arXiv subjects

S. G. Djorgovski

Publications and source records attributed to S. G. Djorgovski.

At least 145 records · Page 8Linked to original sources

arXiv2009

Discovery of the Extremely Energetic Supernova 2008fz

We report on the discovery and initial observations of the energetic type IIn supernova (SN), 2008fz. The optical energy emitted by SN 2008fz (based on the light curve over a 88 day period), is possibly the most ever observed for a supernova (1.4 x 10^51 erg). The event was more luminous than the type IIn SN 2006gy, but exhibited same smooth, slowly evolving light curve. As is characteristic of type IIn SN, the early spectra of 2008fz initially exhibited narrow Balmer lines which were replaced by a broader component at later times. The spectra also show a blue continuum with no signs of Ca or Na absorption, suggesting that there is little extinction due to intragalatic dust in the host or circumstellar material. No host galaxy is identified in prior coadded images reaching R ~ 22. From the supernova's redshift, z=0.133, we place an upper limit on the host of M_R=-17. The presence of the SN within such a faint host follows the majority of recently discovered highly luminous SN. A possible reason for this occurrence is the very high star formation rate occurring in low-mass galaxies in combination with the low metallicity environment, which makes the production of very massive stars possible. We determine the peak absolute magnitude of the event to be M_V = -22.3 from the initial photometry and the redshift distance, placing it among the most luminous supernovae discovered.

astro-ph.HE↗

arXiv2009

Highly Variable Objects in the Palomar-QUEST Survey: A Blazar Search using Optical Variability

We identify 3,113 highly variable objects in 7,200 square degrees of the Palomar-QUEST Survey, which each varied by more than 0.4 magnitudes simultaneously in two broadband optical filters on timescales from hours to roughly 3.5 years. The primary goal of the selection is to find blazars by their well-known violent optical variability. Because most known blazars have been found in radio and/or X-ray wavelengths, a sample discovered through optical variability may have very different selection effects, elucidating the range of behavior possible in these systems. A set of blazars selected in this unusual manner will improve our understanding of the physics behind this extremely variable and diverse class of AGN. The object positions, variability statistics, and color information are available using the Palomar-QUEST CasJobs server. The time domain is just beginning to be explored over large sky areas; we do not know exactly what a violently variable sample will hold. About 20% of the sample has been classified in the literature; over 70% of those objects are known or likely AGN. The remainder largely consists of a variety of variable stars, including a number of RR Lyrae and cataclysmic variables.

astro-ph.CO↗

arXiv2009

Binary Quasars at High Redshift I: 24 New Quasar Pairs at z ~ 3-4

The clustering of quasars on small scales yields fundamental constraints on models of quasar evolution and the buildup of supermassive black holes. This paper describes the first systematic survey to discover high redshift binary quasars. Using color-selection and photometric redshift techniques, we searched 8142 deg^2 of SDSS imaging data for binary quasar candidates, and confirmed them with follow-up spectroscopy. Our sample of 27 high redshift binaries (24 of them new discoveries) at redshifts 2.9 < z < 4.3 with proper transverse separations 10 kpc < R_{\perp} < 650 kpc increases the number of such objects known by an order of magnitude. Eight members of this sample are very close pairs with R_{\perp} < 100 kpc, and of these close systems four are at z > 3.5. The completeness and efficiency of our well-defined selection algorithm are quantified using simulated photometry and we find that our sample is ~ 50% complete. Our companion paper uses this knowledge to make the first measurement of the small scale clustering (R < 1 Mpc/h comoving) of high-redshift quasars. High redshift binaries constitute exponentially rare coincidences of two extreme (M >~ 10^9 Msun) supermassive black holes. At z ~ 4 there is about one close binary per 10 Gpc^3, thus these could be the highest sigma peaks, the analogs of superclusters, in the early Universe.

