Catherine Petry

The FIRST Bright Quasar Survey. II. 60 Nights and 1200 Spectra Later

We have used the Very Large Array (VLA) FIRST survey and the Automated Plate Measuring Facility (APM) catalog of the Palomar Observatory Sky Survey I (POSS-I) plates as the basis for constructing a new radio-selected sample of optically bright quasars. This is the first radio-selected sample that is competitive in size with current optically selected quasar surveys. Using only two basic criteria, radio-optical positional coincidence and optical morphology, quasars and BL Lac objects can be identified with 60% selection efficiency; the efficiency increases to 70% for objects fainter than 17 mag. We show that a more sophisticated selection scheme can predict with better than 85% reliability which candidates will turn out to be quasars. This paper presents the second installment of the FIRST Bright Quasar Survey (FBQS), with a catalog of 636 quasars distributed over 2682 deg2. The quasar sample is characterized and all spectra are displayed. The FBQS detects both radio-loud and radio-quiet quasars out to redshift z > 3. We find a large population of objects of intermediate radio loudness; there is no evidence in our sample for a bimodal distribution of radio characteristics. The sample includes ~29 broad absorption line quasars, both high and low ionization, and a number of new objects with remarkable optical spectra.

An end-to-end simulation framework for the Large Synoptic Survey Telescope

The LSST will, over a 10-year period, produce a multi-color, multi-epoch survey of more than 18000 square degrees of the southern sky. It will generate a multi-petabyte archive of images and catalogs of astrophysical sources from which a wide variety of high-precision statistical studies can be undertaken. To accomplish these goals, the LSST project has developed a suite of modeling and simulation tools for use in validating that the design and the as-delivered components of the LSST system will yield data products with the required statistical properties. In this paper we describe the development, and use of the LSST simulation framework, including the generation of simulated catalogs and images for targeted trade studies, simulations of the observing cadence of the LSST, the creation of large-scale simulations that test the procedures for data calibration, and use of end-to-end image simulations to evaluate the performance of the system as a whole.

The LSST metrics analysis framework (MAF)

We describe the Metrics Analysis Framework (MAF), an open-source python framework developed to provide a user-friendly, customizable, easily-extensible set of tools for analyzing data sets. MAF is part of the Large Synoptic Survey Telescope (LSST) Simulations effort. Its initial goal is to provide a tool to evaluate LSST Operations Simulation (OpSim) simulated surveys to help understand the effects of telescope scheduling on survey performance, however MAF can be applied to a much wider range of datasets. The building blocks of the framework are Metrics (algorithms to analyze a given quantity of data), Slicers (subdividing the overall data set into smaller data slices as relevant for each Metric), and Database classes (to access the dataset and read data into memory). We describe how these building blocks work together, and provide an example of using MAF to evaluate different dithering strategies. We also outline how users can write their own custom Metrics and use these within the framework.

Simulation of autonomous observing with a ground-based telescope: the LSST experience

A survey program with multiple science goals will be driven by multiple technical requirements. On a ground-based telescope, the variability of conditions introduces yet greater complexity. For a program that must be largely autonomous with minimal dwell time for efficiency it may be quite difficult to foresee the achievable performance. Furthermore, scheduling will likely involve self-referential constraints and appropriate optimization tools may not be available. The LSST project faces these issues, and has designed and implemented an approach to performance analysis in its Operations Simulator and associated post-processing packages. The Simulator has allowed the project to present detailed performance predictions with a strong basis from the engineering design and measured site conditions. At present, the Simulator is in regular use for engineering studies and science evaluation, and planning is underway for evolution to an operations scheduling tool. We will describe the LSST experience, emphasizing the objectives, the accomplishments and the lessons learned.

QUASARS AS ABSORPTION PROBES OF THE J0053+1234 REGION

We present UBRI photometry and spectra for 60 quasars found within 1 deg2 centered on the J0053+1234 region, which has been the subject of the Caltech Faint Galaxy Redshift Survey. Candidate quasars were selected by their ultraviolet excess with respect to the stellar locus and confirmed spectroscopically. The quasars span a wide range in brightness (17.5 < B < 21.6) and redshift (0.43 < z < 2.38). These new quasars comprise a grid of absorption probes that can be used to study large-scale structure, as well as the correlation between luminous galaxies, nonluminous halos, and Lyα absorbers in the direction of the deep pencil-beam galaxy survey. Spectra of 14 emission-line galaxies found using the same technique are also presented.

