arXiv subjects

S. G. Djorgovski

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

At least 55 records · Page 3Linked to original sources

arXiv2019

A MegaCam Survey of Outer Halo Satellites. VII. A Single Sérsic Index v/s Effective Radius Relation for Milky Way Outer Halo Satellites

In this work we use structural properties of Milky Way's outer halo ($R_G > 25\,\mathrm{kpc}$) satellites (dwarf spheroidal galaxies, ultra-faint dwarf galaxies and globular clusters) derived from deep, wide-field and homogeneous data, to present evidence of a correlation in the Sérsic index v/s effective radius plane followed by a large fraction of outer halo globular clusters and satellite dwarf galaxies. We show that this correlation can be entirely reproduced by fitting empirical relations in the central surface brightness v/s absolute magnitude and Sérsic index v/s absolute magnitude parameter spaces, and by assuming the existence of two types of outer halo globular clusters: one of high surface brightness (HSB group), with properties similar to inner halo clusters; and another of low surface brightness (LSB group), which share characteristics with dwarf spheroidal and ultra-faint dwarf galaxies. Given the similarities of LSB clusters with dwarf spheroidal and ultra-faint dwarf galaxies, we discuss the possibility that outer halo clusters also originated inside dark matter halos and that tidal forces from different galaxy host's potentials are responsible for the different properties between HSB and LSB clusters.

astro-ph.GA↗

arXiv2019

Gaia GraL: Gaia DR2 Gravitational Lens Systems. III. A systematic blind search for new lensed systems

Aims: In this work, we aim to provide a reliable list of gravitational lens (GL) candidates based on a search performed over the entire Gaia Data Release 2 (Gaia DR2). We also show that the sole astrometric and photometric informations coming from the Gaia satellite yield sufficient insights for supervised learning methods to automatically identify GL candidates with an efficiency that is comparable to methods based on image processing. Methods: We simulated 106,623,188 lens systems composed of more than two images, based on a regular grid of parameters characterizing a non-singular isothermal ellipsoid lens model in the presence of an external shear. These simulations are used as an input for training and testing our supervised learning models consisting of Extremely Randomized Trees. The latter are finally used to assign to each of the 2,129,659 clusters of celestial objects a discriminant value that reflects the ability of our simulations to match the observed relative positions and fluxes from each cluster. Once complemented with additional constraints, these discriminant values allowed us to identify GL candidates out of the list of clusters. Results: We report the discovery of 15 new quadruply-imaged lens candidates with angular separations less than 6" and assess the performance of our approach by recovering 12 out of the 13 known quadruply-imaged systems with all their components detected in Gaia DR2 with a misclassification rate of fortuitous clusters of stars as lens systems that is below one percent. Similarly, the identification capability of our method regarding quadruply-imaged systems where three images are detected in Gaia DR2 is assessed by recovering 10 out of the 13 known quadruply-imaged systems having one of their constituting images discarded. The associated misclassification rate varying then between 5.8% and 20%, depending on the image we decided to remove.

astro-ph.CO↗

arXiv2018

Results of a systematic search for outburst events in 1.4 million galaxies

We present an analysis of nine years of Catalina Surveys optical photometry for 1.4 million spectroscopically confirmed SDSS galaxies. We find 717 outburst events that were not reported by ongoing transient surveys. These events have timescales ranging from weeks to years. More than two thirds of these new events are found in starforming galaxies, while such galaxies only constitute ~20% of our sample. Based on the properties of the hosts and events, we find that almost all of the new events are likely to be associated with regular supernovae. However, a small number of long-timescale events are found among the galaxies containing AGN. These events have similar properties to those recently found in the analyses of light curves of large samples of AGN. Given the lack of such events among the more than a million passive galaxies in the sample, we suggest that the long outbursts are associated with super-massive black holes or their environments.