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arXiv2009

Binary Quasars at High Redshift II: Sub-Mpc Clustering at z ~ 3-4

We present measurements of the small-scale (0.1<~ r <~ 1 Mpc/h) quasar two-point correlation function at z>2.9, for a flux-limited (i<21) sample of 15 binary quasars compiled by Hennawi et al. (2009). The amplitude of the small-scale clustering increases from z ~ 3 to z ~ 4. The small-scale clustering amplitude is comparable to or lower than power-law extrapolations (with slope gamma=2) from the large-scale correlation function of the i<20.2 quasar sample from the Sloan Digital Sky Survey. Using simple prescriptions relating quasars to dark matter halos, we model the observed small-scale clustering with halo occupation models. Reproducing the large-scale clustering amplitude requires that the active fraction of the black holes in the central galaxies of halos is near unity, but the level of small-scale clustering favors an active fraction of black holes in satellite galaxies 0.1 <~ f_s <~ 0.5 at z >~ 3.

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arXiv2009

Exploring the Use of Virtual Worlds as a Scientific Research Platform: The Meta-Institute for Computational Astrophysics (MICA)

We describe the Meta-Institute for Computational Astrophysics (MICA), the first professional scientific organization based exclusively in virtual worlds (VWs). The goals of MICA are to explore the utility of the emerging VR and VWs technologies for scientific and scholarly work in general, and to facilitate and accelerate their adoption by the scientific research community. MICA itself is an experiment in academic and scientific practices enabled by the immersive VR technologies. We describe the current and planned activities and research directions of MICA, and offer some thoughts as to what the future developments in this arena may be.

astro-ph.IM↗

arXiv2009

The Palomar Transient Factory: System Overview, Performance and First Results

The Palomar Transient Factory (PTF) is a fully-automated, wide-field survey aimed at a systematic exploration of the optical transient sky. The transient survey is performed using a new 8.1 square degree camera installed on the 48-inch Samuel Oschin telescope at Palomar Observatory; colors and light curves for detected transients are obtained with the automated Palomar 60-inch telescope. PTF uses eighty percent of the 1.2-m and fifty percent of the 1.5-m telescope time. With an exposure of 60-s the survey reaches a depth of approximately 21.3 in g' and 20.6 in R (5 sigma, median seeing). Four major experiments are planned for the five-year project: 1) a 5-day cadence supernova search; 2) a rapid transient search with cadences between 90 seconds and 1 day; 3) a search for eclipsing binaries and transiting planets in Orion; and 4) a 3-pi sr deep H-alpha survey. PTF provides automatic, realtime transient classification and follow up, as well as a database including every source detected in each frame. This paper summarizes the PTF project, including several months of on-sky performance tests of the new survey camera, the observing plans and the data reduction strategy. We conclude by detailing the first 51 PTF optical transient detections, found in commissioning data.

astro-ph.IM↗

arXiv2009

Skyalert: Real-time Astronomy for You and Your Robots

Skyalert.org is a web application to collect and disseminate observations about time-critical astronomical transients, and to add annotations and intelligent machine-learning to those observations. The information is "pushed" to subscribers, who may be either humans (email, text message etc) or they may be machines that control telescopes. Subscribers can prepare precise "trigger rules" to decide which events should reach them and their robots, rules that may be based on sky position, or on the specific vocabulary of parameters that define a particular type of observation. Our twin thrusts are automation of process, and discrimination of interesting events.

astro-ph.IM↗

arXiv2009

The velocity dispersion and mass-to-light ratio of the remote halo globular cluster NGC 2419

Precise radial velocity measurements from HIRES on the Keck I telescope are presented for 40 stars in the outer halo globular cluster NGC 2419. These data are used to probe the cluster's stellar mass function and search for the presence of dark matter in this cluster. NGC 2419 is one of the best Galactic globular clusters for such a study due to its long relaxation time (T_{r0} ~ 10^{10} yr) and large Galactocentric distance (R_{GC} ~ 90 kpc) -- properties that make significant evolutionary changes in the low-mass end of the cluster mass function unlikely. We find a mean cluster velocity of =-20.3 +- 0.7 km/sec and an internal velocity dispersion of σ= 4.14 +- 0.48 km/sec, leading to a total mass of (9.0 +- 2.2) * 10^5 Msun and a global mass-to-light ratio of M/L_V = 2.05 +- 0.50 in solar units. This mass-to-light ratio is in good agreement with what one would expect for a pure stellar system following a standard mass function at the metallicity of NGC 2419. In addition, the mass-to-light ratio does not appear to rise towards the outer parts of the cluster. Our measurements therefore rule out the presence of a dark matter halo with mass larger than ~10^7 Msun inside the central 500 pc, which is lower than what is found for the central dark matter densities of dSph galaxies. We also discuss the relevance of our measurements for alternative gravitational theories such as MOND, and for possible formation scenarios of ultra-compact dwarf galaxies.