Measuring the effectiveness of simulated LSST observing programs

The Large Synoptic Survey Telescope will record approximately 2.5x10^6 images over a 10-year interval, using 6 optical filters, with a wide variety of cadences on time scales of seconds to years. The observing program will be of a complexity that it can only be realized with heavily automated scheduling. The LSST OpSim team has devised a schedule simulator to support development of that capability. This paper addresses the complex problem of how to measure the success of a schedule simulation for realization of science objectives. Tools called Merit Functions evaluate the patterns and other properties of scheduled image acquisitions.

PC 1643+4631A, B: THE LYMAN-α FOREST AT THE EDGE OF COHERENCE

This is the first measurement and detection of coherence in the intergalactic medium (IGM) at substantially high redshift (z ~ 3.8) and on large physical scales (~2.5 h –1 70 Mpc). We perform the measurement by presenting new observations of the high-redshift quasar pair PC 1643+4631A, B and their Lyα-absorber coincidences. With data collected from Keck I Low Resolution Imaging Spectrometer (LRIS) in a 10,200 s integration, we have full coverage of the Lyα forest over the redshift range 2.6 < z < 3.8 at a resolution of 3.6 A (~220 km s–1). This experiment extends multiple sight line quasar absorber studies to higher redshift, higher opacity, larger transverse separation, and into a regime where coherence across the IGM becomes weak and difficult to detect. Noteworthy features from these spectra are the strong damped Lyα absorbers (DLAs) just blueward of both Lyα emission peaks, each within 1000 km s–1 of the emission redshift but separated by 2500 km s–1 from each other. The coherence is measured by fitting discrete Lyα absorbers and by using pixel flux statistics. The former technique results in 222 Lyα absorbers in the A sight line and 211 in B. Relative to a Monte Carlo pairing test (using symmetric, nearest-neighbor matching) the data exhibit a 4σ excess of pairs at low velocity splitting (Δv < 150 km s–1), thus detecting coherence on transverse scales of ~2.5 h –1 70 Mpc. We use spectra extracted from a smoothed particle hydrodynamic (SPH) simulation to analyze symmetric pair matching, transmission distributions as a function of redshift and compute zero-lag cross-correlations to compare with the quasar pair data. The simulations agree with the data with the same strength (~4σ) at similarly low velocity splitting above random chance pairings. In cross-correlation tests, the simulations agree when the mean flux (as a function of redshift) is assumed to follow the prescription given by Kirkman et al. (2005). While the detection of flux correlation (measured through coincident absorbers and cross-correlation amplitude) is only marginally significant, the agreement between data and simulations is encouraging for future work in which even better quality data will provide the best insight into the overarching structure of the IGM and its understanding as shown by SPH simulations.

An optical to IR sky brightness model for the LSST

To optimize the observing strategy of a large survey such as the LSST, one needs an accurate model of the night sky emission spectrum across a range of atmospheric conditions and from the near-UV to the near-IR. We have used the ESO SkyCalc Sky Model Calculator1, 2 to construct a library of template spectra for the Chilean night sky. The ESO model includes emission from the upper and lower atmosphere, scattered starlight, scattered moonlight, and zodiacal light. We have then extended the ESO templates with an empirical fit to the twilight sky emission as measured by a Canon all-sky camera installed at the LSST site. With the ESO templates and our twilight model we can quickly interpolate to any arbitrary sky position and date and return the full sky spectrum or surface brightness magnitudes in the LSST filter system. Comparing our model to all-sky observations, we find typical residual RMS values of ±0.2-0.3 magnitudes per square arcsecond.

Simulating the LSST OCS for conducting survey simulations using the LSST scheduler

The Operations Simulator was used to prototype the Large Synoptic Survey Telescope (LSST) Scheduler. Currently, the Scheduler is being developed separately to interface with the LSST Observatory Control System (OCS). A new Simulator is under concurrent development to adjust to this new architecture. This requires a package simulating enough of the OCS to allow execution of realistic schedules. This new package is called the Simulated OCS (SOCS). In this paper we detail the SOCS construction plan, package structure, LSST communication middleware platform use, provide some interesting use cases that the separated architecture allows and the software engineering practices used in development.