astro-ph.GA↗

arXiv2018

ALMA resolves the stellar birth explosion in distant quasar 3C298

Galaxies are believed to experience star formation and black hole driven nuclear activity symbiotically. The symbiosis may be more extreme in the distant universe, as far-infrared photometry with the Herschel Space Observatory has found many cases of ultra-luminous cool dust emission in z>1 radio galaxies and quasars, which could have its origin in the central black hole activity, or in extreme starbursts. We here present strong evidence for an extreme circumnuclear starburst in the z=1.439 quasar 3C298. Our unparalleled 0.18 arcsecond resolution ALMA image at rest-frame 410micrometer wavelength shows that the ~40K dust in its host galaxy resides in an asymmetric circumnuclear structure. The morphology of this structure implies a starburst origin and a symbiotic physical relation with the AGN driven radio source. The symbiosis is likely to be a general property of distant massive galaxies.

astro-ph.GA↗

arXiv2018

An Updated Catalog of 4680 Northern Eclipsing Binaries with Algol-Type light curve morphology in the Catalina Sky Surveys

We present an updated catalog of 4680 northern eclipsing binaries (EBs) with Algol-type light curve morphology (i.e., with well-defined beginning and end of primary and secondary eclipses), using data from the Catalina Sky Surveys. Our work includes revised period determinations, phenomenological parameters of the light curves, and system morphology classification based on machine learning techniques. While most of the new periods are in excellent agreement with those provided in the original Catalina catalogs, improved values are now available for ~10% of the stars. A total of 3456 EBs were classified as detached and 449 as semi-detached, while 145 cannot be classified unambiguously into either subtype. The majority of the SD systems seems to be comprised of short-period Algols. By applying color criteria, we searched for K- and M-type dwarfs in these data, and present a subsample of 609 EB candidates for further investigation. We report 119 EBs (2.5% of the total sample) that show maximum quadrature light variations over long timescales, with periods bracketing the range 4.5-18 yrs and fractional luminosity variance of 0.04-0.13. We discuss possible causes for this, making use of models of variable starspot activity in our interpretation of the results.

astro-ph.SR↗

arXiv2018

A highly magnified gravitationally lensed red quasar at z = 2.5 with significant flux anomaly: Uncovering a missing population

We present the discovery of a gravitationally lensed dust-reddened QSO at $z=2.517$ discovered in a survey for red QSOs by infrared selection. $Hubble~Space~Telescope$ imaging in the WFC3/IR F160W and F125W filters reveals a quadruply lensed system in a cusp configuration. We find that compared to the central image of the cusp, the nearby, brightest image is anomalous by a factor of $\sim7-11$. Although the source is extremely bright in the mid-infrared, a magnification by a factor of $\sim50-120$ places an upper limit of 1.35 mJy on the intrinsic mid-infrared brightness, well below the $WISE~W4$ detection limit of 6 mJy. We find that this QSO is moderately reddened, with $E(B-V)=0.7$ and that $\sim1\%$ of the intrinsic spectrum is leaked back into the line of sight resulting in an upturn in its UV spectrum. We conclude that the QSO's reddening is intrinsic and not due to the lens. Consistent with previous red quasar samples, this source exhibits outflows in its spectrum as well as morphological properties suggestive of it being in a merger-driven transitional phase. Depending on how $L_{\rm bol}$ is computed, the quasar's accretion rate may be as high as $0.26~L_{\rm Edd}$. We detect two Lyman limit systems, at $z=2.102$ and $z=2.431$, with absorption by metal lines likely at small impact parameter to the QSO, and a putative lens redshift of $z=0.599$. Given the rarity of quad lenses, the discovery of this source allows detailed study of a less luminous, more typical infrared-selected quasar at high redshift.

