Intraschool Variation in Class Size: Patterns and Implications

Economists attempting to explain the widening of the black-white wage gap in the late 1970's by differences in school quality have been faced with the problem that recent data reveal virtually no gap in the quality of schools attended by blacks and whites using a variety of measures. In this paper, we reexamine racial differences in school quality by considering the effects of using the pupil-teacher ratio, rather than the school's average class size,in an education production function since the pupil-teacher ratio is a rough proxy, at best. We find that while the pupil-teacher ratio and average class size are correlated, the pupil-teacher ratio is systematically less than or equal to the average class size. Part of the difference is due to intraschool allocation of teachers to classes. While the pupil-teacher ratio shows no black-white differences in class size, measures of the school's average class size suggest blacks are in larger classes. Also, the two measures lead to differing estimates of the role of class size in an education production function. We also conclude that school level measures may obscure important within-school variation in class size due to the small class sizes for compensatory education and a kind of aggregation bias results. Not only are blacks in schools with larger average class sizes but they are also in larger classes within schools, conditional on class type. The intraschool class size patterns suggest that using within-school variation in education production functions is not a good solution to aggregation problems due to non-random assignment of students to different sized classes. But once the selection problem has been addressed,smaller classes at the 8th grade lead to larger test score gains from 8th to 10th grade and differences in class size can explain approximately 15% of the black-white gap in educational achievement.

Towards a more realistic population of bright spiral galaxies in cosmological simulations

We present an update to the multiphase SPH galaxy formation code by Scannapieco et al. We include a more elaborate treatment of the production of metals, cooling rates based on individual element abundances, and a scheme for the turbulent diffusion of metals. Our SN feedback model now transfers energy to the ISM in kinetic and thermal form, and we include a prescription for the effects of radiation pressure from massive young stars on the ISM. We calibrate our new code on the well studied Aquarius haloes and then use it to simulate a sample of 16 galaxies with halo masses between 1x10^11 and 3x10^12 M_sun. In general, the stellar masses of the sample agree well with the stellar mass to halo mass relation inferred from abundance matching techniques for redshifts z=0-4. There is however a tendency to overproduce stars at z>4 and to underproduce them at z<0.5 in the least massive haloes. Overly high SFRs at z<1 for the most massive haloes are likely connected to the lack of AGN feedback in our model. The simulated sample also shows reasonable agreement with observed star formation rates, sizes, gas fractions and gas-phase metallicities at z=0-3. Remaining discrepancies can be connected to deviations from predictions for star formation histories from abundance matching. At z=0, the model galaxies show realistic morphologies, stellar surface density profiles, circular velocity curves and stellar metallicities, but overly flat metallicity gradients. 15 out of 16 of our galaxies contain disk components with kinematic disk fraction ranging between 15 and 65 %. The disk fraction depends on the time of the last destructive merger or misaligned infall event. Considering the remaining shortcomings of our simulations we conclude that even higher kinematic disk fractions may be possible for LambdaCDM haloes with quiet merger histories, such as the Aquarius haloes.Comment: 26 pages, 20 figures, accepted for publication in MNRA

Spitzer Observations of Comet 67P/Churyumov-Gerasimenko at 5.5-4.3 AU From the Sun

We report Spitzer Space Telescope observations of comet 67P/Churyumov-Gerasimenko at 5.5 and 4.3 AU from the Sun, post-aphelion. Comet 67P is the primary target of the European Space Agency's Rosetta mission. The Rosetta spacecraft will rendezvous with the nucleus at heliocentric distances similar to our observations. Rotationally resolved observations at 8 and 24 microns (at a heliocentric distance, rh, of 4.8 AU) that sample the size and color-temperature of the nucleus are combined with aphelion R-band light curves observed at the Very Large Telescope (VLT) and yield a mean effective radius of 2.04 +/- 0.11 km, and an R-band geometric albedo of 0.054 +/- 0.006. The amplitudes of the R-band and mid-infrared light curves agree, which suggests that the variability is dominated by the shape of the nucleus. We also detect the dust trail of the comet at 4.8 and 5.5 AU, constrain the grain sizes to be less than or similar to 6 mm, and estimate the impact hazard to Rosetta. We find no evidence for recently ejected dust in our images. If the activity of 67P is consistent from orbit to orbit, then we may expect the Rosetta spacecraft will return images of an inactive or weakly active nucleus as it rendezvous with the comet at rh = 4 AU in 2014.Comment: 19 pages, 2 tables, 10 figures. Accepted for publication in the Astronomical Journa