astro-ph.GA↗

arXiv2009

Thirty Meter Telescope Site Testing I: Overview

As part of the conceptual and preliminary design processes of the Thirty Meter Telescope (TMT), the TMT site testing team has spent the last five years measuring the atmospheric properties of five candidate mountains in North and South America with an unprecedented array of instrumentation. The site testing period was preceded by several years of analyses selecting the five candidates, Cerros Tolar, Armazones and Tolonchar in northern Chile; San Pedro Martir in Baja California, Mexico and the 13 North (13N) site on Mauna Kea, Hawaii. Site testing was concluded by the selection of two remaining sites for further consideration, Armazones and Mauna Kea 13N. It showed that all five candidates are excellent sites for an extremely large astronomical observatory and that none of the sites stands out as the obvious and only logical choice based on its combined properties. This is the first article in a series discussing the TMT site testing project.

astro-ph.IM↗

arXiv2009

Testing Fundamental Physics with Distant Star Clusters: Analysis of Observational Data on Palomar 14

We use the distant outer halo globular cluster Palomar 14 as a test case for classical vs. modified Newtonian dynamics (MOND). Previous theoretical calculations have shown that the line-of-sight velocity dispersion predicted by these theories can differ by up to a factor of three for such sparse, remote clusters like Pal 14. We determine the line-of-sight velocity dispersion of Palomar 14 by measuring radial velocities of 17 red giant cluster members obtained using the Very Large Telescope (VLT) and Keck telescope. The systemic velocity of Palomar 14 is (72.28+-0.12)km/s. The derived velocity dispersion of (0.38+-0.12)km/s of the 16 definite member stars is in agreement with the theoretical prediction for the classical Newtonian case according to Baumgardt et al. (2005). In order to exclude the possibility that a peculiar mass function might have influenced our measurements, we derived the cluster's main sequence mass function down to 0.53Msolar using archival images obtained with the Hubble Space Telescope. We found a mass function slope of 1.27+-0.44, which is, compared to the canonical mass function, a significantly shallower slope. The derived lower limit on the cluster's mass is higher than the theoretically predicted mass in case of MOND. Our data are consistent with a central density of 0.1 Msolar pc^-3. We need no dark matter in Palomar 14. If the cluster is on a circular orbit, our spectroscopic and photometric results argue against MOND, unless this cluster experienced significant mass loss.

astro-ph.GA↗

arXiv2008

The Northern Sky Optical Cluster Survey III: A Cluster Catalog Covering Pi Steradians

We present the complete galaxy cluster catalog from the Northern Sky Optical Cluster Survey, a new, objectively defined catalog of candidate galaxy clusters at z<0.25 drawn from the Digitized Second Palomar Observatory Sky Survey (DPOSS). The data presented here cover the Southern Galactic Cap, as well as the less-well calibrated regions of the Northern Galactic Cap. In addition, due to improvements in our cluster finder and measurement methods, we provide an updated catalog for the well-calibrated Northern Galactic Cap region previously published in Paper II. The complete survey covers 11,411 square degrees, with over 15,000 candidate clusters. We discuss improved photometric redshifts, richnesses and optical luminosities which are provided for each cluster. A variety of substructure measures are computed for a subset of over 11,000 clusters. We also discuss the derivation of dynamical radii r_200 and its relation to cluster richness. A number of consistency checks between the three areas of the survey are also presented, demonstrating the homogeneity of the catalog over disjoint sky areas. We perform extensive comparisons to existing optically and X-ray selected cluster catalogs, and derive new X-ray luminosities and temperatures for a subset of our clusters. We find that the optical and X-ray luminosities are well correlated, even using relatively shallow ROSAT All Sky Survey and DPOSS data. This survey provides a good comparison sample to the MaxBCG catalog based on Sloan Digital Sky Survey Data, and complements that survey at low redshifts 0.07<z<0.1.

