution of the AGN hosts. We perform 2-D surface brightness modeling with GALFIT to yield host galaxy and nuclear point source magnitudes. X-ray selected AGN host galaxy morphologies span a substantial range that peaks between those of early-type, bulge-dominated and late-type, disk-dominated systems. We also measure the asymmetry and concentration of the host galaxies. Unaccounted for, the nuclear point source can significantly bias results of these measured structural parameters, so we subtract the best-fit point source component to obtain images of the underlying host galaxies. Our concentration measurements reinforce the findings of our 2-D morphology fits, placing X-ray AGN hosts between early- and late-type inactive galaxies. AGN host asymmetry distributions are consistent with those of control galaxies. Combined with a lack of excess companion galaxies around AGN, the asymmetry distributions indicate that strong interactions are no more prevalent among AGN than normal galaxies. In light of recent work, these results suggest that the host galaxies of AGN at these X-ray luminosities may be in a transition from disk-dominated to bulge-dominated, but that this transition is not typically triggered by major mergers.

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arXiv2008

Color, 3D simulated images with shapelets

We present a method to simulate color, 3-dimensional images taken with a space-based observatory by building off of the established shapelets pipeline. The simulated galaxies exhibit complex morphologies, which are realistically correlated between, and include, known redshifts. The simulations are created using galaxies from the 4 optical and near-infrared bands (B, V, i and z) of the Hubble Ultra Deep Field (UDF) as a basis set to model morphologies and redshift. We include observational effects such as sky noise and pixelization and can add astronomical signals of interest such as weak gravitational lensing. The realism of the simulations is demonstrated by comparing their morphologies to the original UDF galaxies and by comparing their distribution of ellipticities as a function of redshift and magnitude to wider HST COSMOS data. These simulations have already been useful for calibrating multicolor image analysis techniques and for better optimizing the design of proposed space telescopes.

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arXiv2007

A combined analysis of 3D Weak Lensing, Lyman-alpha forest and WMAP year three data

We present constraints on the amplitude and shape of the matter power spectrum and the density of dark matter within the framework of a standard LambdaCDM model. We use a Markov Chain Monte Carlo approach to combine independent measurements of the three dimensional weak gravitational lensing shear field by the COSMOS survey, of low and high resolution Ly-alpha forest flux power spectrum by SDSS and LUQAS, and of Cosmic Microwave Background temperature and polarization anisotropies by WMAP. We note good agreement between the amplitude of the matter power spectrum on intermediate and small scales as inferred from Ly-alpha forest and lensing data. The Ly-alpha forest data helps to break the sigma_8-Omega_m degeneracy characteristic of weak lensing results, yielding sigma_8 = 0.876 +- 0.048 for COSMOS plus Ly-alpha SDSS data. This is somewhat larger than the value preferred by the WMAP year three CMB data. Combining all three data sets significantly tightens the constraints on sigma_8, the spectral index of primordial density fluctuation n_s, a possible running of the spectral index n_run and the matter density Omega_m. Assuming no running, the joint constraints for COSMOS, SDSS and WMAP are sigma_8 = 0.800 +- 0.023, n_s = 0.971 +- 0.011, Omega_m = 0.247 +- 0.016 (1-sigma error bars).

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arXiv2007

The H$α$ Luminosity Function and Star Formation Rate at $z \approx 0.24$ in the Cosmos 2 Square-Degree Field

