Lensing Object Research Articles

The OGLE view of microlensing towards the Magellanic Clouds - II. OGLE-II Small Magellanic Cloud data★

The primary goal of this paper is to provide evidence that can prove true or false the hypothesis that dark matter in the Galactic halo can clump into stellar-mass compact objects. If such objects exist, they would act as lenses to external sources in the Magellanic Clouds, giving rise to an observable effect of microlensing. We present the results of our search for such events, based on data from the second phase of the OGLE survey (1996–2000) towards the Small Magellanic Cloud (SMC). The data set we used comprises 2.1 million monitored sources distributed over an area of 2.4 deg2. We found only one microlensing event candidate, however its poor-quality light curve limited our discussion of the exact distance to the lensing object. Given a single event, taking blending (crowding of stars) into account for the detection-efficiency simulations and deriving the Hubble Space Telescope (HST)-corrected number of monitored stars, the microlensing optical depth is τ= (1.55 ± 1.55) × 10−7. This result is consistent with the expected SMC self-lensing signal, with no need to introduce dark matter microlenses. Rejecting the unconvincing event leads to an upper limit on the fraction of dark matter in the form of massive compact halo objects (MACHOs) of f < 20 per cent for deflector masses around 0.4 M☉ and f < 11 per cent for masses between 0.003 and 0.2 M☉ (95 per cent confidence limit). Our result indicates that the Milky Way's dark matter is unlikely to be clumpy and to form compact objects in the subsolar-mass range.

The radio-loud active nucleus in the “dark lens” galaxy J1218+2953

Context: There is a possibility that the optically unidentified radio source J1218+2953 may act as a gravitational lens, producing an optical arc ~4" away from the radio position. Until now, the nature of the lensing object has been uncertain since it is not detected in any waveband other than the radio. The estimated high mass-to-light ratio could even allow the total mass of this galaxy to be primarily in the form of dark matter. In this case, J1218+2953 could be the first known example of a "dark lens". Aims: We investigate the nature of J1218+2953 by means of high-resolution radio imaging observations to determine whether there is a radio-loud active galactic nucleus (AGN) in the position of the lensing object. Methods: We report on Very Long Baseline Interferometry (VLBI) observations with the European VLBI Network (EVN) at 1.6 and 5 GHz. Results: Our images, having angular resolutions of ~1 to ~10 milli-arcseconds (mas), reveal a rich and complex radio structure extending to almost 1". Based on its radio spectrum and structure, J1218+2953 can be classified as a compact steep-spectrum (CSS) source, and as a medium-size symmetric object (MSO). The object harbours an AGN. It is also found as an X-ray source in the XMM-Newton EPIC (European Photon Imaging Cameras) instrument serendipitous source catalogue. Conclusions: Rather than being a dark lens, J1218+2953 is most likely a massive, heavily obscured galaxy in which the nuclear activity is currently in an early evolutionary stage.

REST-FRAME OPTICAL SPECTRA OF THREE STRONGLY LENSED GALAXIES ATz∼ 2

We present Keck II NIRSPEC rest-frame optical spectra for three recently discovered lensed galaxies: the Cosmic Horseshoe (z = 2.38), the Clone (z = 2.00), and SDSS J090122.37+181432.3 (z = 2.26). The boost in signal-to-noise ratio (S/N) from gravitational lensing provides an unusually detailed view of the physical conditions in these objects. A full complement of high S/N rest-frame optical emission lines is measured, spanning from rest frame 3600 to 6800 A, including robust detections of fainter lines such as Hγ, [S II]λ6717,6732, and in one instance [Ne III]λ3869. SDSS J090122.37+181432.3 shows evidence for active galactic nucleus activity, and therefore we focus our analysis on star-forming regions in the Cosmic Horseshoe and the Clone. For these two objects, we estimate a wide range of physical properties. Current lensing models for the Cosmic Horseshoe and the Clone allow us to correct the measured Hα luminosity and calculated star formation rate. Metallicities have been estimated with a variety of indicators, which span a range of values of 12+ log(O/H) = 8.3-8.8, between ~0.4 and ~1.5 of the solar oxygen abundance. Dynamical masses were computed from the Hα velocity dispersions and measured half-light radii of the reconstructed sources. A comparison of the Balmer lines enabled measurement of dust reddening coefficients. Variations in the line ratios between the different lensed images are also observed, indicating that the spectra are probing different regions of the lensed galaxies. In all respects, the lensed objects appear fairly typical of ultraviolet-selected star-forming galaxies at z ~ 2. The Clone occupies a position on the emission-line diagnostic diagram of [O III]/Hβ versus [N II]/Hα that is offset from the locations of z ~ 0 galaxies. Our new NIRSPEC measurements may provide quantitative insights into why high-redshift objects display such properties. From the [S II] line ratio, high electron densities (~1000 cm–3) are inferred compared to local galaxies, and [O III]/[O II] line ratios indicate higher ionization parameters compared to the local population. Building on previous similar results at z ~ 2, these measurements provide further evidence (at high S/N) that star-forming regions are significantly different in high-redshift galaxies, compared to their local counterparts.

