We present and describe IM3SHAPE, a new publicly available galaxy shape measurement code for weak... more We present and describe IM3SHAPE, a new publicly available galaxy shape measurement code for weak gravitational lensing shear. IM3SHAPE performs a maximum likelihood fit of a bulgeplus-disc galaxy model to noisy images, incorporating an applied point spread function. We detail challenges faced and choices made in its design and implementation, and then discuss various limitations that affect this and other maximum likelihood methods. We assess the bias arising from fitting an incorrect galaxy model using simple noise-free images and find that it should not be a concern for current cosmic shear surveys. We test IM3SHAPE on the GREAT08 Challenge image simulations, and meet the requirements for upcoming cosmic shear surveys in the case that the simulations are encompassed by the fitted model, using a simple correction for image noise bias. For the fiducial branch of GREAT08 we obtain a negligible additive shear bias and sub-two percent level multiplicative bias, which is suitable for analysis of current surveys. We fall short of the sub-percent level requirement for upcoming surveys, which we attribute to a combination of noise bias and the mis-match between our galaxy model and the model used in the GREAT08 simulations. We meet the requirements for current surveys across all branches of GREAT08, except those with small or high noise galaxies, which we would cut from our analysis. Using the GREAT08 metric we we obtain a score of Q=717 for the usable branches, relative to the goal of Q=1000 for future experiments. The code is freely available from https://bitbucket.org/joezuntz/im3shape.
Monthly Notices of the Royal Astronomical Society, Sep 1, 2007
We derive expressions, in terms of `polar shapelets', for the image distortion operations associa... more We derive expressions, in terms of `polar shapelets', for the image distortion operations associated with weak gravitational lensing. Shear causes galaxy shapes to become elongated, and is sensitive to the second derivative of the projected gravitational potential along their line of sight; flexion bends galaxy shapes into arcs, and is sensitive to the third derivative. Polar shapelets provide a natural representation, in which both shear and flexion transformations are compact. Through this tool, we understand progress in several weak lensing methods. We then exploit various symmetries of shapelets to construct a range of shear estimators with useful properties. Through an analogous investigation, we also explore several flexion estimators. In particular, some of the estimators can be measured simultaneously and independently for every galaxy, and will provide unique checks for systematics in future weak lensing analyses. Using simulated images from the Shear TEsting Programme, we show that we can recover input shears with no significant bias. A complete software package to parametrize astronomical images in terms of polar shapelets, and to perform a full weak lensing analysis, is available on the Internet.
High-precision cosmology with weak gravitational lensing requires a precise measure of the point ... more High-precision cosmology with weak gravitational lensing requires a precise measure of the point spread function across the imaging data where the accuracy to which high spatial frequency variation can be modelled is limited by the stellar number density across the field. We analyse dense stellar fields imaged at the Canada-France-Hawaii Telescope to quantify the degree of high spatial frequency variation in ground-based imaging point spread functions and compare our results to models of atmospheric turbulence. The data show an anisotropic turbulence pattern with an orientation independent of the wind direction and wind speed. We find the amplitude of the high spatial frequencies to decrease with increasing exposure time as t-1/2, and find a negligibly small atmospheric contribution to the point spread function ellipticity variation for exposure times t > 180 s. For future surveys analysing shorter exposure data, this anisotropic turbulence will need to be taken into account as the amplitude of the correlated atmospheric distortions becomes comparable to a cosmological lensing signal on scales less than ˜10 arcmin. This effect could be mitigated, however, by correlating galaxy shear measured on exposures imaged with a time separation greater than 50 s, for which we find the spatial turbulence patterns to be uncorrelated.
We construct a simple, spherical blastwave model to estimate the pressure structure of the interg... more We construct a simple, spherical blastwave model to estimate the pressure structure of the intergalactic medium surrounding hyper-starburst galaxies, and argue that the effects of interaction with star-forming galaxy winds may be approximated at early times by an adiabatically expanding, self-similar `bubble' as described by Weaver et al. (1977) and Ostriker & McKee (1988). This model is used to make observational predictions for the thermal Sunyaev-Zel'dovich effect in the shocked bubble plasma. Radiative cooling losses are explored, and it is found that bremsstrahlung will limit the epoch of adiabatic expansion to $10^7$--$10^8$ years: comparable to total hyper-starburst lifetimes. Prospects for making a first Sunyaev-Zel'dovich detection of galaxy wind bubbles using the Atacama Large Millimeter Array are examined for a number of active hyper-starburst sources in the literature.