To derive a new H$α$ luminosity function and to understand the clustering properties of star-forming galaxies at $z \approx 0.24$, we have made a narrow-band imaging survey for H$α$ emitting galaxies in the HST COSMOS 2 square degree field. We used the narrow-band filter NB816 ($λ_c = 8150$ Å, $Δλ= 120$ Å) and sampled H$α$ emitters with $EW_{\rm obs}(\rm Hα+ [N\textsc{ii}]) > 12$ Åin a redshift range between $z=0.233$ and $z=0.251$ corresponding to a depth of 70 Mpc. We obtained 980 H$α$ emitting galaxies in a sky area of 5540 arcmin$^2$, corresponding to a survey volume of $3.1 \times 10^4 {\rm Mpc^3}$. We derive a H$α$ luminosity function with a best-fit Schechter function parameter set of $α= -1.35^{+0.11}_{-0.13}$, $\logϕ_* = -2.65^{+0.27}_{-0.38}$, and $\log L_* ({\rm erg s^{-1}}) = 41.94^{+0.38}_{-0.23}$. The H$α$ luminosity density is $2.7^{+0.7}_{-0.6} \times 10^{39}$ ergs s$^{-1}$ Mpc$^{-3}$. After subtracting the AGN contribution (15 %) to the H$α$ luminosity density, the star formation rate density is evaluated as $1.8^{+0.7}_{-0.4} \times 10^{-2}$ $M_{\sun}$ yr$^{-1}$ Mpc$^{-3}$. The angular two-point correlation function of H$α$ emitting galaxies of $\log L({\rm Hα}) > 39.8$ is well fit by a power law form of $w(θ) = 0.013^{+0.002}_{-0.001} θ^{-0.88 \pm 0.03}$, corresponding to the correlation function of $ξ(r) = (r/1.9{\rm Mpc})^{-1.88}$. We also find that the H$α$ emitters with higher H$α$ luminosity are more strongly clustered than those with lower luminosity.

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arXiv2007

The angular correlations of galaxies in the COSMOS field

We present measurements of the two-point galaxy angular correlation function w(θ) in the COSMOS field. Independent determinations of w(θ) as a function of magnitude limit are presented for both the HST ACS catalog and also for the ground-based data from Subaru and the CFHT. Despite having significantly different masks, these three determinations agree well. At bright magnitudes (IAB<22), our data generally match very well with existing measurements and with mock catalogs based on semi-analytic galaxy formation calculations of Kitzbichler and White from the Millennium Simulation. The exception is that our result is at the upper end of the expected cosmic variance scatter for θ> 10 arcmin, which we attribute to a particularly rich structure known to exist at z~0.8. For fainter samples, however, the level of clustering is somewhat higher than reported by some previous studies: in all three catalogues we find w(θ=1')~0.014 at a median IAB magnitude of 24. At these very faintest magnitudes, our measurements agree well with the latest determinations from the Canada-France Legacy Survey. This level of clustering is approximately double what is predicted by the semi-analytic catalogs (at all angles). The semi-analytic results allow an estimate of cosmic variance, which is too small to account for the discrepancy. We therefore conclude that the mean amplitude of clustering at this level is higher than previously estimated.

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arXiv2007

The First Release COSMOS Optical and Near-IR Data and Catalog

We present imaging data and photometry for the COSMOS survey in 15 photometric bands between 0.3um and 2.4um. These include data taken on the Subaru 8.3m telescope, the KPNO and CTIO 4m telescopes, and the CFHT 3.6m telescope. Special techniques are used to ensure that the relative photometric calibration is better than 1% across the field of view. The absolute photometric accuracy from standard star measurements is found to be 6%. The absolute calibration is corrected using galaxy spectra, providing colors accurate to 2% or better. Stellar and galaxy colors and counts agree well with the expected values. Finally, as the first step in the scientific analysis of these data we construct panchromatic number counts which confirm that both the geometry of the universe and the galaxy population are evolving.

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arXiv2007

The COSMOS Survey: Hubble Space Telescope / Advanced Camera for Surveys (HST/ACS) Observations and Data Processing

We describe the details of the Hubble Space Telescope (HST) Advanced Camera for Surveys / Wide Field Channel (ACS/WFC) observations of the COSMOS field, including the data calibration and processing procedures. We obtained a total of 583 orbits of HST ACS/WFC imaging in the F814W filter, covering a field that is 1.64 square degrees in area, the largest contiguous field ever imaged with HST. The median exposure depth across the field is 2028 seconds (one HST orbit), achieving a limiting point-source depth AB(F814W) = 27.2 (5 sigma). We also present details about the astrometric image registration, distortion removal and image combination using MultiDrizzle, as well as motivating the choice of our final pixel scale (30 milliarcseconds per pixel), based on the requirements for weak lensing science. The final set of images are publicly available through the archive sites at IPAC and STScI, along with further documentation on how they were produced.

