Chiron: A Robust Recommendation System with Graph Regularizer

Recommendation systems have been widely used by commercial service providers for giving suggestions to users. Collaborative filtering (CF) systems, one of the most popular recommendation systems, utilize the history of behaviors of the aggregate user-base to provide individual recommendations and are effective when almost all users faithfully express their opinions. However, they are vulnerable to malicious users biasing their inputs in order to change the overall ratings of a specific group of items. CF systems largely fall into two categories - neighborhood-based and (matrix) factorization-based - and the presence of adversarial input can influence recommendations in both categories, leading to instabilities in estimation and prediction. Although the robustness of different collaborative filtering algorithms has been extensively studied, designing an efficient system that is immune to manipulation remains a significant challenge. In this work we propose a novel "hybrid" recommendation system with an adaptive graph-based user/item similarity-regularization - "Chiron". Chiron ties the performance benefits of dimensionality reduction (through factorization) with the advantage of neighborhood clustering (through regularization). We demonstrate, using extensive comparative experiments, that Chiron is resistant to manipulation by large and lethal attacks

The Near-Infrared Number Counts and Luminosity Functions of Local Galaxies

This study presents a wide-field near-infrared (K-band) survey in two fields; SA 68 and Lynx 2. The survey covers an area of 0.6 deg.$^2$, complete to K=16.5. A total of 867 galaxies are detected in this survey of which 175 have available redshifts. The near-infrared number counts to K=16.5 mag. are estimated from the complete photometric survey and are found to be in close agreement with other available studies. The sample is corrected for incompleteness in redshift space, using selection function in the form of a Fermi-Dirac distribution. This is then used to estimate the local near-infrared luminosity function of galaxies. A Schechter fit to the infrared data gives: M$^\ast_K = -25.1 \pm 0.3$, $\alpha = -1.3\pm 0.2$ and $\phi^\ast =(1.5\pm 0.5)\times 10^{-3}$ Mpc$^{-3}$ (for H$_0=50$ Km/sec/Mpc and q$_0=0.5$). When reduced to $\alpha=-1$, this agrees with other available estimates of the local IRLF. We find a steeper slope for the faint-end of the infrared luminosity function when compared to previous studies. This is interpreted as due to the presence of a population of faint but evolved (metal rich) galaxies in the local Universe. However, it is not from the same population as the faint blue galaxies found in the optical surveys. The characteristic magnitude ($M^\ast_K$) of the local IRLF indicates that the bright red galaxies ($M_K\sim -27$ mag.) have a space density of $\le 5\times 10^{-5}$ Mpc$^{-3}$ and hence, are not likely to be local objects.Comment: 24 pages, 8 figures, AASTEX 4.0, published in ApJ 492, 45

The Evolution of the Optical and Near-Infrared Galaxy Luminosity Functions and Luminosity Densities to z~2

Using Hubble Space Telescope and ground-based U through K- band photometry from the Great Observatories Origins Deep Survey (GOODS), we measure the evolution of the luminosity function and luminosity density in the rest-frame optical (UBR) to z ~ 2, bridging the poorly explored ``redshift desert'' between z~1 and z~2. We also use deep near-infrared observations to measure the evolution in the rest-frame J-band to z~1. Compared to local measurements from the SDSS, we find a brightening of the characteristic magnitude, (M*), by ~2.1, \~0.8 and ~0.7 mag between z=0.1 and z=1.9, in U, B, and R bands, respectively. The evolution of M* in the J-band is in the opposite sense, showing a dimming between redshifts z=0.4 and z=0.9. This is consistent with a scenario in which the mean star formation rate in galaxies was higher in the past, while the mean stellar mass was lower, in qualitative agreement with hierarchical galaxy formation models. We find that the shape of the luminosity function is strongly dependent on spectral type and that there is strong evolution with redshift in the relative contribution from the different spectral types to the luminosity density. We find good agreement in the luminosity function derived from an R-selected and a K-selected sample at z~1, suggesting that optically selected surveys of similar depth (R < 24) are not missing a significant fraction of objects at this redshift relative to a near-infrared-selected sample. We compare the rest-frame B-band luminosity functions from z~0--2 with the predictions of a semi-analytic hierarchical model of galaxy formation, and find qualitatively good agreement. In particular, the model predicts at least as many optically luminous galaxies at z~1--2 as are implied by our observations.Comment: 43 pages; 15 Figures; 5 Tables, Accepted for publication in Ap.