Strong lensing of submillimetre galaxies: a tracer of foreground structure?

The steep source counts and negative K-corrections of bright submillimetre galaxies (SMGs) suggest that a significant fraction of those observed at high flux densities may be gravitationally lensed, and that the lensing objects may often lie at redshifts above 1, where clusters of galaxies are difficult to detect through other means. In this case follow-up of bright SMGs may be used to identify dense structures along the line of sight. Here we investigate the probability for SMGs to experience strong lensing, using the latest N-body simulations and observed source flux and redshift distributions. We find that almost all high redshift sources with a flux density above 100mJy will be lensed, if they are not relatively local galaxies. We also give estimates of the fraction of sources experiencing strong lensing as a function of observed flux density. This has implications for planning follow-up observations for bright SMGs discovered in future surveys with SCUBA-2 and other instruments. The largest uncertainty in these calculations is the maximum allowed lensing amplification, which is dominated by the presently unknown spatial extent of SMGs.

THE CLUSTER LENS ACO 1703: REDSHIFT CONTRAST AND THE INNER PROFILE

ACO 1703 is a cluster recently found to have a variety of strongly lensed objects: there is a quintuply-imaged system at z=0.888 and several other lensed objects from z=2.2 to 3.0 (the cluster itself is at z=0.28). It is not difficult to model the lens, as previous work has already done. However, lens models are generically non-unique. We generate ensembles of models to explore the non-uniqueness. When the full range of source redshifts is included, all models are close to \rho \propto r^{-1} out to 200 kpc. But if the quint is omitted, both shallower and steeper models (e.g., \rho \propto r^{-2}) are possible. The reason is that the redshift contrast between the quint and the other sources gives a good measurement of the enclosed mass at two different radii, thus providing a good estimate of the mass profile in between. This result supports universal profiles and explains why single-model approaches can give conflicting results. The mass map itself is elongated in the NW-SE direction, like the galaxy distribution. An overdensity in both mass and light is also apparent to the SE, which suggests meso-structure.

Strong lensing optical depths in a CDM universe

We investigate strong gravitational lensing in the concordance ΛCDM cosmology by carrying out ray tracing along past light cones through the Millennium Simulation, the largest simulation of cosmic structure formation ever carried out. We extend previous ray-tracing methods in order to take full advantage of the large volume and the excellent spatial and mass resolution of the simulation. As a function of source redshift we evaluate the probability that an image will be highly magnified, will be highly elongated or will be one of a set of multiple images. We show that such strong lensing events can almost always be traced to a single dominant lensing object and we study the mass and redshift distribution of these primary lenses. We fit analytic models to the simulated dark haloes in order to study how our optical depth measurements are affected by the limited resolution of the simulation and of the lensing planes that we construct from it. We conclude that such effects lead us to underestimate total strong lensing cross-sections by about 15 per cent. This is smaller than the effects expected from our neglect of the baryonic components of galaxies. Finally we investigate whether strong lensing is enhanced by material in front of or behind the primary lens. Although strong lensing lines of sight are indeed biased towards higher than average mean densities, this additional matter typically contributes only a few per cent of the total surface density.

