Stanislav G. Djorgovski

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 (OTs) with timescales of minutes to years, the CRTS analyses data from the Catalina Sky Survey which repeatedly covers 26,000 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 Keyhole Markup Language through Google Sky. The initial discoveries include over 350 unique OTs rising more than 2 mag from past measurements. Sixty two of these are classified as supernovae (SNe), based on light curves, prior deep imaging and spectroscopic data. Seventy seven are due to cataclysmic variables (CVs; only 13 previously known), while an additional 100 transients were too infrequently sampled to distinguish between faint CVs and SNe. The remaining OTs include active galactic nucleus, 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 SN 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.

Probing the Outer Galactic halo with RR Lyrae from the Catalina Surveys

We present analysis of 12,227 type-ab RR Lyraes (RRLs) found among the 200 million public light curves in Catalina Surveys Data Release 1. These stars span the largest volume of the Milky Way ever surveyed with RRLs, covering ~20,000 deg2 of the sky (0° 1500 of the RRLs. Using the accurate distances derived for the RRLs, we show the paths of the Sagittarius tidal streams crossing the sky at heliocentric distances from 20 to 60 kpc. By selecting samples of Galactic halo RRLs, we compare their velocity, metallicity, and distance with predictions from a recent detailed N-body model of the Sagittarius system. We find that there are some significant differences between the distances and structures predicted and our observations.

Identification of a galaxy responsible for a high-redshift Lyman-α absorption system

DAMPEDLyman-α systems are high-column-density intergalactic clouds of hydrogen, the existence of which is inferred from absorption lines appearing in the emission spectra of distant quasars. The galaxies believed to be responsible for these absorption systems have been suggested as possible progenitors of the normal disk galaxies observed in the local Universe1. Indeed, Lyman-α systems appear to contain a substantial fraction of the baryons known to exist in galaxies today2,3. Here we report the optical detection of a galaxy (designated DLA2233 + 131) associated with a known4 damped Lyman-α absorption system at a redshift of z = 3.150. The properties of this galaxy correspond closely to those expected of a young disk galaxy in the early stages of formation, and show no evidence for an active nucleus. This finding gives strong support to the idea that damped Lyman-α systems represent a population of young galaxies at high red-shifts.

1608+656 - A QUADRUPLE-LENS SYSTEM FOUND IN THE CLASS GRAVITATIONAL LENS SURVEY

The first phase of a large gravitational lens survey using the Very Large Array at a wavelength of 3.6 cm has been completed, yielding images for 3258 radio sources. The Cosmic Lens All-Sky Survey (CLASS) is designed to locate gravitational lens systems consisting of multiply imaged compact components with separations greater than 02. We report here the first discovery of a gravitational lens from the survey: 1608+656, a quadruply imaged object with a maximum separation of 21. Images from the Palomar 5 m and Keck 10 m telescopes show the lensed images and the lensing galaxy. An optical spectrum obtained with the Palomar 5 m telescope indicates a redshift of z=0.6304 for the lensing galaxy. No conclusive redshift for the lensed object has been determined, although a single strong emission line is found at 9240 A in the Keck low-resolution imaging spectrograph spectrum. The two most likely identifications for this line are Hβ (z=0.90) and Mg II (z=2.30). The preliminary lens model derived from the radio image reproduces the observed configuration and relative fluxes of the images, as well as the position, shape, and orientation of the lensing galaxy. Because a simple mass model is able to fit the observations, we argue that this lens system is promising for determining H0.

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

Precise radial velocity measurements from high-resolution echelle spectrometer 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 clusters 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〈v_r〉= −20.3 ± 0.7 km s^−1 and an internal velocity dispersion of σ = 4.14 ± 0.48 km s^−1, leading to a total mass of (9.0 ± 2.2) × 10^5 M⊙ 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 M⊙ 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 Modified Newtonian Dynamics, and for possible formation scenarios of ultracompact dwarf galaxies.

Evidence for a Milky Way Tidal Stream Reaching Beyond 100?kpc

We present the analysis of 1207 RR Lyrae found in photometry taken by the Catalina Surveys Mount Lemmon telescope. By combining accurate distances for these stars with measurements for ~14,000 type-ab RR Lyrae from the Catalina Schmidt telescope, we reveal an extended association that reaches Galactocentric distances beyond 100 kpc and overlaps the Sagittarius stream system. This result confirms earlier evidence for the existence of an outer halo tidal stream resulting from a disrupted stellar system. By comparing the RR Lyrae source density with that expected based on halo models, we find the detection has ~8σ significance. We investigate the distances, radial velocities, metallicities, and period-amplitude distribution of the RR Lyrae. We find that both radial velocities and distances are inconsistent with current models of the Sagittarius stream. We also find tentative evidence for a division in source metallicities for the most distant sources. Following prior analyses, we compare the locations and distances of the RR Lyrae with photometrically selected candidate horizontal branch stars and find supporting evidence that this structure spans at least 60° of the sky. We investigate the prospects of an association between the stream and the unusual globular cluster NGC 2419.