Publications of the Astronomical Society of the Pacific, Aug 18, 2013
Weak gravitational lensing observations are a key science driver for the NASA Wide Field Infrared... more Weak gravitational lensing observations are a key science driver for the NASA Wide Field Infrared Survey Telescope (WFIRST). To validate the performance of the WFIRST infrared detectors, we have performed a laboratory emulation of weak gravitational lensing measurements. Our experiments used a custom precision projector system to image a target mask composed of a grid of pinholes, emulating stellar point sources, onto a 1.7 micron cut-off Teledyne HgCdTe/H2RG detector. We used a 880nm LED illumination source and f/22 pupil stop to produce undersampled point spread functions similar to those expected from WFIRST. We also emulated the WFIRST image reconstruction strategy, using the IMage COMbination (IMCOM) algorithm to derive oversampled images from dithered, undersampled input images. We created shear maps for this data and computed shear correlation functions to mimic a real weak lensing analysis. After removing only 2nd order polynomial fits to the shear maps, we found that the correlation functions could be reduced to O(10^-6). This places a conservative upper limit on the detector-induced bias to the correlation function (under our test conditions). This bias is two orders of magnitude lower than the expected weak lensing signal. Restricted to scales relevant to dark energy analyses (sky separations > 0.5 arcmin), the bias is O(10^-7): comparable to the requirement for future weak lensing missions to avoid biasing cosmological parameter estimates. Our experiment will need to be upgraded and repeated under different configurations to fully characterize the shape measurement performance of WFIRST IR detectors.
Monthly Notices of the Royal Astronomical Society, 2015
We present first results from the third GRavitational lEnsing Accuracy Testing (GREAT3) challenge... more We present first results from the third GRavitational lEnsing Accuracy Testing (GREAT3) challenge, the third in a sequence of challenges for testing methods of inferring weak gravitational lensing shear distortions from simulated galaxy images. GREAT3 was divided into experiments to test three specific questions, and included simulated space-and ground-based data with constant or cosmologically-varying shear fields. The simplest (control) experiment included parametric galaxies with a realistic distribution of signal-to-noise, size, and ellipticity, and a complex point spread function (PSF). The other experiments tested the additional impact of realistic galaxy morphology, multiple exposure imaging, and the uncertainty about a spatially-varying PSF; the last two questions will be explored in Paper II. The 24 participating teams competed to estimate lensing shears to within systematic error tolerances for upcoming Stage-IV dark energy surveys, making 1525 submissions overall. GREAT3 saw considerable variety and innovation in the types of methods applied. Several teams now meet or exceed the targets in many of the tests conducted (to within the statistical errors). We conclude that the presence of realistic galaxy morphology in simulations changes shear calibration biases by ∼ 1 per cent for a wide range of methods. Other effects such as truncation biases due to finite galaxy postage stamps, and the impact of galaxy type as measured by the Sérsic index, are quantified for the first time. Our results generalize previous studies regarding sensitivities to galaxy size and signal-to-noise, and to PSF properties such as seeing and defocus. Almost all methods' results support the simple model in which additive shear biases depend linearly on PSF ellipticity.
ABSTRACT Interaction of charges in CCDs with the already accumulated charge distribution causes b... more ABSTRACT Interaction of charges in CCDs with the already accumulated charge distribution causes both a flux dependence of the point-spread function (an increase of observed size with flux, also known as the brighter/fatter effect) and pixel-to-pixel correlations of the noise in flat fields. We describe these effects in the Dark Energy Camera (DECam) with charge dependent shifts of effective pixel borders, i.e. the Antilogus et al. (2014) model, which we fit to measurements of flat-field noise correlations. The latter fall off approximately as a power-law r^-2.5 with pixel separation r, are isotropic except for an asymmetry in the direct neighbors along rows and columns, are stable in time, and are weakly dependent on wavelength. They show variations from chip to chip at the 20% level that correlate with the silicon resistivity. The charge shifts predicted by the model cause biased shape measurements, primarily due to their effect on bright stars, at levels exceeding weak lensing science requirements. We measure the flux dependence of star images and show that the effect can be mitigated by applying the reverse charge shifts at the pixel level during image processing. Differences in stellar size, however, remain significant due to residuals at larger distance from the centroid.