astro-ph.GA↗

arXiv2018

A MegaCam Survey of Outer Halo Satellites. I. Description of the Survey

We describe a deep, systematic imaging study of satellites in the outer halo of the Milky Way. Our sample consists of 58 stellar overdensities --- i.e., substructures classified as either globular clusters, classical dwarf galaxies, or ultra-faint dwarf galaxies --- that are located at Galactocentric distances of R$_{\rm GC}$ > 25 kpc (outer halo) and out to ~400 kpc. This includes 44 objects for which we have acquired deep, wide-field, $g-$ and $r-$band imaging with the MegaCam mosaic cameras on the 3.6m Canada-France-Hawaii Telescope and the 6.5m Magellan-Clay telescope. These data are supplemented by archival imaging, or published $gr$ photometry, for an additional 14 objects, most of which were discovered recently in the Dark Energy Survey (DES). We describe the scientific motivation for our survey, including sample selection, observing strategy, data reduction pipeline, calibration procedures, and the depth and precision of the photometry. The typical 5$σ$ point-source limiting magnitudes for our MegaCam imaging --- which collectively covers an area of ~52 deg$^{2}$ --- are $g_{\rm lim}$ ~25.6 and $r_{\rm lim}$ ~25.3 AB mag. These limits are comparable to those from the coadded DES images and are roughly a half-magnitude deeper than will be reached in a single visit with LSST. Our photometric catalog thus provides the deepest and most uniform photometric database of Milky Way satellites available for the foreseeable future. In other papers in this series, we have used these data to explore the blue straggler populations in these objects, their density distributions, star formation histories, scaling relations and possible foreground structures.

astro-ph.GA↗

arXiv2018

A MegaCam Survey of Outer Halo Satellites. III. Photometric and Structural Parameters

We present structural parameters from a wide-field homogeneous imaging survey of Milky Way satellites carried out with the MegaCam imagers on the 3.6m Canada-France-Hawaii Telescope (CFHT) and 6.5m Magellan-Clay telescope. Our survey targets an unbiased sample of "outer halo" satellites (i.e., substructures having Galactocentric distances greater than 25 kpc) and includes classical dSph galaxies, ultra-faint dwarfs, and remote globular clusters. We combine deep, panoramic $gr$ imaging for 44 satellites and archival $gr$ imaging for 14 additional objects (primarily obtained with the DECam instrument as part of the Dark Energy Survey), to measure photometric and structural parameters for 58 outer halo satellites. This is the largest and most uniform analysis of Milky Way satellites undertaken to date and represents roughly three quarters ($58/81 \simeq$72\%) of all known outer halo satellites. We use a maximum-likelihood method to fit four density laws to each object in our survey: exponential, Plummer, King and Sersic models. We examine systematically the isodensity contour maps and color magnitude diagrams for each of our program objects, present a comparison with previous results, and tabulate our best-fit photometric and structural parameters, including ellipticities, position angles, effective radii, Sersic indices, absolute magnitudes, and surface brightness measurements. We investigate the distribution of outer halo satellites in the size-magnitude diagram, and show that the current sample of outer halo substructures spans a wide range in effective radius, luminosity and surface brightness, with little evidence for a clean separation into star cluster and galaxy populations at the faintest luminosities and surface brightnesses.

astro-ph.GA↗

arXiv2018

Gaia GraL II - Gaia DR2 Gravitational Lens Systems: The Known Multiply Imaged Quasars

Thanks to its spatial resolution the ESA/Gaia space mission offers a unique opportunity to discover new multiply-imaged quasars and to study the already known lensed systems at sub-milliarcsecond astrometric precisions. In this paper, we address the detection of the known multiply-imaged quasars from the Gaia Data Release 2 and determine the astrometric and photometric properties of the individually detected images found in the Gaia DR2 catalogue. We have compiled an exhaustive list of quasar gravitational lenses from the literature to search for counterparts in the Gaia Data Release 2. We then analyze the astrometric and photometric properties of these Gaia's detections. To highlight the tremendous potential of Gaia at the sub-milliarcsecond level we finally perform a simple Bayesian modeling of the well-known gravitational lens system HE0435-1223, using Gaia Data Release 2 and HST astrometry. From 478 known multiply imaged quasars, 200 have at least one image found in the Gaia Data Release 2. Among the 41 known quadruply-imaged quasars of the list, 26 have at least one image in the Gaia Data Release 2, 12 of which are fully detected, 6 have three counterparts, 7 have two and 1 has only one. As expected, the modeling of HE0435-1223 shows that the model parameters are significantly better constrained when using Gaia astrometry compared to HST astrometry, in particular the relative positions of the background quasar source and the centroid of the deflector. The Gaia sub-milliarcsecond astrometry also significantly reduces the parameter correlations. Besides providing an up-to-date list of multiply imaged quasars and their detection in the Gaia DR2, this paper shows that more complex modeling scenarios will certainly benefit from Gaia sub-milliarcsecond astrometry.

