Diversity of Online Community Activities

Web sites where users create and rate content as well as form networks with other users display long-tailed distributions in many aspects of behavior. Using behavior on one such community site, Essembly, we propose and evaluate plausible mechanisms to explain these behaviors. Unlike purely descriptive models, these mechanisms rely on user behaviors based on information available locally to each user. For Essembly, we find the long-tails arise from large differences among user activity rates and qualities of the rated content, as well as the extensive variability in the time users devote to the site. We show that the models not only explain overall behavior but also allow estimating the quality of content from their early behaviors.Comment: 14 page

Foraging Distance of the Argentine Ant in California Vineyards.

Argentine ants, Linepithema humile (Mayr) (Hymenoptera: Formicidae), form mutualisms with hemipteran pests in crop systems. In vineyards, they feed on honeydew produced by mealybugs and soft scales, which they tend and protect from natural enemies. Few options for controlling Argentine ants are available; one of the more effective approaches is to use liquid baits containing a low dose of an insecticide. Knowledge of ant foraging patterns is required to estimate how many bait stations to deploy per unit area. To measure how far ants move liquid bait in vineyards, we placed bait stations containing sugar water and a protein marker in plots for 6 d, and then collected ants along transects extending away from bait stations. The ants moved an average of 16.08 m and 12.21 m from bait stations in the first and second years of the study, respectively. Marked ants were found up to 63 m from bait stations; however, proportions of marked ants decreased exponentially as distance from the bait station increased. Results indicate that Argentine ants generally forage at distancesvineyards, thus suggesting that insecticide bait stations must be deployed at intervals of 36 m or less to control ants. We found no effect of insecticide on distances that ants moved the liquid bait, but this may have been because bait station densities were too low to affect the high numbers of Argentine ants that were present at the study sites

Cosmic homogeneity demonstrated with luminous red galaxies

We test the homogeneity of the Universe at $z\sim 0.3$ with the Luminous Red Galaxy (LRG) spectroscopic sample of the Sloan Digital Sky Survey. First, the mean number $N(R)$ of LRGs within completely surveyed LRG-centered spheres of comoving radius $R$ is shown to be proportional to $R^3$ at radii greater than $R\sim 70 h^{-1} \mathrm{Mpc}$. The test has the virtue that it does not rely on the assumption that the LRG sample has a finite mean density; its results show, however, that there \emph{is} such a mean density. Secondly, the survey sky area is divided into 10 disjoint solid angular regions and the fractional rms density variations of the LRG sample in the redshift range $0.2<z<0.35$ among these ($\sim 2\times10^7 h^{-3} \mathrm{Mpc^3}$) regions is found to be 7 percent of the mean density. This variance is consistent with typical biased \lcdm models and puts very strong constraints on the quality of SDSS photometric calibration.Comment: submitted to Ap

The External Shear Acting on Gravitational Lens B 1422+231

In a number of multiply imaged quasar systems, a significant contribution to the lensing potential is provided by groups and clusters of galaxies associated with the primary lens. As part of an ongoing effort to gather observational data on these systems, we present spectroscopy and near-infrared and optical photometry of galaxies in the field of the quadruple lens system B 1422+231. The spectra show that the primary lens and five nearby galaxies belong to a compact group at z = 0.338. The median projected radius of this group is 35 h^{-1} kpc and its velocity dispersion is 550 km/s. A straightforward application of the virial theorem yields a group mass of 1.4 x 10^{13} h^{-1} M(sun), which provides sufficient external shear to produce the observed image configuration. This data rules out a class of models and improves the system's prospects for a measurement of the Hubble constant.Comment: 16 pages including 3 tables, 2 eps figures and 2 jpeg images. Submitted to the Astronomical Journa

Quantum heuristic algorithm for traveling salesman problem

We propose a quantum heuristic algorithm to solve a traveling salesman problem by generalizing Grover search. Sufficient conditions are derived to greatly enhance the probability of finding the tours with extremal costs, reaching almost to unity and they are shown characterized by statistical properties of tour costs. In particular for a Gaussian distribution of the tours along the cost we show that the quantum algorithm exhibits the quadratic speedup of its classical counterpart, similarly to Grover search.Comment: Published versio

The Dual Origin of Stellar Halos II: Chemical Abundances as Tracers of Formation History