A systematic search for close supermassive black hole binaries in the Catalina Real-time Transient Survey

Hierarchical assembly models predict a population of supermassive black hole (SMBH) binaries. These are not resolvable by direct imaging but may be detectable via periodic variability (or nanohertz frequency gravitational waves). Following our detection of a 5.2-year periodic signal in the quasar PG 1302−102, we present a novel analysis of the optical variability of 243 500 known spectroscopically confirmed quasars using data from the Catalina Real-time Transient Survey (CRTS) to look for close (<0.1 pc) SMBH systems. Looking for a strong Keplerian periodic signal with at least 1.5 cycles over a baseline of nine years, we find a sample of 111 candidate objects. This is in conservative agreement with theoretical predictions from models of binary SMBH populations. Simulated data sets, assuming stochastic variability, also produce no equivalent candidates implying a low likelihood of spurious detections. The periodicity seen is likely attributable to either jet precession, warped accretion discs or periodic accretion associated with a close SMBH binary system. We also consider how other SMBH binary candidates in the literature appear in CRTS data and show that none of these are equivalent to the identified objects. Finally, the distribution of objects found is consistent with that expected from a gravitational-wave-driven population. This implies that circumbinary gas is present at small orbital radii and is being perturbed by the black holes. None of the sources is expected to merge within at least the next century. This study opens a new unique window to study a population of close SMBH binaries that must exist according to our current understanding of galaxy and SMBH evolution.

Discovery of the Extremely Energetic Supernova 2008fz

We report on the discovery and initial observations of the energetic type IIn supernova 2008fz. This object was discovered at redshift z = 0.133 and reached an apparent magnitude of V ~ 17. After correcting for Galactic extinction and redshift, we determine the peak absolute magnitude of the event to be M_V = –22.3, placing it among the most luminous supernovae discovered. The optical energy emitted by SN 2008fz (based on the light curve over an 88 day period) is possibly the most ever observed for a supernova (>1.4 × 10^(51) erg). The event was more luminous than the type IIn SN 2006gy, but exhibited the same smooth, slowly evolving light curve. As is characteristic of type IIn supernova, the early spectra of SN 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 dust in the host or circumstellar material. No host galaxy is identified in prior co-added images reaching R ~ 22. From the supernovas redshift, we place an upper limit on the brightness of the host of M_R ~ –17 (similar to the brightness of the Small Magellanic Cloud). The presence of the supernova within such a faint galaxy follows the majority of recently discovered highly luminous supernovae. A possible reason for this is the combination of a high star formation rate in low-mass galaxies with a low-metallicity environment.

Cataclysmic variables from the Catalina Real-time Transient Survey

We present 855 cataclysmic variable candidates detected by the Catalina Real-time Transient Survey (CRTS) of which at least 137 have been spectrosc opically confirmed and 705 are new discoveries. The sources were identified from the ana lysis of five years of data, and come from an area covering three quarters of the sky. We study the amplitude distribution of the dwarf novae CVs discovered by CRTS during outburst, and fi nd that in quiescence they are typically two magnitudes fainter compared to the spectr os opic CV sample identified by SDSS. However, almost all CRTS CVs in the SDSS footprint have ugriz photometry. We analyse the spatial distribution of the CVs and find evidence that many of the systems lie at scale heights beyond those expected for a Galactic thin disc population. We compare the outburst rates of newly discovered CRTS CVs with the previously known CV population, and find no evidence for a difference between them. However, we fin d that significant evidence for a systematic difference in orbital period distribution . We discuss the CVs found below the orbital period minimum and argue that many more are yet to be identified among the full CRTS CV sample. We cross-match the CVs with archival X-ray ca talogs and find that most of the systems are dwarf novae rather than magnetic CVs.

The SKICAT System for Processing and Analyzing Digital Imaging Sky Surveys

We describe the design and implementation of a software system for producing, managing, and analyzing catalogs from the digital scans of the Second Palomar Observatory Sky Surveys. The system (SKICAT) integrates new and existing packages for performing the full sequence of tasks from raw pixel processing, to object classification, to the matching of multiple, overlapping Schmidt plates and CCD calibration frames. We describe the relevant details of constructing SKITCAT plate, CCD, matched, and object catalogs. Plate and CCD catalogs are generated from images, while the latter are derived from existing catalogs. A pair of programs complete the majroity of plate and CCD processing in an automated, pipeline fashion, with the user required to execute a minimal number of pre- and post-processing procedures. We apply a modified version of FOCAS for the detection and photometry, and new software for matching catalogs on an object by object basis. SKICAT employs modern machine learning techniques, such as decision trees, to perform automatic star-galaxy-artifact classification with a > 90% accuracy down to ~1m above the plate detection limit. The system also provides a variety of tools for interactively querying and analyzing the resulting object catalogs.