Monthly Notices of the Royal Astronomical Society, 2014
We present the cluster mass-richness scaling relation calibrated by a weak lensing analysis of > ... more We present the cluster mass-richness scaling relation calibrated by a weak lensing analysis of > ∼ 18,000 galaxy cluster candidates in the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS). Detected using the 3D-Matched-Filter cluster-finder of Milkeraitis et al., these cluster candidates span a wide range of masses, from the small group scale up to ∼ 10 15 M ⊙ , and redshifts 0.2 z 0.9. The total significance of the shear measurement amounts to 54σ. We compare cluster masses determined using weak lensing shear and magnification, finding the measurements in individual richness bins to yield 1σ compatibility, but with magnification estimates biased low. This first direct mass comparison yields important insights for improving the systematics handling of future lensing magnification work. In addition, we confirm analyses that suggest cluster miscentring has an important effect on the observed 3D-Matched-Filter halo profiles, and we quantify this by fitting for projected cluster centroid offsets, which are typically ∼ 0.4 arcmin. We bin the cluster candidates as a function of redshift, finding similar cluster masses and richness across the full range up to z ∼ 0.9. We measure the 3D-MF mass-richness scaling relation M 200 = M 0 (N 200 /20) β . We find a normalization M 0 ∼ (2.7 +0.5 −0.4 ) × 10 13 M ⊙ , and a logarithmic slope of β ∼ 1.4 ± 0.1, both of which are in 1σ agreement with results from the magnification analysis. We find no evidence for a redshift-dependence of the normalization. The CFHTLenS 3D-Matched-Filter cluster catalogue is now available at cfhtlens.org.
Monthly Notices of the Royal Astronomical Society, 2015
We measure the weak-lensing masses and galaxy distributions of four massive galaxy clusters obser... more We measure the weak-lensing masses and galaxy distributions of four massive galaxy clusters observed during the Science Verification phase of the Dark Energy Survey with the purpose of 1) validating the DECam imager for the task of measuring weak-lensing shapes, and 2) utilizing DECam's large field of view to map out the clusters and their environments over 90 arcmin. We conduct a series of rigorous tests on astrometry, photometry, image quality, PSF modeling, and shear measurement accuracy to single out flaws in the data and also to identify the optimal data processing steps and parameters. We find Science Verification data from DECam to be suitable for lensing analyses. The PSF is generally well-behaved, but the modeling is rendered difficult by a flux-dependent PSF width. We employ photometric redshifts to distinguish between foreground and background galaxies, and a red-sequence cluster finder to provide cluster richness estimates and cluster-galaxy distributions. By fitting NFW profiles to the clusters in this study, we determine weak-lensing masses that are in agreement with previous work. For Abell 3261, we provide the first estimates of redshift, weak-lensing mass, and richness. In addition, the cluster-galaxy distributions indicate the existence of filaments attached to 1E 0657-56 and RXC J2248.7-4431, stretching out as far as 1 degree (approximately 20 Mpc), showcasing the potential of DECam and DES for detailed studies of degree-scale features on the sky.
Monthly Notices of the Royal Astronomical Society, 2015
We present weak lensing and X-ray analysis of 12 low mass clusters from the CFHTLenS and XMM-CFHT... more We present weak lensing and X-ray analysis of 12 low mass clusters from the CFHTLenS and XMM-CFHTLS surveys. We combine these systems with high-mass systems from CCCP and low-mass systems from COSMOS to obtain a sample of 70 systems, which we divide into subsamples of 15 merging and 55 relaxed systems. We measure L X -T X , M-L X and M-T X scaling relations and find in all cases that the power-law slopes of the full, merging and relaxed subsamples are consistent. For the M-T X we find slopes consistent with the self-similar model, whereas L X -T X results in steeper and M-L X in flatter relations. We find a marginal trend for larger scatter and lower normalisation in the M-L X and M-T X relations for the merging subsample, which we attribute to triaxiality and substructure. We explore the effects of X-ray cross-calibration and find that Chandra calibration leads to flatter L X -T X and M-T X relations. We also utilise the three surveys making up the sample as overlapping mass bins. For COSMOS and CFHTLS we find slopes consistent with the relation fitted to the full sample, whereas the high mass CCCP sample favours flatter slopes. We also find that intermediate mass systems have a higher mass for their luminosity. Unfortunately our sample does not enable direct measurement of a break at low masses, but we find a trend for enhanced intrinsic scatter in mass at low masses.