Selection of high-z supernovae candidates

Deep, ground based, optical wide-field supernova searches are capable of detecting a large number of supernovae over a broad redshift range up to z~1.5. While it is practically unfeasible to obtain spectroscopic redshifts of all the supernova candidates right after the discovery, we show that the magnitudes and colors of the host galaxies, as well as the supernovae, can be used to select high-z supernova candidates, for subsequent spectroscopic and photometric follow-up. Using Monte-Carlo simulations we construct criteria for selecting galaxies in well-defined redshift bands. For example, with a selection criteria using B-R and R-I colors we are able to pick out potential host galaxies for which z>0.85 with 80% confidence level and with a selection efficiency of 64-86%. The method was successfully tested using real observations from the HDF. Similarly, we show that that the magnitude and colors of the supernova discovery data can be used to constrain the redshift. With a set of cuts based on V-R and R-I in a search to m_I~25, supernovae at z~1 can be selected in a redshift interval sigma_z <0.15.Comment: 33 pages, 13 figures, accepted for publication in PASP (March 2002 issue

On the Determination of Star Formation Rates in Evolving Galaxy Populations

The redshift dependence of the luminosity density in certain wavebands (e.g. UV and H-alpha) can be used to infer the history of star formation in the populations of galaxies producing this luminosity. This history is a useful datum in studies of galaxy evolution. It is therefore important to understand the errors that attend the inference of star formation rate densities from luminosity densities. This paper explores the self-consistency of star formation rate diagnostics by reproducing commonly used observational procedures in a model with known galaxy populations, evolutionary histories and spectral emission properties. The study reveals a number of potential sources of error in the diagnostic processes arising from the differential evolution of different galaxy types. We argue that multi-wavelength observations can help to reduce these errors.Comment: 13 pages (including 5 encapsulated postscript figures), aastex, accepted for publication in Ap

Radio observations of the CDF-South: a possible link between radio emission and star formation in X-ray selected AGN

We explore the nature of the radio emission of X-ray selected AGN by combining deep radio (1.4GHz; 60micro-Jy) and X-ray data with multiwavelength (optical, mid-infrared) observations in the Extended Chandra Deep Field South (E-CDFS). The fraction of radio detected X-ray sources increases from 9% in the E-CDFS to 14% in the central region of this field, which has deeper X-ray coverage from the 1Ms CDFS. We find evidence that the radio emission of up to 60% of the hard X-ray/radio matched AGN is likely associated with star-formation in the host galaxy. Firstly, the mid-IR (24micron) properties of these sources are consistent with the infrared/radio correlation of starbursts. Secondly, most of them are found in galaxies with blue rest-frame optical colours (U-V), suggesting a young stellar population. On the contrary, X-ray/radio matched AGN which are not detected in the mid-infrared have red U-V colours suggesting their radio emission is associated with AGN activity. We also find no evidence for a population of heavily obscured radio-selected AGN that are not detected in X-rays. Finally, we do no confirm previous claims for a correlation between radio emission and X-ray obscuration. Assuming that the radio continuum measures star-formation, this finding is against models where the dust and gas clouds associated with circumnuclear starbursts are spherically blocking our view to the central engine.Comment: Accepted by A&

Deprojection of luminosity functions of galaxies in the Coma cluster

We use a simple analytic model to deproject 2-d luminosity functions (LF) of galaxies in the Coma cluster measured by Beijersbergen et al. 2002. We demonstrate that the shapes of the LFs change after deprojection. It is therefore essential to correct LFs for projection effects. The deprojected LFs of the central area have best-fitting Schechter parameters of M^{*}_U=-18.31^{+0.08}_{-0.08} and \alpha_U=-1.27^{+0.018}_{-0.018}, M^{*}_B=-19.79^{+0.14}_{-0.15} and \alpha_B=-1.44^{+0.016}_{-0.016} and M^{*}_r=-21.77^{+0.20}_{-0.28} and \alpha_r=-1.27^{+0.012}_{-0.012}. The corrections are not significant enough to change the previously observed trend of increasing faint end slopes with increasing distance to the cluster center. The weighted U, B, and r band slopes of the deprojected LFs show a slightly weaker steepening with increasing projected cluster radius.Comment: Accepted for publication in A&A as a Research Not