Search

Binary supermassive black holes (BSBHs) are expected to be a generic byproduct from hierarchical galaxy formation. The final coalescence of BSBHs is thought to be the loudest gravitational wave (GW) siren, yet no confirmed BSBH is known in the GW-dominated regime. While periodic quasars have been proposed as BSBH candidates, the physical origin of the periodicity has been largely uncertain. Here we report discovery of a periodicity (P=1607$\pm$7 days) at 99.95% significance (with a global p-value of ~$10^{-3}$ accounting for the look elsewhere effect) in the optical light curves of a redshift 1.53 quasar, SDSS J025214.67-002813.7. Combining archival Sloan Digital Sky Survey data with new, sensitive imaging from the Dark Energy Survey, the total ~20-yr time baseline spans ~4.6 cycles of the observed 4.4-yr (restframe 1.7-yr) periodicity. The light curves are best fit by a bursty model predicted by hydrodynamic simulations of circumbinary accretion disks. The periodicity is likely caused by accretion rate modulation by a milli-parsec BSBH emitting GWs, dynamically coupled to the circumbinary accretion disk. A bursty hydrodynamic variability model is statistically preferred over a smooth, sinusoidal model expected from relativistic Doppler boost, a kinematic effect proposed for PG1302-102. Furthermore, the frequency dependence of the variability amplitudes disfavors Doppler boost, lending independent support to the circumbinary accretion variability hypothesis. Given our detection rate of one BSBH candidate from circumbinary accretion variability out of 625 quasars, it suggests that future large, sensitive synoptic surveys such as the Vera C. Rubin Observatory Legacy Survey of Space and Time may be able to detect hundreds to thousands of candidate BSBHs from circumbinary accretion with direct implications for Laser Interferometer Space Antenna., Comment: Accepted to MNRAS, 17 pages, 9 figures

Periodically variable quasars have been suggested as close binary supermassive black holes. We present a systematic search for periodic light curves in 625 spectroscopically confirmed quasars with a median redshift of 1.8 in a 4.6 deg$^2$ overlapping region of the Dark Energy Survey Supernova (DES-SN) fields and the Sloan Digital Sky Survey Stripe 82 (SDSS-S82). Our sample has a unique 20-year long multi-color ($griz$) light curve enabled by combining DES-SN Y6 observations with archival SDSS-S82 data. The deep imaging allows us to search for periodic light curves in less luminous quasars (down to $r{\sim}$23.5 mag) powered by less massive black holes (with masses $\gtrsim10^{8.5}M_{\odot}$) at high redshift for the first time. We find five candidates with significant (at $>$99.74% single-frequency significance in at least two bands with a global p-value of $\sim$7$\times10^{-4}$--3$\times10^{-3}$ accounting for the look-elsewhere effect) periodicity with observed periods of $\sim$3--5 years (i.e., 1--2 years in rest frame) having $\sim$4--6 cycles spanned by the observations. If all five candidates are periodically variable quasars, this translates into a detection rate of ${\sim}0.8^{+0.5}_{-0.3}$% or ${\sim}1.1^{+0.7}_{-0.5}$ quasar per deg$^2$. Our detection rate is 4--80 times larger than those found by previous searches using shallower surveys over larger areas. This discrepancy is likely caused by differences in the quasar populations probed and the survey data qualities. We discuss implications on the future direct detection of low-frequency gravitational waves. Continued photometric monitoring will further assess the robustness and characteristics of these candidate periodic quasars to determine their physical origins., Comment: 21 pages, 8 figures, MNRAS accepted