astro-ph.IM↗

arXiv2018

Luminous WISE-selected Obscured, Unobscured, and Red Quasars in Stripe 82

We present a spectroscopically complete sample of 147 infrared-color-selected AGN down to a 22 $μ$m flux limit of 20 mJy over the $\sim$270 deg$^2$ of the SDSS Stripe 82 region. Most of these sources are in the QSO luminosity regime ($L_{\rm bol} \gtrsim 10^{12} L_\odot$) and are found out to $z\simeq3$. We classify the AGN into three types, finding: 57 blue, unobscured Type-1 (broad-lined) sources; 69 obscured, Type-2 (narrow-lined) sources; and 21 moderately-reddened Type-1 sources (broad-lined and $E(B-V) > 0.25$). We study a subset of this sample in X-rays and analyze their obscuration to find that our spectroscopic classifications are in broad agreement with low, moderate, and large amounts of absorption for Type-1, red Type-1 and Type-2 AGN, respectively. We also investigate how their X-ray luminosities correlate with other known bolometric luminosity indicators such as [O III] line luminosity ($L_{\rm [OIII]}$) and infrared luminosity ($L_{6 μ{\rm m}}$). While the X-ray correlation with $L_{\rm [OIII]}$ is consistent with previous findings, the most infrared-luminous sources appear to deviate from established relations such that they are either under-luminous in X-rays or over-luminous in the infrared. Finally, we examine the luminosity function (LF) evolution of our sample, and by AGN type, in combination with the complementary, infrared-selected, AGN sample of Lacy et al. (2013), spanning over two orders of magnitude in luminosity. We find that the two obscured populations evolve differently, with reddened Type-1 AGN dominating the obscured AGN fraction ($\sim$30%) for $L_{5 μ{\rm m}} > 10^{45}$ erg s$^{-1}$, while the fraction of Type-2 AGN with $L_{5 μ{\rm m}} < 10^{45}$ erg s$^{-1}$ rises sharply from 40% to 80% of the overall AGN population.

astro-ph.GA↗

arXiv2018

A Mid-IR Selected Changing-Look Quasar and Physical Scenarios for Abrupt AGN Fading

We report a new changing-look quasar, WISE~J105203.55+151929.5 at $z=0.303$, found by identifying highly mid-IR variable quasars in the WISE/NEOWISE data stream. Compared to multi-epoch mid-IR photometry of a large sample of SDSS-confirmed quasars, WISE J1052+1519 is an extreme photometric outlier, fading by more than a factor of two at $3.4$ and $4.6 μ$m since 2009. Swift target-of-opportunity observations in 2017 show even stronger fading in the soft X-rays compared to the ROSAT detection of this source in 1995, with at least a factor of fifteen decrease. We obtained second-epoch spectroscopy with the Palomar telescope in 2017 which, when compared with the 2006 archival SDSS spectrum, reveals that the broad H$β$ emission has vanished and that the quasar has become significantly redder. The two most likely interpretations for this dramatic change are source fading or obscuration, where the latter is strongly disfavored by the mid-IR data. We discuss various physical scenarios that could cause such changes in the quasar luminosity over this timescale, and favor changes in the innermost regions of the accretion disk that occur on the thermal and heating/cooling front timescales. We discuss possible physical triggers that could cause these changes, and predict the multiwavelength signatures that could distinguish these physical scenarios.

astro-ph.GA↗

arXiv2018

Gaia DR2 Gravitational Lens Systems I: New lensed quasar candidates around known quasars