Monthly Notices of the Royal Astronomical Society, 2015
We address the problem of separating stars from galaxies in future large photometric surveys. We ... more We address the problem of separating stars from galaxies in future large photometric surveys. We focus our analysis on simulations of the Dark Energy Survey (DES). In the first part of the paper, we derive the science requirements on star/galaxy separation, for measurement of the cosmological parameters with the Gravitational Weak Lensing and Large Scale Structure probes. These requirements are dictated by the need to control both the statistical and systematic errors on the cosmological parameters, and by Point Spread Function calibration. We formulate the requirements in terms of the completeness and purity provided by a given star/galaxy classifier. In order to achieve these requirements at faint magnitudes, we propose a new method for star/galaxy separation in the second part of the paper. We first use Principal Component Analysis to outline the correlations between the objects parameters and extract from it the most relevant information. We then use the reduced set of parameters as input to an Artificial Neural Network. This multi-parameter approach improves upon purely morphometric classifiers (such as the classifier implemented in SExtractor), especially at faint magnitudes: it increases the purity by up to 20% for stars and by up to 12% for galaxies, at i-magnitude fainter than 23.
Monthly Notices of the Royal Astronomical Society, 2015
We present new constraints on the relationship between galaxies and their host dark matter halos,... more We present new constraints on the relationship between galaxies and their host dark matter halos, measured from the location of the peak of the stellar-to-halo mass ratio (SHMR), up to the most massive galaxy clusters at redshift z ∼ 0.8 and over a volume of nearly 0.1 Gpc 3 . We use a unique combination of deep observations in the CFHTLenS/VIPERS field from the near-UV to the near-IR, supplemented by ∼ 60 000 secure spectroscopic redshifts, analysing galaxy clustering, galaxy-galaxy lensing and the stellar mass function. We interpret our measurements within the halo occupation distribution (HOD) framework, separating the contributions from central and satellite galaxies. We find that the SHMR for the central galaxies peaks at M h,peak = 1.9 +0.2 −0.1 × 10 12 M ⊙ with an amplitude of 0.025, which decreases to ∼ 0.001 for massive halos (M h > 10 14 M ⊙ ). Compared to central galaxies only, the total SHMR (including satellites) is boosted by a factor 10 in the high-mass regime (cluster-size halos), a result consistent with cluster analyses from the literature based on fully independent methods. After properly accounting for differences in modelling, we have compared our results with a large number of results from the literature up to z = 1: we find good general agreement, independently of the method used, within the typical stellar-mass systematic errors at low to intermediate mass (M ⋆ < 10 11 M ⊙ ) and the statistical errors above. We have also compared our SHMR results to semi-analytic simulations and found that the SHMR is tilted compared to our measurements in such a way that they over-(under-) predict star formation efficiency in central (satellite) galaxies.
ABSTRACT GALSIM is a collaborative, open-source project aimed at providing an image simulation to... more ABSTRACT GALSIM is a collaborative, open-source project aimed at providing an image simulation tool of enduring benefit to the astronomical community. It provides a software library for generating images of astronomical objects such as stars and galaxies in a variety of ways, efficiently handling image transformations and operations such as convolution and rendering at high precision. We describe the GALSIM software and its capabilities, including necessary theoretical background. We demonstrate that the performance of GALSIM meets the stringent requirements of high precision image analysis applications such as weak gravitational lensing, for current datasets and for the Stage IV dark energy surveys of the Large Synoptic Survey Telescope, ESA&#39;s Euclid mission, and NASA&#39;s WFIRST-AFTA mission. The GALSIM project repository is public and includes the full code history, all open and closed issues, installation instructions, documentation, and wiki pages (including a Frequently Asked Questions section). The GALSIM repository can be found at https://github.com/GalSim-developers/GalSim .
... Within the next few year wide-area survey telescopes will begin imaging huge volumes of deep ... more ... Within the next few year wide-area survey telescopes will begin imaging huge volumes of deep space, with the measurement of the gravitational lensing signal being given high ... Worthy of more than mention are William Brown, Mairi Brookes, Sophie Qureshi and Dan Smith. ...
ABSTRACT The Precision Projector (PP) is a joint project between Caltech and NASA’s Jet Propulsio... more ABSTRACT The Precision Projector (PP) is a joint project between Caltech and NASA’s Jet Propulsion Laboratory (JPL) that tests image sensors for upcoming astronomy missions. Our principal instrument - an Offner-based re-imaging system (a.k.a. “the Projector”) - casts precisely controlled images and spectra onto CCD, CMOS or IR detectors. With known input signals and a model for the output, the measured images allow us to characterize detectors and quantitatively understand their non-idealities. The projector can also emulate astronomical data (e.g. stars and galaxies) in order to assess the impact of the detectors on real data and forecast mission performance. The primary goal of the PP, which began in 2008, is to emulate weak gravitational lensing surveys by projecting galaxy images at the cadence and intensity of the WFIRST mission. We have since expanded our infrastructure and personnel to address other astronomical applications that rely on inferences from small numbers of pixels (e.g. exoplanet searches and adaptive optics) and to include the ability to project spectra.