We report the identification of a low-mass AGN, DES J0218$-$0430, in a redshift $z = 0.823$ galaxy in the Dark Energy Survey (DES) Supernova field. We select DES J0218$-$0430 as an AGN candidate by characterizing its long-term optical variability alone based on DES optical broad-band light curves spanning over 6 years. An archival optical spectrum from the fourth phase of the Sloan Digital Sky Survey shows both broad Mg II and broad H$\beta$ lines, confirming its nature as a broad-line AGN. Archival XMM-Newton X-ray observations suggest an intrinsic hard X-ray luminosity of $L_{{\rm 2-12\,keV}}\sim7.6\pm0.4\times10^{43}$ erg s$^{-1}$, which exceeds those of the most X-ray luminous starburst galaxies, in support of an AGN driving the optical variability. Based on the broad H$\beta$ from SDSS spectrum, we estimate a virial BH mass of $M_{\bullet}\approx10^{6.43}$-$10^{6.72}M_{\odot}$ (with the error denoting 1$\sigma$ statistical uncertainties only), consistent with the estimation from OzDES, making it the lowest mass AGN with redshift $>$ 0.4 detected in optical. We estimate the host galaxy stellar mass to be $M_{\ast}\sim10^{10.5\pm0.3}M_{\odot}$ based on modeling the multi-wavelength spectral energy distribution. DES J0218$-$0430 extends the $M_{\bullet}$-$M_{\ast}$ relation observed in luminous AGNs at $z\sim1$ to masses lower than being probed by previous work. Our work demonstrates the feasibility of using optical variability to identify low-mass AGNs at higher redshift in deeper synoptic surveys with direct implications for the upcoming Legacy Survey of Space and Time at Vera C. Rubin Observatory., Comment: 13 pages, 8 figures, 1 table, accepted to MNRAS

We present a detailed characterization of the 849 broad-line quasars from the Sloan Digital Sky Survey Reverberation Mapping (SDSS-RM) project. Our quasar sample covers a redshift range of 0.1

We perform a systematic search for long-term extreme variability quasars (EVQs) in the overlapping Sloan Digital Sky Survey (SDSS) and 3-Year Dark Energy Survey (DES) imaging, which provide light curves spanning more than 15 years. We identified ~1000 EVQs with a maximum g band magnitude change of more than 1 mag over this period, about 10% of all quasars searched. The EVQs have L_bol~10^45-10^47 erg/s and L/L_Edd~0.01-1. Accounting for selection effects, we estimate an intrinsic EVQ fraction of ~30-50% among all g<~22 quasars over a baseline of ~15 years. These EVQs are good candidates for so-called "changing-look quasars", where a spectral transition between the two types of quasars (broad-line and narrow-line) is observed between the dim and bright states. We performed detailed multi-wavelength, spectral and variability analyses for the EVQs and compared to their parent quasar sample. We found that EVQs are distinct from a control sample of quasars matched in redshift and optical luminosity: (1) their UV broad emission lines have larger equivalent widths; (2) their Eddington ratios are systematically lower; and (3) they are more variable on all timescales. The intrinsic difference in quasar properties for EVQs suggest that internal processes associated with accretion are the main driver for the observed extreme long-term variability. However, despite their different properties, EVQs seem to be in the tail of a continuous distribution of quasar properties, rather than standing out as a distinct population. We speculate that EVQs are normal quasars accreting at relatively low accretion rates, where the accretion flow is more likely to experience instabilities that drive the factor of few changes in flux on multi-year timescales., Comment: submitted to ApJ; comments welcome

As astronomers increasingly exploit the information available in the time domain, spectroscopic variability in particular opens broad new channels of investigation. Here we describe the selection algorithms for all targets intended for repeat spectroscopy in the Time Domain Spectroscopic Survey (TDSS), part of the extended Baryon Oscillation Spectroscopic Survey within the Sloan Digital Sky Survey-IV. Also discussed are the scientific rationale and technical constraints leading to these target selections. The TDSS includes a large "Repeat Quasar Spectroscopy" (RQS) program delivering ~13,000 repeat spectra of confirmed SDSS quasars, and several smaller "Few-Epoch Spectroscopy" (FES) programs targeting specific classes of quasars as well as stars. The RQS program aims to provide a large and diverse quasar data set for studying variations in quasar spectra on timescales of years, a comparison sample for the FES quasar programs, and opportunity for discovering rare, serendipitous events. The FES programs cover a wide variety of phenomena in both quasars and stars. Quasar FES programs target broad absorption line quasars, high signal-to-noise ratio normal broad line quasars, quasars with double-peaked or very asymmetric broad emission line profiles, binary supermassive black hole candidates, and the most photometrically variable quasars. Strongly variable stars are also targeted for repeat spectroscopy, encompassing many types of eclipsing binary systems, and classical pulsators like RR Lyrae. Other stellar FES programs allow spectroscopic variability studies of active ultracool dwarf stars, dwarf carbon stars, and white dwarf/M dwarf spectroscopic binaries. We present example TDSS spectra and describe anticipated sample sizes and results., Comment: 26 pages, 11 figures, replaced with accepted version to AJ

