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

Photometric Redshifts and Photometry Errors

We examine the impact of non-Gaussian photometry errors on photometric redshift performance. We find that they greatly increase the scatter, but this can be mitigated to some extent by incorporating the correct noise model into the photometric redshift estimation process. However, the remaining scatter is still equivalent to that of a much shallower survey with Gaussian photometry errors. We also estimate the impact of non-Gaussian errors on the spectroscopic sample size required to verify the photometric redshift rms scatter to a given precision. Even with Gaussian {\it photometry} errors, photometric redshift errors are sufficiently non-Gaussian to require an order of magnitude larger sample than simple Gaussian statistics would indicate. The requirements increase from this baseline if non-Gaussian photometry errors are included. Again the impact can be mitigated by incorporating the correct noise model, but only to the equivalent of a survey with much larger Gaussian photometry errors. However, these requirements may well be overestimates because they are based on a need to know the rms, which is particularly sensitive to tails. Other parametrizations of the distribution may require smaller samples.Comment: submitted to ApJ

Task and Socioemotional Interaction as Contexts Affecting the Agreement-Attraction Relationship

The majority of the research supports the hypothesis that agreement is a major variable in attraction, but several recent studies have shown other conditions may also be important. Following Wright\u27s (1969a) friendship model, Wright and Crawford (1969) have shown that males are oriented toward both task and social-emotional situations, while females are oriented primarily toward social- emotional situations. The present study was designed to investigate the role of agreement within these two situational variables. It was hypothesized that, for males in a task situation, agreement would yield greater attraction than disagreement. For females, greater attraction was predicted for agreeing pairs than for disagreeing pairs in a social-emotional situation. No prediction was made for females in a task situation. Subjects were same-sex pairs who were initially strangers. Each subject completed a value questionnaire and received feedback regarding the amount of agreement with his partner. The pair then participated in either a project oriented task condition or a discussion oriented social-emotional condition without task involvement. At the conclusion of the session, each subject described his partner with a person-perception questionnaire. An analysis of variance was performed on the data. The results showed that males find it difficult to get along with new acquaintances no matter what the situation. Females find it relatively difficult to get along in a task situation and relatively easy to get along in a social-emotional situation. The only significant effect for agreement was found for females in the task situation. None of the specific hypotheses of the study were confirmed. The findings were discussed in terms of cultural sex differences between men and women. Implications for other models of attraction were discussed. It was concluded that agreement may not be as general a determinant of attraction as previous research had indicated. Sex and situational variables must also be considered in predicting attraction

Ubercalibration of the Deep Lens Survey

We describe the internal photometric calibration of the Deep Lens Survey, which consists of five widely separated fields observed by two different observatories. Adopting the global linear least-squares ("ubercal") approach developed for the Sloan Digital Sky Survey (SDSS), we derive flatfield corrections for all observing runs, which indicate that the original sky flats were nonuniform by up to 0.13 mag peak to valley in \z band, and by up to half that amount in {\it BVR}. We show that application of these corrections reduces spatial nonuniformities in corrected exposures to the 0.01-0.02 mag level. We conclude with some lessons learned in applying ubercal to a survey structured very differently from SDSS, with isolated fields, multiple observatories, and shift-and-stare rather than drift-scan imaging. Although the size of the error caused by using sky or dome flats is instrument- and wavelength-dependent, users of wide-field cameras should not assume that it is small. Pipeline developers should facilitate routine application of this procedure, and surveys should include it in their plans from the outset.Comment: accepted to MNRA

Spurious Shear from the Atmosphere in Ground-Based Weak Lensing Observations

Weak lensing observations have the potential to be even more powerful than cosmic microwave background (CMB) observations in constraining cosmological parameters. However, the practical limits to weak lensing observations are not known. Most theoretical studies of weak lensing constraints on cosmology assume that the only limits are shot noise on small scales, and cosmic variance on large scales. For future large surveys, shot noise will be so low that other, systematic errors will likely dominate. Here we examine a potential source of additive systematic error for ground-based observations: spurious power induced by the atmosphere. We show that this limit will not be a significant factor even in future massive surveys such as LSST.Comment: accepted to ApJ Letter

Cross-correlation Tomography: Measuring Dark Energy Evolution with Weak Lensing

A cross-correlation technique of lensing tomography is presented to measure the evolution of dark energy in the universe. The variation of the weak lensing shear with redshift around massive foreground objects like bright galaxies and clusters depends solely on ratios of angular diameter distances. Use of the massive foreground halos allow us to compare relatively high, linear shear values in the same part of the sky, thus largely eliminating the dominant source of systematic error in cosmological weak lensing measurements. The statistic we use does not rely on knowledge of the foreground mass distribution and is only shot-noise limited. We estimate the constraints that deep lensing surveys with photometric redshifts can provide on the dark energy density Omega, the equation of state parameter w and its redshift derivative w'. The accuracies on w and w' are: sigma(w) ~ 0.02 fsky^{-1/2} and sigma(w') ~ 0.05 fsky^{-1/2}, where fsky is the fraction of sky covered by the survey and sigma(Omega)=0.03 is assumed in the marginalization. Combining our cross-correlation method with standard lensing tomography, which has complementary degeneracies, will allow measurement of the dark energy parameters with significantly better accuracy.Comment: 5 pages, 2 figures, submitted to PRL. Error in shear signal corrected - parameter constraints about a factor of 2 wors