The effect of imperfect corrections of PSF anisotropy on cosmic shear measurements

Current measurements of the weak lensing signal induced by large scale structure provide useful constraints on a range of cosmological parameters. However, the ultimate succes of this technique depends on the accuracy with which one can correct for the effect of the Point Spread Function (PSF). In this paper we examine the accuracy of the PSF anisotropy correction using images of fields with a large number of stars. The ellipticity correlation function of the residuals is studied to quantify the effect of imperfect corrections for PSF anisotropy on cosmic shear studies. These imperfections occur on the chip scale and consequently the systematic signal decreases rapidly with increasing angular scale. Separation of the signal into ``E'' (curl-free) and ``B'' (curl) components can help to identify the presence of residual systematics, but in general, the amplitude of the ``B''-mode is different from that of the ``E''-mode. The study of fields with many stars can be beneficial in finding a proper description of the variation of PSF anisotropy, and consequently help to significantly improve the accuracy with which the cosmic shear signal can be measured. We show that with such an approach it is feasible that the accuracy of future cosmic shear studies is limited by the statistical noise introduced by the intrinsic shapes of the sources. In particular, the prospects for accurate measurements of the cosmic shear signal on scales larger than ~10 arcminutes are excellent.Comment: submitted to MNRAS, 8 page

Is There a Learning Gap?

This semester I student taught within a School Corporation in Northwest Indiana. I taught at two different elementary schools, School A and School B. Although the curriculum is the same across both schools, the students at each school are different. School A has a higher income rate and also has lower enrollment for ethnic students. School B has more students enrolled in the special education program and more students enrolled in the free and reduced lunch program. My goal is to figure out if students from both schools will learn the same given the same instruction

Dynamics of Merging Galaxy Clusters from Simulated Analogs

Merging galaxy clusters may provide a unique window into the behavior of dark matter and the evolution of member galaxies. To interpret these natural collider experiments we must account for how much time has passed since pericenter passage (TSP), the maximum relative speed of the merging subclusters, merger phase (outbound after first pericenter or returning for second pericenter), and other dynamical parameters that are not directly observable. These quantities are often inferred from staged simulations or analytical timing arguments that include neither substructure, large-scale structure, nor a cosmologically motivated range of impact parameters. We include all these effects by extracting dynamical parameters from analog systems in a cosmological n-body simulation, and we present constraints for 11 observed systems. The TSP and viewing angles we derive are consistent with those of staged hydrodynamical simulations, but we find lower maximum speeds. Compared to the analytical MCMAC method we find lower TSP, and viewing angles that put the separation vector closer to the plane of the sky; we attribute this to the MCMAC assumption of zero pericenter distance. We discuss potential extensions to the basic analog method as well as complementarities between methods.Comment: 17 pages, 15 figures. This version accepted to ApJ improves on v0 by demonstrating recovery of correct parameters in cases where the correct answer is know

Patient Perception of Negative Non-Invasive Prenatal Testing Results

Non-invasive prenatal testing (NIPT) uses cell-free fetal DNA to assess for fetal aneuploidy during pregnancy. NIPT has higher detection rates and positive predictive values than previous methods; however, NIPT is not diagnostic. Studies suggest patients may underestimate the limitations of prenatal screening. Therefore, we conducted a prospective cross-sectional study of ninety-four women from genetic counseling clinics in Houston, Texas to assess patient understanding of the residual risk for aneuploidy after receiving a negative NIPT. The majority of participants (66%) understood the residual risk for Down syndrome following negative NIPT; however, 34% of participants indicated that negative NIPT completely eliminated the risk. Individuals with at least four years of college education were more likely to understand that NIPT does not eliminate the chance of trisomy 13/18 (p=0.012) and sex chromosome abnormality (p=0.039), and were more likely to understand which conditions NIPT tests for (p=0.021), compared to women with less formal education. These data demonstrate that despite the recent implementation of NIPT into obstetric practice, the majority of women are aware of its limitations after genetic counseling. However, clinicians may need to consider alternative ways to communicate the limitations of NIPT to those women with less formal education to ensure understanding

Precision Weak Gravitational Lensing Using Velocity Fields: Fisher Matrix Analysis

Weak gravitational lensing measurements based on photometry are limited by shape noise, the variance in the unknown unlensed orientations of the source galaxies. If the source is a disk galaxy with a well-ordered velocity field, however, velocity field data can support simultaneous inference of the shear, inclination, and position angle, virtually eliminating shape noise. We use the Fisher Information Matrix formalism to forecast the precision of this method in the idealized case of a perfectly ordered velocity field defined on an infinitesimally thin disk. For nearly face-on targets one shear component, $\gamma_\times$, can be constrained to $0.003\frac{90}{I_0}\frac{25}{n_{\rm pix}}$ where $I_0$ is the S/N of the central intensity pixel and $n_{\rm pix}$ is the number of pixels across a diameter enclosing 80\% of the light. This precision degrades with inclination angle, by a factor of three by $i{=}50^\circ$. Uncertainty on the other shear component, $\gamma_+$, is about 1.5 (7) times larger than the $\gamma_\times$ uncertainty for targets at $i=10^\circ$ ($50^\circ$). For arbitrary galaxy position angle on the sky, these forecasts apply not to $\gamma_+$ and $\gamma_\times$ as defined on the sky, but to two eigenvectors in $(\gamma_+, \gamma_\times,\mu)$ space where $\mu$ is the magnification. We also forecast the potential of less expensive partial observations of the velocity field such as slit spectroscopy. We conclude by outlining some ways in which real galaxies depart from our idealized model and thus create random or systematic uncertainties not captured here. In particular, our forecast $\gamma_\times$ precision is currently limited only by the data quality rather than scatter in galaxy properties because the relevant type of scatter has yet to be measured.Comment: Accepted to ApJ, 17 pages, 14 figures. Diff from v1: added Sec 3.1 on degeneracies and Appendix with simulations confirming Fisher result