We study the time lags between the continuum emission of quasars at different wavelengths, based on more than four years of multi-band ($g$, $r$, $i$, $z$) light-curves in the Pan-STARRS Medium Deep Fields. As photons from different bands emerge from different radial ranges in the accretion disk, the lags constrain the sizes of the accretion disks. We select 240 quasars with redshifts $z \approx 1$ or $z \approx 0.3$ that are relatively emission line free. The light curves are sampled from day to month timescales, which makes it possible to detect lags on the scale of the light crossing time of the accretion disks. With the code JAVELIN, we detect typical lags of several days in the rest frame between the $g$ band and the $riz$ bands. The detected lags are $\sim 2-3$ times larger than the light crossing time estimated from the standard thin disk model, consistent with the recently measured lag in NGC5548 and micro-lensing measurements of quasars. The lags in our sample are found to increase with increasing luminosity. Furthermore, the increase in lags going from $g-r$ to $g-i$ and then to $g-z$ is slower than predicted in the thin disk model, particularly for high luminosity quasars. The radial temperature profile in the disk must be different from what is assumed. We also find evidence that the lags decrease with increasing line ratios between ultraviolet FeII lines and MgII, which may point to changes in the accretion disk structure at higher metallicity., Comment: 18 pages, 17 Figures, 3 Tables, submitted to ApJ

Using the Pan-STARRS1 survey, we derive limiting magnitude, spatial completeness and density maps that we use to probe the three dimensional structure and estimate the stellar mass of the so-called Monoceros Ring. The Monoceros Ring is an enormous and complex stellar sub-structure in the outer Milky Way disk. It is most visible across the large Galactic Anticenter region, 120 < l < 240 degrees, -30 < b < +40 degrees. We estimate its stellar mass density profile along every line of sight in 2 X 2 degree pixels over the entire 30,000 square degree Pan-STARRS1 survey using the previously developed MATCH software. By parsing this distribution into a radially smooth component and the Monoceros Ring, we obtain its mass and distance from the Sun along each relevant line of sight. The Monoceros Ring is significantly closer to us in the South (6 kpc) than in the North (9 kpc). We also create 2D cross sections parallel to the Galactic plane that show 135 degrees of the Monoceros Ring in the South and 170 degrees of the Monoceros Ring in the North. We show that the Northern and Southern structures are also roughly concentric circles, suggesting that they may be a wave rippling from a common origin. Excluding the Galactic plane, we observe an excess stellar mass of 4 million solar masses across 120 < l < 240 degrees. If we interpolate across the Galactic plane, we estimate that this region contains 8 million solar masses. If we assume (somewhat boldly) that the Monoceros Ring is a set of two Galactocentric rings, its total stellar mass is 60 million solar masses. Finally, if we assume that it is a set of two circles centered at a point 4 kpc from the Galactic center in the anti-central direction, as our data suggests, we estimate its stellar mass to be 40 million solar masses., Comment: 24 pages, 23 figures, accepted by ApJ