We present and describe IM3SHAPE, a new publicly available galaxy shape measurement code for weak... more We present and describe IM3SHAPE, a new publicly available galaxy shape measurement code for weak gravitational lensing shear. IM3SHAPE performs a maximum likelihood fit of a bulgeplus-disc galaxy model to noisy images, incorporating an applied point spread function. We detail challenges faced and choices made in its design and implementation, and then discuss various limitations that affect this and other maximum likelihood methods. We assess the bias arising from fitting an incorrect galaxy model using simple noise-free images and find that it should not be a concern for current cosmic shear surveys. We test IM3SHAPE on the GREAT08 Challenge image simulations, and meet the requirements for upcoming cosmic shear surveys in the case that the simulations are encompassed by the fitted model, using a simple correction for image noise bias. For the fiducial branch of GREAT08 we obtain a negligible additive shear bias and sub-two percent level multiplicative bias, which is suitable for analysis of current surveys. We fall short of the sub-percent level requirement for upcoming surveys, which we attribute to a combination of noise bias and the mis-match between our galaxy model and the model used in the GREAT08 simulations. We meet the requirements for current surveys across all branches of GREAT08, except those with small or high noise galaxies, which we would cut from our analysis. Using the GREAT08 metric we we obtain a score of Q=717 for the usable branches, relative to the goal of Q=1000 for future experiments. The code is freely available from https://bitbucket.org/joezuntz/im3shape.
Monthly Notices of the Royal Astronomical Society, Sep 1, 2007
We derive expressions, in terms of `polar shapelets', for the image distortion operations associa... more We derive expressions, in terms of `polar shapelets', for the image distortion operations associated with weak gravitational lensing. Shear causes galaxy shapes to become elongated, and is sensitive to the second derivative of the projected gravitational potential along their line of sight; flexion bends galaxy shapes into arcs, and is sensitive to the third derivative. Polar shapelets provide a natural representation, in which both shear and flexion transformations are compact. Through this tool, we understand progress in several weak lensing methods. We then exploit various symmetries of shapelets to construct a range of shear estimators with useful properties. Through an analogous investigation, we also explore several flexion estimators. In particular, some of the estimators can be measured simultaneously and independently for every galaxy, and will provide unique checks for systematics in future weak lensing analyses. Using simulated images from the Shear TEsting Programme, we show that we can recover input shears with no significant bias. A complete software package to parametrize astronomical images in terms of polar shapelets, and to perform a full weak lensing analysis, is available on the Internet.
High-precision cosmology with weak gravitational lensing requires a precise measure of the point ... more High-precision cosmology with weak gravitational lensing requires a precise measure of the point spread function across the imaging data where the accuracy to which high spatial frequency variation can be modelled is limited by the stellar number density across the field. We analyse dense stellar fields imaged at the Canada-France-Hawaii Telescope to quantify the degree of high spatial frequency variation in ground-based imaging point spread functions and compare our results to models of atmospheric turbulence. The data show an anisotropic turbulence pattern with an orientation independent of the wind direction and wind speed. We find the amplitude of the high spatial frequencies to decrease with increasing exposure time as t-1/2, and find a negligibly small atmospheric contribution to the point spread function ellipticity variation for exposure times t > 180 s. For future surveys analysing shorter exposure data, this anisotropic turbulence will need to be taken into account as the amplitude of the correlated atmospheric distortions becomes comparable to a cosmological lensing signal on scales less than ˜10 arcmin. This effect could be mitigated, however, by correlating galaxy shear measured on exposures imaged with a time separation greater than 50 s, for which we find the spatial turbulence patterns to be uncorrelated.
We construct a simple, spherical blastwave model to estimate the pressure structure of the interg... more We construct a simple, spherical blastwave model to estimate the pressure structure of the intergalactic medium surrounding hyper-starburst galaxies, and argue that the effects of interaction with star-forming galaxy winds may be approximated at early times by an adiabatically expanding, self-similar `bubble' as described by Weaver et al. (1977) and Ostriker & McKee (1988). This model is used to make observational predictions for the thermal Sunyaev-Zel'dovich effect in the shocked bubble plasma. Radiative cooling losses are explored, and it is found that bremsstrahlung will limit the epoch of adiabatic expansion to $10^7$--$10^8$ years: comparable to total hyper-starburst lifetimes. Prospects for making a first Sunyaev-Zel'dovich detection of galaxy wind bubbles using the Atacama Large Millimeter Array are examined for a number of active hyper-starburst sources in the literature.