Fully cosmological, high resolution N-Body + SPH simulations are used to investigate the chemical abundance trends of stars in simulated stellar halos as a function of their origin. These simulations employ a physically motivated supernova feedback recipe, as well as metal enrichment, metal cooling and metal diffusion. As presented in an earlier paper, the simulated galaxies in this study are surrounded by stellar halos whose inner regions contain both stars accreted from satellite galaxies and stars formed in situ in the central regions of the main galaxies and later displaced by mergers into their inner halos. The abundance patterns ([Fe/H] and [O/Fe]) of halo stars located within 10 kpc of a solar-like observer are analyzed. We find that for galaxies which have not experienced a recent major merger, in situ stars at the high [Fe/H] end of the metallicity distribution function are more [alpha/Fe]-rich than accreted stars at similar [Fe/H]. This dichotomy in the [O/Fe] of halo stars at a given [Fe/H] results from the different potential wells within which in situ and accreted halo stars form. These results qualitatively match recent observations of local Milky Way halo stars. It may thus be possible for observers to uncover the relative contribution of different physical processes to the formation of stellar halos by observing such trends in the halo populations of the Milky Way, and other local L* galaxies.Comment: Version accepted for publication in ApJ Part 1. This version of the paper has been extended to include a detailed discussion of numerical issue

Selection and photometric properties of K+A galaxies

Two different simple measurements of galaxy star formation rate with different timescales are compared empirically on $156,395$ fiber spectra of galaxies with $r<17.77$ mag taken from the Sloan Digital Sky Survey in the redshift range $0.05<z<0.20$: a ratio \Aamp / \Kamp found by fitting a linear sum of an average old stellar poplulation spectrum (\Kamp) and average A-star spectrum (\Aamp) to the galaxy spectrum, and the equivalent width (EW) of the \Halpha emission line. The two measures are strongly correlated, but there is a small clearly separated population of outliers from the median correlation that display excess \Aamp /\Kamp relative to \Halpha EW. These ``K+A'' (or ``E+A'') galaxies must have dramatically decreased their star-formation rates over the last $\sim 1$ Gyr. The K+A luminosity distribution is very similar to that of the total galaxy population. The K+A population appears to be bulge-dominated, but bluer and higher surface-brightness than normal bulge-dominated galaxies; it appears that K+A galaxies will fade with time into normal bulge-dominated galaxies. The inferred rate density for K+A galaxy formation is $\sim 10^{-4} h^3 Mpc^{-3} Gyr^{-1}$ at redshift $z\sim 0.1$. These events are taking place in the field; K+A galaxies don't primarily lie in the high-density environments or clusters typical of bulge-dominated populations.Comment: submitted to Ap

Chemistry on hot astrochemical dust surfaces: Sulfur in AGB outflows

Astrochemical models treat dust surfaces as ice covered. We investigate the effects of implementing increased bare dust binding energies of CO and S-bearing species on the chemistry in the outflows of asymptotic giant branch (AGB) stars. We demonstrate the potential for improving agreement with observations in the outflow of IK Tau. Increasing the binding energies to measured and computationally derived values in high mass-loss AGB outflows increased the production of daughter species. Switching from a high binding energy on bare dust to weaker binding to ice, the gas phase abundance increased at a radius in agreement with observations of IK Tau, suggesting that displacement of bound species could contribute to this observational puzzle. Using a strong binding to bare dust, a gas phase increase was not observed, however parent species concentrations had to be increased by around a factor of four to explain observed concentrations

The overdensities of galaxy environments as a function of luminosity and color

We study the mean environments of galaxies in the Sloan Digital Sky Survey as a function of rest-frame luminosity and color. Overdensities in galaxy number are estimated in $8 h^{-1} \mathrm{Mpc}$ and $1 h^{-1} \mathrm{Mpc}$ spheres centered on $125,000$ galaxies taken from the SDSS spectroscopic sample. We find that, at constant color, overdensity is independent of luminosity for galaxies with the blue colors of spirals. This suggests that, at fixed star-formation history, spiral-galaxy mass is a very weak function of environment. Overdensity does depend on luminosity for galaxies with the red colors of early types; both low-luminosity and high-luminosity red galaxies are found to be in highly overdense regions.Comment: submitted to ApJ