The Time-Domain Spectroscopic Survey (TDSS) is an SDSS-IV eBOSS subproject primarily aimed at obtaining identification spectra of ~220,000 optically-variable objects systematically selected from SDSS/Pan-STARRS1 multi-epoch imaging. We present a preview of the science enabled by TDSS, based on TDSS spectra taken over ~320 deg^2 of sky as part of the SEQUELS survey in SDSS-III, which is in part a pilot survey for eBOSS in SDSS-IV. Using the 15,746 TDSS-selected single-epoch spectra of photometrically variable objects in SEQUELS, we determine the demographics of our variability-selected sample, and investigate the unique spectral characteristics inherent in samples selected by variability. We show that variability-based selection of quasars complements color-based selection by selecting additional redder quasars, and mitigates redshift biases to produce a smooth quasar redshift distribution over a wide range of redshifts. The resulting quasar sample contains systematically higher fractions of blazars and broad absorption line quasars than from color-selected samples. Similarly, we show that M-dwarfs in the TDSS-selected stellar sample have systematically higher chromospheric active fractions than the underlying M-dwarf population, based on their H-alpha emission. TDSS also contains a large number of RR Lyrae and eclipsing binary stars with main-sequence colors, including a few composite-spectrum binaries. Finally, our visual inspection of TDSS spectra uncovers a significant number of peculiar spectra, and we highlight a few cases of these interesting objects. With a factor of ~15 more spectra, the main TDSS survey in SDSS-IV will leverage the lessons learned from these early results for a variety of time-domain science applications., Comment: 17 pages, 14 figures, submitted to ApJ

We present a reverberation mapping (RM) experiment that combines broad- and intermediate-band photometry; it is the first such attempt targeting a sample of 13 quasars at $0.20.2$., Comment: 12 pages, 9 figures, accepted for publication in ApJ

We report the discovery of a new "changing-look" quasar, SDSS J101152.98+544206.4, through repeat spectroscopy from the Time Domain Spectroscopic Survey. This is an addition to a small but growing set of quasars whose blue continua and broad optical emission lines have been observed to decline by a large factor on a time scale of approximately a decade. The 5100 Angstrom monochromatic continuum luminosity of this quasar drops by a factor of > 9.8 in a rest-frame time interval of < 9.7 years, while the broad H-alpha luminosity drops by a factor of 55 in the same amount of time. The width of the broad H-alpha line increases in the dim state such that the black hole mass derived from the appropriate single-epoch scaling relation agrees between the two epochs within a factor of 3. The fluxes of the narrow emission lines do not appear to change between epochs. The light curve obtained by the Catalina Sky Survey suggests that the transition occurs within a rest-frame time interval of approximately 500 days. We examine three possible mechanisms for this transition suggested in the recent literature. An abrupt change in the reddening towards the central engine is disfavored by the substantial difference between the timescale to obscure the central engine and the observed timescale of the transition. A decaying tidal disruption flare is consistent with the decay rate of the light curve but not with the prolonged bright state preceding the decay, nor can this scenario provide the power required by the luminosities of the emission lines. An abrupt drop in the accretion rate onto the supermassive black hole appears to be the most plausible explanation for the rapid dimming., Comment: Submitted to MNRAS

The uncertain origin of the recently-discovered `changing-looking' quasar phenomenon -- in which a luminous quasar dims significantly to a quiescent state in repeat spectroscopy over ~10 year timescales -- may present unexpected challenges to our understanding of quasar accretion. To better understand this phenomenon, we take a first step to building a sample of changing-look quasars with a systematic but simple archival search for these objects in the Sloan Digital Sky Survey Data Release 12. By leveraging the >10 year baselines for objects with repeat spectroscopy, we uncover two new changing-look quasars, and a third discovered previously. Decomposition of the multi-epoch spectra and analysis of the broad emission lines suggest that the quasar accretion disk emission dims due to rapidly decreasing accretion rates (by factors of >2.5), while disfavoring changes in intrinsic dust extinction for the two objects where these analyses are possible. Broad emission line energetics also support intrinsic dimming of quasar emission as the origin for this phenomenon rather than transient tidal disruption events or supernovae. Although our search criteria included quasars at all redshifts and transitions from either quasar-like to galaxy-like states or the reverse, all of the clear cases of changing-look quasars discovered were at relatively low-redshift (z ~ 0.2 - 0.3) and only exhibit quasar-like to galaxy-like transitions., Comment: 15 pages, 8 figures. Updated to accepted version