Publications of the Astronomical Society of the Pacific, Aug 18, 2013
Weak gravitational lensing observations are a key science driver for the NASA Wide Field Infrared... more Weak gravitational lensing observations are a key science driver for the NASA Wide Field Infrared Survey Telescope (WFIRST). To validate the performance of the WFIRST infrared detectors, we have performed a laboratory emulation of weak gravitational lensing measurements. Our experiments used a custom precision projector system to image a target mask composed of a grid of pinholes, emulating stellar point sources, onto a 1.7 micron cut-off Teledyne HgCdTe/H2RG detector. We used a 880nm LED illumination source and f/22 pupil stop to produce undersampled point spread functions similar to those expected from WFIRST. We also emulated the WFIRST image reconstruction strategy, using the IMage COMbination (IMCOM) algorithm to derive oversampled images from dithered, undersampled input images. We created shear maps for this data and computed shear correlation functions to mimic a real weak lensing analysis. After removing only 2nd order polynomial fits to the shear maps, we found that the correlation functions could be reduced to O(10^-6). This places a conservative upper limit on the detector-induced bias to the correlation function (under our test conditions). This bias is two orders of magnitude lower than the expected weak lensing signal. Restricted to scales relevant to dark energy analyses (sky separations > 0.5 arcmin), the bias is O(10^-7): comparable to the requirement for future weak lensing missions to avoid biasing cosmological parameter estimates. Our experiment will need to be upgraded and repeated under different configurations to fully characterize the shape measurement performance of WFIRST IR detectors.
Monthly Notices of the Royal Astronomical Society, 2015
We present first results from the third GRavitational lEnsing Accuracy Testing (GREAT3) challenge... more We present first results from the third GRavitational lEnsing Accuracy Testing (GREAT3) challenge, the third in a sequence of challenges for testing methods of inferring weak gravitational lensing shear distortions from simulated galaxy images. GREAT3 was divided into experiments to test three specific questions, and included simulated space-and ground-based data with constant or cosmologically-varying shear fields. The simplest (control) experiment included parametric galaxies with a realistic distribution of signal-to-noise, size, and ellipticity, and a complex point spread function (PSF). The other experiments tested the additional impact of realistic galaxy morphology, multiple exposure imaging, and the uncertainty about a spatially-varying PSF; the last two questions will be explored in Paper II. The 24 participating teams competed to estimate lensing shears to within systematic error tolerances for upcoming Stage-IV dark energy surveys, making 1525 submissions overall. GREAT3 saw considerable variety and innovation in the types of methods applied. Several teams now meet or exceed the targets in many of the tests conducted (to within the statistical errors). We conclude that the presence of realistic galaxy morphology in simulations changes shear calibration biases by ∼ 1 per cent for a wide range of methods. Other effects such as truncation biases due to finite galaxy postage stamps, and the impact of galaxy type as measured by the Sérsic index, are quantified for the first time. Our results generalize previous studies regarding sensitivities to galaxy size and signal-to-noise, and to PSF properties such as seeing and defocus. Almost all methods' results support the simple model in which additive shear biases depend linearly on PSF ellipticity.
ABSTRACT Interaction of charges in CCDs with the already accumulated charge distribution causes b... more ABSTRACT Interaction of charges in CCDs with the already accumulated charge distribution causes both a flux dependence of the point-spread function (an increase of observed size with flux, also known as the brighter/fatter effect) and pixel-to-pixel correlations of the noise in flat fields. We describe these effects in the Dark Energy Camera (DECam) with charge dependent shifts of effective pixel borders, i.e. the Antilogus et al. (2014) model, which we fit to measurements of flat-field noise correlations. The latter fall off approximately as a power-law r^-2.5 with pixel separation r, are isotropic except for an asymmetry in the direct neighbors along rows and columns, are stable in time, and are weakly dependent on wavelength. They show variations from chip to chip at the 20% level that correlate with the silicon resistivity. The charge shifts predicted by the model cause biased shape measurements, primarily due to their effect on bright stars, at levels exceeding weak lensing science requirements. We measure the flux dependence of star images and show that the effect can be mitigated by applying the reverse charge shifts at the pixel level during image processing. Differences in stellar size, however, remain significant due to residuals at larger distance from the centroid.