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arXiv2007

Large Structures and Galaxy Evolution in COSMOS at z < 1.1

We present the first identification of large-scale structures (LSS) at z $< 1.1$ in the Cosmic Evolution Survey (COSMOS). The structures are identified from adaptive smoothing of galaxy counts in the pseudo-3d space ($α,δ$,z) using the COSMOS photometric redshift catalog. The technique is tested on a simulation including galaxies distributed in model clusters and a field galaxy population -- recovering structures on all scales from 1 to 20\arcmin without {\it a priori} assumptions for the structure size or density profile. Our procedure makes {\bf no} {\it a priori} selection on galaxy spectral energy distribution (SED, for example the Red Sequence), enabling an unbiased investigation of environmental effects on galaxy evolution. The COSMOS photometric redshift catalog yields a sample of $1.5\times10^5$ galaxies with redshift accuracy, $Δz_{FWHM}/(1+z) \leq 0.1$ at z $< 1.1$ down to I$_{AB} \leq 25$ mag. Using this sample of galaxies, we identify 42 large-scale structures and clusters. abstract truncated for astroph 25 line limit -- see preprint

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arXiv2007

COSMOS : Hubble Space Telescope Observations

The Cosmic Evolution Survey (COSMOS) was initiated with an extensive allocation (590 orbits in Cycles 12-13) using the Hubble Space Telescope (HST) for high resolution imaging. Here we review the characteristics of the HST imaging with the Advanced Camera for Surveys (ACS) and parallel observations with NICMOS and WFPC2. A square field (1.8$\sq$°) has been imaged with single-orbit ACS I-F814W exposures with 50% completeness for sources 0.5\arcsec in diameter at I$_{AB} $ = 26.0 mag. The ACS imaging is a key part of the COSMOS survey, providing very high sensitivity and high resolution (0.09\arcsec FWHM, 0.05\arcsec pixels) imaging and detecting 1.2 million objects to a limiting magnitude of 26.5 (AB). These images yield resolved morphologies for several hundred thousand galaxies. The small HST PSF also provides greatly enhanced sensitivity for weak lensing investigations of the dark matter distribution.

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arXiv2007

Weak Gravitational Lensing with COSMOS: Galaxy Selection and Shape Measurements

With a primary goal of conducting precision weak lensing measurements from space, the COSMOS survey has imaged the largest contiguous area observed by the Hubble Space Telescope (HST) to date using the Advanced Camera for Surveys (ACS). This is the first paper in a series where we describe our strategy for addressing the various technical challenges in the production of weak lensing measurements from the COSMOS data. The COSMOS ACS catalog is constructed from 575 ACS/WFC tiles (1.64 deg^2) and contains a total 1.2x10^6 objects to a limiting magnitude of F814W=26.5. This catalog is made publicly available. The shapes of galaxies have been measured and corrected for the distortion induced by the time varying ACS Point Spread Function and for Charge Transfer Efficiency effects. Next, simulated images are used to derive the shear susceptibility factors that are necessary in order to transform shape measurements into unbiased shear estimators. Finally, for each galaxy, we derive a shape measurement error and utilize this quantity to extract the intrinsic shape noise of the galaxy sample. Interestingly, our results indicate that the intrinsic shape noise varies little with either size, magnitude or redshift. Representing a number density of 66 galaxies per arcmin^2, the final COSMOS weak lensing catalog contains 3.9x10^5 galaxies with accurate shape measurements. The properties of the COSMOS weak lensing catalog described throughout this paper will provide key input numbers for the preparation and design of next-generation wide field space missions.

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