We present the selection algorithm and anticipated results for the Time Domain Spectroscopic Survey (TDSS). TDSS is an SDSS-IV eBOSS subproject that will provide initial identification spectra of approximately 220,000 luminosity-variable objects (variable stars and AGN) across 7,500 square degrees selected from a combination of SDSS and multi-epoch Pan-STARRS1 photometry. TDSS will be the largest spectroscopic survey to explicitly target variable objects, avoiding pre-selection on the basis of colors or detailed modeling of specific variability characteristics. Kernel Density Estimate (KDE) analysis of our target population performed on SDSS Stripe 82 data suggests our target sample will be 95% pure (meaning 95% of objects we select have genuine luminosity variability of a few magnitudes or more). Our final spectroscopic sample will contain roughly 135,000 quasars and 85,000 stellar variables, approximately 4,000 of which will be RR Lyrae stars which may be used as outer Milky Way probes. The variability-selected quasar population has a smoother redshift distribution than a color-selected sample, and variability measurements similar to those we develop here may be used to make more uniform quasar samples in large surveys. The stellar variable targets are distributed fairly uniformly across color space, indicating that TDSS will obtain spectra for a wide variety of stellar variables including pulsating variables, stars with significant chromospheric activity, cataclysmic variables and eclipsing binaries. TDSS will serve as a pathfinder mission to identify and characterize the multitude of variable objects that will be detected photometrically in even larger variability surveys such as LSST., Comment: 22 pages, 18 figures, accepted by ApJ

We explore the practical feasibility of AGN broad-band reverberation mapping and present first results. We lay out and apply a rigorous approach for stochastic reverberation mapping of unevenly sampled multi-broad-band flux measurements, assuming that the broad-line region (BLR) line flux is contributing up to 15 % in some bands, and is directly constrained by one spectroscopical epoch. The approach describes variations of the observed flux as the continuum, modeled as a stochastic Gaussian process, and emission line contribution, modeled as a scaled, smoothed and delayed version of the continuum. This approach is capable not only to interpolate in time between measurements, but also to determine confidence limits on continuum -- line emission delays. This approach is applied to SDSS observations in 'Stripe 82' (S82) providing flux measurements precise to 2 % at $\sim$ 60 epochs over $\sim$ 10 years. The strong annual variations in the epoch sampling prove a serious limitation in practice. Also, suitable redshift ranges must be identified, where strong broad emission line contribute to one filter, but not to another. Through generating and evaluating problem-specific mock data, we verify that S82-like data can constrain $\tau_{\mathrm{delay}}$ for a simple transfer function model. In application to real data, we estimate $\tau_{\mathrm{delay}}$ for 323 AGN with $0.225 < z < 0.846$, combining information for different objects through the ensemble-scaling relationships for BLR size and BH mass. Our analysis tentatively indicates a 1.7 times larger BLR size of H$\alpha$ and MgII compared to Kaspi2000 and Vestergaard2002, but the seasonal data sampling casts doubt on the robustness of the inference.

We present a panoptic view of the stellar structure in the Galactic disk's outer reaches commonly known as the Monoceros Ring, based on data from Pan-STARRS1. These observations clearly show the large extent of the stellar overdensities on both sides of the Galactic disk, extending between b = -25 and b = +35 degrees and covering over 130 degrees in Galactic longitude. The structure exhibits a complex morphology with both stream-like features and a sharp edge to the structure in both the north and the south. We compare this map to mock observations of two published simulations aimed at explaining such structures in the outer stellar disk, one postulating an origin as a tidal stream and the other demonstrating a scenario where the disk is strongly distorted by the accretion of a satellite. These morphological comparisons of simulations can link formation scenarios to observed structures, such as demonstrating that the distorted-disk model can produce thin density features resembling tidal streams. Although neither model produces perfect agreement with the observations--the tidal stream predicts material at larger distances which is not detected while in the distorted disk model the midplane is warped to an excessive degree--future tuning of the models to accommodate these latest data may yield better agreement., Comment: 10 pages, 7 figures. Accepted to ApJ. Key figures and data can be found at http://dept.astro.lsa.umich.edu/~ctslater/MRi/