Monthly Notices of the Royal Astronomical Society, 2014
We present the cluster mass-richness scaling relation calibrated by a weak lensing analysis of > ... more We present the cluster mass-richness scaling relation calibrated by a weak lensing analysis of > ∼ 18,000 galaxy cluster candidates in the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS). Detected using the 3D-Matched-Filter cluster-finder of Milkeraitis et al., these cluster candidates span a wide range of masses, from the small group scale up to ∼ 10 15 M ⊙ , and redshifts 0.2 z 0.9. The total significance of the shear measurement amounts to 54σ. We compare cluster masses determined using weak lensing shear and magnification, finding the measurements in individual richness bins to yield 1σ compatibility, but with magnification estimates biased low. This first direct mass comparison yields important insights for improving the systematics handling of future lensing magnification work. In addition, we confirm analyses that suggest cluster miscentring has an important effect on the observed 3D-Matched-Filter halo profiles, and we quantify this by fitting for projected cluster centroid offsets, which are typically ∼ 0.4 arcmin. We bin the cluster candidates as a function of redshift, finding similar cluster masses and richness across the full range up to z ∼ 0.9. We measure the 3D-MF mass-richness scaling relation M 200 = M 0 (N 200 /20) β . We find a normalization M 0 ∼ (2.7 +0.5 −0.4 ) × 10 13 M ⊙ , and a logarithmic slope of β ∼ 1.4 ± 0.1, both of which are in 1σ agreement with results from the magnification analysis. We find no evidence for a redshift-dependence of the normalization. The CFHTLenS 3D-Matched-Filter cluster catalogue is now available at cfhtlens.org.
Monthly Notices of the Royal Astronomical Society, 2015
We measure the weak-lensing masses and galaxy distributions of four massive galaxy clusters obser... more We measure the weak-lensing masses and galaxy distributions of four massive galaxy clusters observed during the Science Verification phase of the Dark Energy Survey with the purpose of 1) validating the DECam imager for the task of measuring weak-lensing shapes, and 2) utilizing DECam's large field of view to map out the clusters and their environments over 90 arcmin. We conduct a series of rigorous tests on astrometry, photometry, image quality, PSF modeling, and shear measurement accuracy to single out flaws in the data and also to identify the optimal data processing steps and parameters. We find Science Verification data from DECam to be suitable for lensing analyses. The PSF is generally well-behaved, but the modeling is rendered difficult by a flux-dependent PSF width. We employ photometric redshifts to distinguish between foreground and background galaxies, and a red-sequence cluster finder to provide cluster richness estimates and cluster-galaxy distributions. By fitting NFW profiles to the clusters in this study, we determine weak-lensing masses that are in agreement with previous work. For Abell 3261, we provide the first estimates of redshift, weak-lensing mass, and richness. In addition, the cluster-galaxy distributions indicate the existence of filaments attached to 1E 0657-56 and RXC J2248.7-4431, stretching out as far as 1 degree (approximately 20 Mpc), showcasing the potential of DECam and DES for detailed studies of degree-scale features on the sky.
Monthly Notices of the Royal Astronomical Society, 2015
We present weak lensing and X-ray analysis of 12 low mass clusters from the CFHTLenS and XMM-CFHT... more We present weak lensing and X-ray analysis of 12 low mass clusters from the CFHTLenS and XMM-CFHTLS surveys. We combine these systems with high-mass systems from CCCP and low-mass systems from COSMOS to obtain a sample of 70 systems, which we divide into subsamples of 15 merging and 55 relaxed systems. We measure L X -T X , M-L X and M-T X scaling relations and find in all cases that the power-law slopes of the full, merging and relaxed subsamples are consistent. For the M-T X we find slopes consistent with the self-similar model, whereas L X -T X results in steeper and M-L X in flatter relations. We find a marginal trend for larger scatter and lower normalisation in the M-L X and M-T X relations for the merging subsample, which we attribute to triaxiality and substructure. We explore the effects of X-ray cross-calibration and find that Chandra calibration leads to flatter L X -T X and M-T X relations. We also utilise the three surveys making up the sample as overlapping mass bins. For COSMOS and CFHTLS we find slopes consistent with the relation fitted to the full sample, whereas the high mass CCCP sample favours flatter slopes. We also find that intermediate mass systems have a higher mass for their luminosity. Unfortunately our sample does not enable direct measurement of a break at low masses, but we find a trend for enhanced intrinsic scatter in mass at low masses.
Monthly Notices of the Royal Astronomical Society, 2015
We address the problem of separating stars from galaxies in future large photometric surveys. We ... more We address the problem of separating stars from galaxies in future large photometric surveys. We focus our analysis on simulations of the Dark Energy Survey (DES). In the first part of the paper, we derive the science requirements on star/galaxy separation, for measurement of the cosmological parameters with the Gravitational Weak Lensing and Large Scale Structure probes. These requirements are dictated by the need to control both the statistical and systematic errors on the cosmological parameters, and by Point Spread Function calibration. We formulate the requirements in terms of the completeness and purity provided by a given star/galaxy classifier. In order to achieve these requirements at faint magnitudes, we propose a new method for star/galaxy separation in the second part of the paper. We first use Principal Component Analysis to outline the correlations between the objects parameters and extract from it the most relevant information. We then use the reduced set of parameters as input to an Artificial Neural Network. This multi-parameter approach improves upon purely morphometric classifiers (such as the classifier implemented in SExtractor), especially at faint magnitudes: it increases the purity by up to 20% for stars and by up to 12% for galaxies, at i-magnitude fainter than 23.