Context. Strong gravitationally lensed quasars are among the most interesting and useful observable extragalactic phenomena. Because their study constitutes a unique tool in various fields of astronomy, they are highly sought, not without difficulty. Indeed, even in this era of all-sky surveys, their recognition remains a great challenge, with barely a few hundred currently known systems. Aims. In this work we aim to detect new strongly lensed quasar candidates in the recently published Gaia Data Release 2 (DR2), which is the highest spatial resolution astrometric and photometric all-sky survey, attaining effective resolutions from 0.4" to 2.2". Methods. We cross-matched a merged list of quasars and candidates with the Gaia DR2 and found 1,839,143 counterparts within 0.5". We then searched matches with more than two Gaia DR2 counterparts within 6". We further narrowed the resulting list using astrometry and photometry compatibility criteria between the Gaia DR2 counterparts. A supervised machine learning method, Extremely Randomized Trees, is finally adopted to assign to each remaining system a probability of being lensed. Results. We report the discovery of three quadruply-imaged quasar candidates that are fully detected in Gaia DR2. These are the most promising new quasar lens candidates from Gaia DR2 and a simple singular isothermal ellipsoid lens model is able to reproduce their image positions to within $\sim$1 mas. This letter demonstrates the gravitational lens discovery potential of Gaia.

astro-ph.GA↗

arXiv2018

A survey for dwarf galaxy remnants around fourteen globular clusters in the outer halo

We report the results of a systematic photometric survey of the peripheral regions of a sample of fourteen globular clusters in the outer halo of the Milky Way at distances d_GC>25 kpc from the Galactic centre. The survey is aimed at searching for the remnants of the host satellite galaxies where these clusters could originally have been formed before being accreted onto the Galactic halo. The limiting surface brightness varies within our sample, but reaches muV_lim=30-32 mag arcsec^-2. For only two globular clusters (NGC 7492 and Whiting 1; already suggested to be associated with the Sagittarius galaxy) we detect extended stellar populations that cannot be associated with either the clusters themselves or with the surrounding Galactic field population. We show that the lack of substructures around globular clusters at these Galactocentric distances is still compatible with the predictions of cosmological simulations whereby in the outer halo the Galactic globular cluster system is built up through hierarchical accretion at early epochs.

astro-ph.GA↗

arXiv2018

SNhunt151: an explosive event inside a dense cocoon

SNhunt151 was initially classified as a supernova (SN) impostor (nonterminal outburst of a massive star). It exhibited a slow increase in luminosity, lasting about 450 d, followed by a major brightening that reaches M_V ~ -18 mag. No source is detected to M_V > -13 mag in archival images at the position of SNhunt151 before the slow rise. Low-to-mid-resolution optical spectra obtained during the pronounced brightening show very little evolution, being dominated at all times by multicomponent Balmer emission lines, a signature of interaction between the material ejected in the new outburst and the pre-existing circumstellar medium. We also analyzed mid-infrared images from the Spitzer Space Telescope, detecting a source at the transient position in 2014 and 2015. Overall, SNhunt151 is spectroscopically a Type IIn SN, somewhat similar to SN2009ip. However, there are also some differences, such as a slow pre-discovery rise, a relatively broad light-curve peak showing a longer rise time (~ 50 d) and a slower decline, along with a negligible change in the temperature around the peak (T < 10^4 K). We suggest that SNhunt151 is the result of an outburst, or a SN explosion, within a dense circumstellar nebula, similar to those embedding some luminous blue variables like Eta Carinae and originating from past mass-loss events.

astro-ph.SR↗

arXiv2017

The maximum isotropic energy of gamma-ray bursts

The most energetic gamma-ray bursts (GRBs) are remarkable sources releasing huge amounts of energy on short timescales. Their prompt emission, which usually lasts few seconds, is so bright that it is visible across the whole observable universe. Studying these extreme events may provide clues on the nature of GRB progenitors and on the physical processes at work in relativistic jets. In this paper, we study the bright end of the isotropic energy distribution of long GRBs. We use two samples of long GRBs with redshift detected by Fermi/GBM or Konus-Wind, two instruments which measure the spectral shape and the energetics of the prompt emission accurately. We focus on GRBs within a range of redshifts z = 1 -- 5, a volume that contains a large number of energetic GRBs, and we propose a simple method to reconstruct the bright end of the GRB energy distribution from the observed one. We find that the GRB energy distribution cannot be described by a simple power law but requires a strong cutoff above $1-3 \times 10^{54}$ erg. We attribute this feature to an intrinsic limit on the energy per unit of solid angle radiated by gamma-ray bursts.

