The environments of intermediate-redshift QSOs: 0.3 < z < 0.7

An angular correlation of low significance (2 sigma) is observed between 0.3 < z < 0.5 QSOs and V < 23 galaxies. Overall, the cross-correlation function between 82 intermediate-redshift (0.3 < z < 0.7), X-ray selected QSOs and V < 24 galaxies is investigated, but no signal is detected for the z > 0.5 QSOs. After converting to an excess of galaxies physically associated with the QSO, this lack of strong correlation is shown to be consistent with the clustering of normal galaxies at the same moderate redshifts. Combined with previous observations, these results imply that the environments of radio-quiet QSOs do not undergo significant evolution with respect to the galaxy population over a wide range of redshifts (0 < z < 1.5). This is in marked contrast to the rapid increase in the richness of the environments associated with radio-loud QSOs over the same redshift range.Comment: 12 pages, 8 figures, MNRAS TeX macro, to appear to MNRA

A Simple Method for Computing the Non-Linear Mass Correlation Function with Implications for Stable Clustering

We propose a simple and accurate method for computing analytically the mass correlation function for cold dark matter and scale-free models that fits N-body simulations over a range that extends from the linear to the strongly non-linear regime. The method, based on the dynamical evolution of the pair conservation equation, relies on a universal relation between the pair-wise velocity and the smoothed correlation function valid for high and low density models, as derived empirically from N-body simulations. An intriguing alternative relation, based on the stable-clustering hypothesis, predicts a power-law behavior of the mass correlation function that disagrees with N-body simulations but conforms well to the observed galaxy correlation function if negligible bias is assumed. The method is a useful tool for rapidly exploring a wide span of models and, at the same time, raises new questions about large scale structure formation.Comment: 10 pages, 3 figure

Implicit Bias in Daily Perceptions and Legal Judgments

In today’s demonstration, we explored the audience’s positive and negative associations with blacks and whites. The demonstration is an adaptation of the Implicit Association Test (www.projectimplicit.net), a computer-based task designed to explore mental connections between various concepts. Participants were presented with a list of concepts (stereotypically black and white names, pleasant and unpleasant concepts) in a column down the middle of a screen along with the response categories (black/white or Pleasant/Unpleasant) along the left and right sides. When reading a word, participants were asked to categorize it by slapping the knee (left or right) that corresponds to the category displayed on the left or right side of the screen. Their task was to do this as fast as possible, correcting any mistakes before going forward, and raising their hand after completing the last word in the list. We then noted the amount of time for a critical mass of participants (approximately seventy-five percent) to complete each trial. In simple trials, either names or valenced words are displayed. In two critical trials, both names and valenced words were presented simultaneously in the middle of the screen. The trials differ based on the pairings of the response categories, and these trials tend to show a drastic difference in completion times. When participants were required to use the same hand to categorize black names and unpleasant words or white names and pleasant words, they tended to respond almost twice as quickly as when using the same hand to categorize black names and pleasant words or white names and unpleasant words. While there are individual differences among the respondents, the demonstration provided audible and visceral evidence of the trend. We then asked the audience to interrogate the methodological and cultural implications of the task, ultimately concluding that the test potentially reveals the fairly widespread implicit negative associations with blacks. With this working hypothesis, we next discussed the nature of racial bias and its implications for judgment in legal domains

The environments of z<0.3 QSOs

Based on a cross-correlation analysis of X-ray selected QSOs with faint (B<20.5) galaxies, we find a 5sigma galaxy excess around low-redshift (z<0.3) QSOs with an amplitude identical to that of the galaxy angular correlation function. The similarity between QSO-galaxy clustering and galaxy-galaxy clustering suggests that QSOs are unbiased with respect to galaxies and make useful tracers of large-scale structure in the Universe.Comment: uuencoded compressed postscript. The preprint is also available at http://www.ast.cam.ac.uk/preprint/PrePrint.htm

Correlation Function of Superclusters of Galaxies

We present a study of the two-point correlation function of superclusters of galaxies. The largest catalogs are used. The results show negligible correlation less than 0.1-0.2 for separations up to 500-600 h^{-1} Mpc. Small correlations are obtained using various estimates and samples. Seemingly there are no structures of superclusters of galaxies.Comment: 19 pages, 3 figures, 4 tables. To appear in 1998 ApJ, 506, No. 2 (Oct 20

Passive Evolution: Are the Faint Blue Galaxy Counts Produced by a Population of Eternally Young Galaxies?

A constant age population of blue galaxies, postulated in the model of Gronwall & Koo (1995), seems to provide an attractive explanation of the excess of very blue galaxies in the deep galaxy counts. Such a population may be generated by a set of galaxies with cycling star formation rates, or at the other extreme, be maintained by the continual formation of new galaxies which fade after they reach the age specified in the Gronwall and Koo model. For both of these hypotheses, we have calculated the luminosity functions including the respective selection criteria, the redshift distributions, and the number counts in the B_J and K bands. We find a substantial excess in the number of galaxies at low redshift (0 < z < 0.05) over that observed in the CFH redshift survey (Lilly et al. 1995) and at the faint end of the Las Campanas luminosity function (Lin et al. 1996). Passive or mild evolution fails to account for the deep galaxy counts because of the implications for low redshift determinations of the I-selected redshift distribution and the r-selected luminosity function in samples where the faded counterparts of the star-forming galaxies would be detectable.Comment: 11 pages, LaTeX type (aaspp4.sty), 3 Postscript figures, submitted to ApJ Letter

The Apparent and Intrinsic Shape of the APM Galaxy Clusters

We estimate the distribution of intrinsic shapes of APM galaxy clusters from the distribution of their apparent shapes. We measure the projected cluster ellipticities using two alternative methods. The first method is based on moments of the discrete galaxy distribution while the second is based on moments of the smoothed galaxy distribution. We study the performance of both methods using Monte Carlo cluster simulations covering the range of APM cluster distances and including a random distribution of background galaxies. We find that the first method suffers from severe systematic biases, whereas the second is more reliable. After excluding clusters dominated by substructure and quantifying the systematic biases in our estimated shape parameters, we recover a corrected distribution of projected ellipticities. We use the non-parametric kernel method to estimate the smooth apparent ellipticity distribution, and numerically invert a set of integral equations to recover the corresponding distribution of intrinsic ellipticities under the assumption that the clusters are either oblate or prolate spheroids. The prolate spheroidal model fits the APM cluster data best.Comment: 8 pages, including 7 figures, accepted for publication in MNRA

Child Abuse Evidence: New Perspectives from Law, Medicine, Psychology & Statistics: Question and Answer Session

A transcript of the Question and Answer session during the University of Michigan Journal of Law Reform Symposium, Child Abuse Evidence: New Perspectives from Law, Medicine, Psychology & Statistics