We report the discovery of two new dwarf galaxies, Lacerta I/Andromeda XXXI (Lac I/And XXXI) and Cassiopeia III/Andromeda XXXII (Cas III/And XXXII), in stacked Pan-STARRS1 r_P1- and i_P1-band imaging data. Both are luminous systems (M_V ~ -12) located at projected distances of 20.3{\deg} and 10.5{\deg} from M31. Lac I and Cas III are likely satellites of the Andromeda galaxy with heliocentric distances of 756^{+44}_{-28} kpc and 772^{+61}_{-56} kpc, respectively, and corresponding M31-centric distances of 275+/-7 kpc and 144^{+6}_{-4} kpc . The brightest of recent Local Group member discoveries, these two new dwarf galaxies owe their late discovery to their large sizes (r_h = 4.2^{+0.4}_{-0.5} arcmin or 912^{+124}_{-93} pc for Lac I; r_h = 6.5^{+1.2}_{-1.0} arcmin or 1456+/-267 pc for Cas III), and consequently low surface brightness (\mu_0 ~ 26.0 mag/arcsec^2), as well as to the lack of a systematic survey of regions at large radii from M31, close to the Galactic plane. This latter limitation is now alleviated by the 3{\pi} Pan-STARRS1 survey, which could lead to the discovery of other distant Andromeda satellite dwarf galaxies., Comment: 7 pages, 5 figures. Accepted for publication in ApJ

We present the discovery of the first high redshift (z > 5.7) quasar from the Panoramic Survey Telescope and Rapid Response System 1 (Pan-STARRS1 or PS1). This quasar was initially detected as an i dropoutout in PS1, confirmed photometrically with the SAO Widefield InfraRed Camera (SWIRC) at Arizona's Multiple Mirror Telescope (MMT) and the Gamma-Ray Burst Optical/Near-Infrared Detector (GROND) at the MPG 2.2 m telescope in La Silla. The quasar was verified spectroscopically with the the MMT Spectrograph, Red Channel and the Cassegrain Twin Spectrograph (TWIN) at the Calar Alto 3.5 m telescope. It has a redshift of 5.73, an AB z magnitude of 19.4, a luminosity of 3.8 x 10^47 erg/s and a black hole mass of 6.9 x 10^9 solar masses. It is a Broad Absorption Line quasar with a prominent Ly-beta peak and a very blue continuum spectrum. This quasar is the first result from the PS1 high redshift quasar search that is projected to discover more than a hundred i dropout quasars, and could potentially find more than 10 z dropout (z > 6.8) quasars., Comment: 8 pages, 7 figures

We present a tomographic cosmological weak lensing analysis of the HST COSMOS Survey. Applying our lensing-optimized data reduction, principal component interpolation for the ACS PSF, and improved modelling of charge-transfer inefficiency, we measure a lensing signal which is consistent with pure gravitational modes and no significant shape systematics. We carefully estimate the statistical uncertainty from simulated COSMOS-like fields obtained from ray-tracing through the Millennium Simulation. We test our pipeline on simulated space-based data, recalibrate non-linear power spectrum corrections using the ray-tracing, employ photometric redshifts to reduce potential contamination by intrinsic galaxy alignments, and marginalize over systematic uncertainties. We find that the lensing signal scales with redshift as expected from General Relativity for a concordance LCDM cosmology, including the full cross-correlations between different redshift bins. For a flat LCDM cosmology, we measure sigma_8(Omega_m/0.3)^0.51=0.75+-0.08 from lensing, in perfect agreement with WMAP-5, yielding joint constraints Omega_m=0.266+0.025-0.023, sigma_8=0.802+0.028-0.029 (all 68% conf.). Dropping the assumption of flatness and using HST Key Project and BBN priors only, we find a negative deceleration parameter q_0 at 94.3% conf. from the tomographic lensing analysis, providing independent evidence for the accelerated expansion of the Universe. For a flat wCDM cosmology and prior w in [-2,0], we obtain w<-0.41 (90% conf.). Our dark energy constraints are still relatively weak solely due to the limited area of COSMOS. However, they provide an important demonstration for the usefulness of tomographic weak lensing measurements from space. (abridged), Comment: 26 pages, 25 figures, matches version accepted for publication by Astronomy and Astrophysics