astro-ph.HE↗

arXiv2017

`Zwicky's Nonet': a compact merging ensemble of nine galaxies and 4C 35.06, a peculiar radio galaxy with dancing radio jets

We report the results of our radio, optical and infra-red studies of a peculiar radio source 4C~35.06, an extended radio-loud AGN at the center of galaxy cluster Abell 407 ($z=0.047$). The central region of this cluster hosts a remarkably tight ensemble of nine galaxies, the spectra of which resemble those of passive red ellipticals, embedded within a diffuse stellar halo of $\sim$1~arcmin size. This system (named the `Zwicky's Nonet') provides unique and compelling evidence for a multiple-nucleus cD galaxy precursor. Multifrequency radio observations of 4C~35.06 with the Giant Meterwave Radio Telescope (GMRT) at 610, 235 and 150 MHz reveal a system of 400~kpc scale helically twisted and kinked radio jets and outer diffuse lobes. The outer extremities of jets contain extremely steep spectrum (spectral index -1.7 to -2.5) relic/fossil radio plasma with a spectral age of a few$\,\times (10^7 - 10^8)$ yr. Such ultra-steep spectrum relic radio lobes without definitive hot-spots are rare, and they provide an opportunity to understand the life-cycle of relativistic jets and physics of black hole mergers in dense environments. We interpret our observations of this radio source in the context of the growth of its central black hole, triggering of its AGN activity and jet precession, all possibly caused by galaxy mergers in this dense galactic system. A slow conical precession of the jet axis due to gravitational perturbation between interacting black holes is invoked to explain the unusual jet morphology.

astro-ph.GA↗

arXiv2017

Supernovae 2016bdu and 2005gl, and their link with SN 2009ip-like transients: another piece of the puzzle

Supernova (SN) 2016bdu is an unusual transient resembling SN 2009ip. SN 2009ip-like events are characterized by a long-lasting phase of erratic variability which ends with two luminous outbursts a few weeks apart. The second outburst is significantly more luminous (about 3 mag) than the first. In the case of SN 2016bdu, the first outburst (Event A) reached an absolute magnitude M(r) ~ -15.3 mag, while the second one (Event B) occurred over one month later and reached M(r) ~ -18 mag. By inspecting archival data, a faint source at the position of SN 2016bdu is detectable several times in the past few years. We interpret these detections as signatures of a phase of erratic variability, similar to that experienced by SN 2009ip between 2008 and mid-2012, and resembling the currently observed variability of the luminous blue variable SN 2000ch in NGC 3432. Spectroscopic monitoring of SN 2016bdu during the second peak initially shows features typical of a SN IIn. One month after the Event B maximum, the spectra develop broad Balmer lines with P Cygni profiles and broad metal features. At these late phases, the spectra resemble those of a typical Type II SN. All members of this SN 2009ip-like group are remarkably similar to the Type IIn SN 2005gl. For this object, the claim of a terminal SN explosion is supported by the disappearance of the progenitor star. The similarity with SN 2005gl suggests that all members of this family may finally explode as genuine SNe, although the unequivocal detection of nucleosynthesised elements in their nebular spectra is still missing.

astro-ph.SR↗

arXiv2017

Understanding extreme quasar optical variability with CRTS: I. Major AGN flares

There is a large degree of variety in the optical variability of quasars and it is unclear whether this is all attributable to a single (set of) physical mechanism(s). We present the results of a systematic search for major flares in AGN in the Catalina Real-time Transient Survey as part of a broader study into extreme quasar variability. Such flares are defined in a quantitative manner as being atop of the normal, stochastic variability of quasars. We have identified 51 events from over 900,000 known quasars and high probability quasar candidates, typically lasting 900 days and with a median peak amplitude of $Δm = 1.25$ mag. Characterizing the flare profile with a Weibull distribution, we find that nine of the sources are well described by a single-point single-lens model. This supports the proposal by Lawrence et al. (2016) that microlensing is a plausible physical mechanism for extreme variability. However, we attribute the majority of our events to explosive stellar-related activity in the accretion disk: superluminous supernovae, tidal disruption events, and mergers of stellar mass black holes.

astro-ph.GA↗