astro-ph↗

arXiv2008

Cosmological Studies with Radio Galaxies and Supernovae

Physical sizes of extended radio galaxies can be employed as a cosmological "standard ruler", using a previously developed method. Eleven new radio galaxies are added to our previous sample of nineteen sources, forming a sample of thirty objects with redshifts between 0 and 1.8. This sample of radio galaxies are used to obtain the best fit cosmological parameters in a quintessence model in a spatially flat universe, a cosmological constant model that allows for non-zero space curvature, and a rolling scalar field model in a spatially flat universe. Results obtained with radio galaxies are compared with those obtained with different supernova samples, and with combined radio galaxy and supernova samples. Results obtained with different samples are consistent, suggesting that neither method is seriously affected by systematic errors. Best fit radio galaxy and supernovae model parameters determined in the different cosmological models are nearly identical, and are used to determine dimensionless coordinate distances to supernovae and radio galaxies, and distance moduli to the radio galaxies. The distance moduli to the radio galaxies can be combined with supernovae samples to increase the number of sources, particularly high-redshift sources, in the samples. The constraints obtained here with the combined radio galaxy plus supernovae data set in the rolling scalar field model are quite strong. The best fit parameter values suggest a value of omega is less than about 0.35, and the model parameter alpha is close to zero; that is, a cosmological constant provides a good description of the data. We also obtain new constraints on the physics of engines that power the large-scale radio emission.

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arXiv2008

The Environments of High Redshift QSOs

We present a sample of $i_{775}$-dropout candidates identified in five Hubble Advanced Camera for Surveys fields centered on Sloan Digital Sky Survey QSOs at redshift $z\sim 6$. Our fields are as deep as the Great Observatory Origins Deep Survey (GOODS) ACS images which are used as a reference field sample. We find them to be overdense in two fields, underdense in two fields, and as dense as the average density of GOODS in one field. The two excess fields show significantly different color distributions from that of GOODS at the 99% confidence level, strengthening the idea that the excess objects are indeed associated with the QSO. The distribution of $i_{775}$-dropout counts in the five fields is broader than that derived from GOODS at the 80% to 96% confidence level, depending on which selection criteria were adopted to identify $i_{775}$-dropouts; its width cannot be explained by cosmic variance alone. Thus, QSOs seem to affect their environments in complex ways. We suggest the picture where the highest redshift QSOs are located in very massive overdensities and are therefore surrounded by an overdensity of lower mass halos. Radiative feedback by the QSO can in some cases prevent halos from becoming galaxies, thereby generating in extreme cases an underdensity of galaxies. The presence of both enhancement and suppression is compatible with the expected differences between lines of sight at the end of reionization as the presence of residual diffuse neutral hydrogen would provide young galaxies with shielding from the radiative effects of the QSO.

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arXiv2008

New Approaches to Object Classification in Synoptic Sky Surveys

Digital synoptic sky surveys pose several new object classification challenges. In surveys where real-time detection and classification of transient events is a science driver, there is a need for an effective elimination of instrument-related artifacts which can masquerade as transient sources in the detection pipeline, e.g., unremoved large cosmic rays, saturation trails, reflections, crosstalk artifacts, etc. We have implemented such an Artifact Filter, using a supervised neural network, for the real-time processing pipeline in the Palomar-Quest (PQ) survey. After the training phase, for each object it takes as input a set of measured morphological parameters and returns the probability of it being a real object. Despite the relatively low number of training cases for many kinds of artifacts, the overall artifact classification rate is around 90%, with no genuine transients misclassified during our real-time scans. Another question is how to assign an optimal star-galaxy classification in a multi-pass survey, where seeing and other conditions change between different epochs, potentially producing inconsistent classifications for the same object. We have implemented a star/galaxy multipass classifier that makes use of external and a priori knowledge to find the optimal classification from the individually derived ones. Both these techniques can be applied to other, similar surveys and data sets.