Optical Galaxy Clusters in the Deep Lens Survey

We present the first sample of 882 optically selected galaxy clusters in the Deep Lens Survey (DLS), selected with the Bayesian Cluster Finder. We create mock DLS data to assess completeness and purity rates, and find that both are at least $70\%$ within 0.1$\le z \le$ 1.2 for clusters with $M_{200}\ge 1.2\times 10^{14}M_{\odot}$. We verified the integrity of the sample by performing several comparisons with other optical, weak lensing, X-ray and spectroscopic surveys which overlap the DLS footprint: the estimated redshifts are consistent with the spectroscopic redshifts of known clusters (for $z>0.25$ where saturation in the DLS is not an issue); our richness estimates in combination with a previously calibrated richness-mass relation yields individual cluster mass estimates consistent with available SHeLS dynamical mass estimates; synthetic mass maps made from the optical mass estimates are correlated ($>3\sigma$ significance) with the weak lensing mass maps; and the mass function thus derived is consistent with theoretical predictions for the CDM scenario. With the verified sample we investigated correlations between the brightest cluster galaxies (BCG) properties and the host cluster properties within a broader range in redshift (0.25 $\le z \le$ 0.8) and mass ($\ge2.4\times 10^{14}M_{\odot}$) than in previous work. We find that the slope of the BCG magnitude-redshift relation throughout this redshift range is consistent with that found at lower redshifts. This result supports an extrapolation to higher redshift of passive evolution of the BCG within the hierarchical scenario.Comment: Paper accepted for publication in MNRAS, Table 1 will be available online or under reques

Brightest Cluster Galaxy Alignments in Merging Clusters

The orientations of brightest cluster galaxies (BCGs) and their host clusters tend to be aligned, but the mechanism driving this is not clear. To probe the role of cluster mergers in this process, we quantify alignments of 38 BCGs in 22 clusters undergoing major mergers (up to $\sim 1$ Gyr after first pericenter). We find alignments entirely consistent with those of clusters in general. This suggests that alignments are robust against major cluster mergers. If, conversely, major cluster mergers actually help orient the BCG, such a process is acting quickly because the orientation is in place within $\sim 1$ Gyr after first pericenter.Comment: accepted to Ap

Overconfidence in Photometric Redshift Estimation

We describe a new test of photometric redshift performance given a spectroscopic redshift sample. This test complements the traditional comparison of redshift {\it differences} by testing whether the probability density functions $p(z)$ have the correct {\it width}. We test two photometric redshift codes, BPZ and EAZY, on each of two data sets and find that BPZ is consistently overconfident (the $p(z)$ are too narrow) while EAZY produces approximately the correct level of confidence. We show that this is because EAZY models the uncertainty in its spectral energy distribution templates, and that post-hoc smoothing of the BPZ $p(z)$ provides a reasonable substitute for detailed modeling of template uncertainties. Either remedy still leaves a small surplus of galaxies with spectroscopic redshift very far from the peaks. Thus, better modeling of low-probability tails will be needed for high-precision work such as dark energy constraints with the Large Synoptic Survey Telescope and other large surveys.Comment: accepted to MNRA

Shedding Light on the Matter of Abell 781

The galaxy cluster Abell 781 West has been viewed as a challenge to weak gravitational lensing mass calibration, as Cook and dell'Antonio (2012) found that the weak lensing signal-to-noise in three independent sets of observations was consistently lower than expected from mass models based on X-ray and dynamical measurements. We correct some errors in statistical inference in Cook and dell'Antonio (2012) and show that their own results agree well with the dynamical mass and exhibit at most 2.2--2.9$\sigma$ low compared to the X-ray mass, similar to the tension between the dynamical and X-ray masses. Replacing their simple magnitude cut with weights based on source photometric redshifts eliminates the tension between lensing and X-ray masses; in this case the weak lensing mass estimate is actually higher than, but still in agreement with, the dynamical estimate. A comparison of lensing analyses with and without photometric redshifts shows that a 1--2$\sigma$ chance alignment of low-redshift sources lowers the signal-to-noise observed by all previous studies which used magnitude cuts rather than photometric redshifts. The fluctuation is unexceptional, but appeared to be highly significant in Cook and dell'Antonio (2012) due to the errors in statistical interpretation.Comment: 7 pages, submitted to MNRA