We have searched 4.5 square degrees of archival HST/ACS images for cosmic strings, identifying close pairs of similar, faint galaxies and selecting groups whose alignment is consistent with gravitational lensing by a long, straight string. We find no evidence for cosmic strings in five large-area HST treasury surveys (covering a total of 2.22 square degrees), or in any of 346 multi-filter guest observer images (1.18 square degrees). Assuming that simulations ccurately predict the number of cosmic strings in the universe, this non-detection allows us to place upper limits on the unitless Universal cosmic string tension of G mu/c^2 < 2.3 x 10^-6, and cosmic string density of Omega_s < 2.1 x 10^-5 at the 95% confidence level (marginalising over the other parameter in each case). We find four dubious cosmic string candidates in 318 single filter guest observer images (1.08 square degrees), which we are unable to conclusively eliminate with existing data. The confirmation of any one of these candidates as cosmic strings would imply G mu/c^2 ~ 10^-6 and Omega_s ~ 10^-5. However, we estimate that there is at least a 92% chance that these string candidates are random alignments of galaxies. If we assume that these candidates are indeed false detections, our final limits on G mu/c^2 and Omega_s fall to 6.5 x 10^-7 and 7.3 x 10^-6. Due to the extensive sky coverage of the HST/ACS image archive, the above limits are universal. They are quite sensitive to the number of fields being searched, and could be further reduced by more than a factor of two using forthcoming HST data., Comment: 21 pages, 18 figures

We present a stable procedure for defining and measuring the two point angular autocorrelation function, w, of faint (25 < V < 29), barely resolved and unresolved sources in the HST GOODS and UDF datasets. We construct catalogs that include close pairs and faint detections. We show for the first time that on subarcsecond scales, the correlation function exceeds unity. This correlation function is well fit by a power law with index of 2.5 and a characteristic angular scale that decrease slowly with magnitude. This is very different from the purely gravitationalcorrelation function of brighter galaxies which has a index of 0.7 and a characteristic angular scale which decreases quickly with magnitude. This observed clustering probably reflects the presence of giant star-forming regions within galactic-scale potential wells. Its measurement enables a new approach to measuring the redshift distribution of the faintest sources in the sky., Comment: 13 pages, 11 figures

We use current theoretical estimates for the density of long cosmic strings to predict the number of strong gravitational lensing events in astronomical imaging surveys as a function of angular resolution and survey area. We show that angular resolution is the most important factor, and that interesting limits on the dimensionless string tension Gmu/c^2 can be obtained by existing and planned surveys. At the resolution of the Hubble Space Telescope (0.14"), it is sufficient to survey of order a few square degrees -- well within reach of the current HST archive -- to probe the regime Gmu/c^2 ~ 10^{-7}. If lensing by cosmic strings is not detected, such a survey would improve the limit on the string tension by a factor of two over that available from the cosmic microwave background. Future high resolution imaging surveys, covering a few hundred square degrees or more, either from space in the optical or from large-format radio telescopes on the ground, would be able to further lower this limit to Gmu/c^2 < 10^{-8}., Comment: 7 pages, 3 figures, accepted for publication in MNRAS following in-press correction