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arXiv2008

Towards Real-time Classification of Astronomical Transients

Exploration of time domain is now a vibrant area of research in astronomy, driven by the advent of digital synoptic sky surveys. While panoramic surveys can detect variable or transient events, typically some follow-up observations are needed; for short-lived phenomena, a rapid response is essential. Ability to automatically classify and prioritize transient events for follow-up studies becomes critical as the data rates increase. We have been developing such methods using the data streams from the Palomar-Quest survey, the Catalina Sky Survey and others, using the VOEventNet framework. The goal is to automatically classify transient events, using the new measurements, combined with archival data (previous and multi-wavelength measurements), and contextual information (e.g., Galactic or ecliptic latitude, presence of a possible host galaxy nearby, etc.); and to iterate them dynamically as the follow-up data come in (e.g., light curves or colors). We have been investigating Bayesian methodologies for classification, as well as discriminated follow-up to optimize the use of available resources, including Naive Bayesian approach, and the non-parametric Gaussian process regression. We will also be deploying variants of the traditional machine learning techniques such as Neural Nets and Support Vector Machines on datasets of reliably classified transients as they build up.

astro-ph↗

arXiv2008

First Results from the Catalina Real-time Transient Survey

We report on the results from the first six months of the Catalina Real-time Transient Survey (CRTS). In order to search for optical transients with timescales of minutes to years, the CRTS analyses data from the Catalina Sky Survey which repeatedly covers twenty six thousand of square degrees on the sky. The CRTS provides a public stream of transients that are bright enough to be followed up using small telescopes. Since the beginning of the survey, all CRTS transients have been made available to astronomers around the world in real-time using HTML tables, RSS feeds and VOEvents. As part of our public outreach program the detections are now also available in KML through Google Sky. The initial discoveries include over 350 unique optical transients rising more than two magnitudes from past measurements. Sixty two of these are classified as supernovae, based on light curves, prior deep imaging and spectroscopic data. Seventy seven are due to cataclysmic variables (only 13 previously known), while an additional 100 transients were too infrequently sampled to distinguish between faint CVs and SNe. The remaining optical transients include AGN, Blazars, high proper motions stars, highly variable stars (such as UV Ceti stars) and transients of an unknown nature. Our results suggest that there is a large population of SNe missed by many current supernova surveys because of selection biases. These objects appear to be associated with faint host galaxies. We also discuss the unexpected discovery of white dwarf binary systems through dramatic eclipses.

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arXiv2008

An Exploratory Search for z~6 Quasars in the UKIDSS Early Data Release

We conducted an exploratory search for quasars at z~ 6 - 8, using the Early Data Release from United Kingdom Infrared Deep Sky survey (UKIDSS) cross-matched to panoramic optical imagery. High redshift quasar candidates are chosen using multi-color selection in i,z,Y,J,H and K bands. After removal of apparent instrumental artifacts, our candidate list consisted of 34 objects. We further refined this list with deeper imaging in the optical for ten of our candidates. Twenty-five candidates were followed up spectroscopically in the near-infrared and in the optical. We confirmed twenty-five of our spectra as very low-mass main-sequence stars or brown dwarfs, which were indeed expected as the main contaminants of this exploratory search. The lack of quasar detection is not surprising: the estimated probability of finding a single z>6 quasar down to the limit of UKIDSS in the 27.3 square degrees of the EDR is <5%. We find that the most important limiting factor in this work is the depth of the available optical data. Experience gained in this pilot project can help refine high-redshift quasar selection criteria for subsequent UKIDSS data releases.

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arXiv2008

The Origins and the Early Evolution of Quasars and Supermassive Black Holes

The relationship between galaxies and supermassive black holes (SMBH) found in their cores plays a key role in the formation and evolution of both of these major constituents of the universe, as well as the evolution of the intergalactic medium. Neither can be fully understood on their own, and studies of galaxy and SMBH co-formation and co-evolution are now among the central topics of research in cosmology. Yet the very origins, and the early growth phases of the SMBH are still not firmly established. We review our current understanding of the relevant processes and their astrophysical and cosmological context, with an emphasis on the observability of the SMBH growth mechanisms at high redshifts, and their leftover progeny at low redshifts.

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