Monthly Notices of the Royal Astronomical Society, 2015
We present new constraints on the relationship between galaxies and their host dark matter halos,... more We present new constraints on the relationship between galaxies and their host dark matter halos, measured from the location of the peak of the stellar-to-halo mass ratio (SHMR), up to the most massive galaxy clusters at redshift z ∼ 0.8 and over a volume of nearly 0.1 Gpc 3 . We use a unique combination of deep observations in the CFHTLenS/VIPERS field from the near-UV to the near-IR, supplemented by ∼ 60 000 secure spectroscopic redshifts, analysing galaxy clustering, galaxy-galaxy lensing and the stellar mass function. We interpret our measurements within the halo occupation distribution (HOD) framework, separating the contributions from central and satellite galaxies. We find that the SHMR for the central galaxies peaks at M h,peak = 1.9 +0.2 −0.1 × 10 12 M ⊙ with an amplitude of 0.025, which decreases to ∼ 0.001 for massive halos (M h > 10 14 M ⊙ ). Compared to central galaxies only, the total SHMR (including satellites) is boosted by a factor 10 in the high-mass regime (cluster-size halos), a result consistent with cluster analyses from the literature based on fully independent methods. After properly accounting for differences in modelling, we have compared our results with a large number of results from the literature up to z = 1: we find good general agreement, independently of the method used, within the typical stellar-mass systematic errors at low to intermediate mass (M ⋆ < 10 11 M ⊙ ) and the statistical errors above. We have also compared our SHMR results to semi-analytic simulations and found that the SHMR is tilted compared to our measurements in such a way that they over-(under-) predict star formation efficiency in central (satellite) galaxies.
ABSTRACT GALSIM is a collaborative, open-source project aimed at providing an image simulation to... more ABSTRACT GALSIM is a collaborative, open-source project aimed at providing an image simulation tool of enduring benefit to the astronomical community. It provides a software library for generating images of astronomical objects such as stars and galaxies in a variety of ways, efficiently handling image transformations and operations such as convolution and rendering at high precision. We describe the GALSIM software and its capabilities, including necessary theoretical background. We demonstrate that the performance of GALSIM meets the stringent requirements of high precision image analysis applications such as weak gravitational lensing, for current datasets and for the Stage IV dark energy surveys of the Large Synoptic Survey Telescope, ESA&#39;s Euclid mission, and NASA&#39;s WFIRST-AFTA mission. The GALSIM project repository is public and includes the full code history, all open and closed issues, installation instructions, documentation, and wiki pages (including a Frequently Asked Questions section). The GALSIM repository can be found at https://github.com/GalSim-developers/GalSim .
... Within the next few year wide-area survey telescopes will begin imaging huge volumes of deep ... more ... Within the next few year wide-area survey telescopes will begin imaging huge volumes of deep space, with the measurement of the gravitational lensing signal being given high ... Worthy of more than mention are William Brown, Mairi Brookes, Sophie Qureshi and Dan Smith. ...
ABSTRACT The Precision Projector (PP) is a joint project between Caltech and NASA’s Jet Propulsio... more ABSTRACT The Precision Projector (PP) is a joint project between Caltech and NASA’s Jet Propulsion Laboratory (JPL) that tests image sensors for upcoming astronomy missions. Our principal instrument - an Offner-based re-imaging system (a.k.a. “the Projector”) - casts precisely controlled images and spectra onto CCD, CMOS or IR detectors. With known input signals and a model for the output, the measured images allow us to characterize detectors and quantitatively understand their non-idealities. The projector can also emulate astronomical data (e.g. stars and galaxies) in order to assess the impact of the detectors on real data and forecast mission performance. The primary goal of the PP, which began in 2008, is to emulate weak gravitational lensing surveys by projecting galaxy images at the cadence and intensity of the WFIRST mission. We have since expanded our infrastructure and personnel to address other astronomical applications that rely on inferences from small numbers of pixels (e.g. exoplanet searches and adaptive optics) and to include the ability to project spectra.
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Papers by Barnaby Rowe