Deciphering Quasar Millilensing – How to Distinguish Between Dark and Luminous Lenses

AbstractDwarf-galaxy sized dark matter halos may be detectable when a bright background source is gravitationally lensed on scales of milliarcseconds. Such effects can be caused by dark galaxies as well as luminous ones. Here we discuss conditions for detecting a luminous lensing object through direct imaging, via measurements of the point spread function (PSF) to subtract the quasar from the image, revealing the quasar host galaxy and any luminous lens. The technique applies to both types of millilensing effect, i.e. magnification and image splitting. We show that luminous dwarf-galaxy lenses should be detectable with this method up to redshifts z~0.2.

Local Melting of Glass Material and Its Application to Direct Fusion Welding by Ps-laser Pulses

Fusion welding of glass by picosecond laser pulses is reported where laser energy absorbed by nonlinear absorption is utilized as a heat source for local internal glass melting. Laser pulses with 10ps duration with repetition rates up to 500kHz were focused within borosilicate glass, Schott D263, by an objective lense of NA 0.7. The nonlinear absorptivity of the laser pulses was determined at different pulse energies, repetition rates, traveling velocities and depth of the focus position. The transient and time-averaged temperature distribution in fusion welding of glass was calculated using a thermal conduction model where heat is instantaneously deposited at different repetition rates in the rectangular solid moving linearly at a constant velocity. The experimental dimensions of the melt features agreed well with the calculated values in a velocity range of 1~100mm/s. Excellent seam welding of two glass plates is demonstrated without cracking and thermal distortion. 10ps laser pulses provided higher melting and joining efficiencies than sub-ps pulses due to the higher nonlinear absorptivity by the larger contribution of the avalanche ionization, and also provided the highest efficiencies in the existing fusion welding techniques.

An S-shaped arc in the galaxy cluster RX J0054.0-2823

Context. The center of the galaxy cluster RX J0054.0-2823 at z = 0.292 is a dynamically active region that includes an interacting system of three galaxies surrounded by a large halo of intra-cluster light. Aims. We report here the discovery of an S-shaped feature of total length 11 �� in the central region of this cluster and discuss its physical nature. Methods. We test the gravitational lensing assumption by doing a mass modeling of the central part of the galaxy cluster. Results. We very naturally reproduce position and form of this S-shaped feature as a gravitationally lensed background object at redshift between 0.5 and 1.0. Conclusions. We conclude that the lensing nature is the very probable explanation for this S-shaped arc; the ultimate proof will be the spectroscopic confirmation by measuring the high redshift of this elongated feature with surface brightness V ∼ 24 mag arcsec −2 .

Gravitational lensing and the maximum number of images

We show that in a configuration of point masses, replacing one of the point deflectors by a spherically symmetric distributed mass only introduces one extra image. This extra image is located inside the radius of the distributed mass added. We extend a result available for a set of n point masses, by showing that the maximum number of images that n distributed lensing objects located on a plane can create is Nmax = 6(n − 1) + 1 for n ≥ 2. Our result is generic in that the mass density for each deflector is only constrained to diverge slower than 1/r as r approaches the center of the mass.

Lens‐Aided Multi‐Angle Spectroscopy (LAMAS) Reveals Small‐Scale Outflow Structure in Quasars

Spectral differences between lensed quasar image components are common. Since lensing is intrinsically achromatic, these differences are typically explained as the effect of either microlensing, or as light path time delays sampling intrinsic quasar spectral variability. Here we advance a novel third hypothesis: some spectral differences are due to small line-of-sight differences through quasar disk wind outflows. In particular, we propose that variable spectral differences seen only in component A of the widest separation lens SDSS J1004+4112 are due to differential absorption along the sight lines. The absorber properties required by this hypothesis are akin to known broad absorption line (BAL) outflows but must have a broader, smoother velocity profile. We interpret the observed C IV emission-line variability as further evidence for spatial fine structure transverse to the line of sight. Since outflows are likely to be rotating, such absorber fine structure can consistently explain some of the UV and X-ray variability seen in AGNs. The implications are many: (1) Spectroscopic differences in other lensed objects may be due to this lens-aided multi-angle spectroscopy (LAMAS). (2) Outflows have fine structure on size scales of arcseconds, as seen from the nucleus. (3) Assuming either broad absorption line region sizes proposed in recent wind models, or typically assumed continuum emission region sizes, LAMAS and/or variability provide broadly consistent absorber size scale estimates of ~1015 cm. (4) Very broad smooth absorption may be ubiquitous in quasar spectra, even when no obvious troughs are seen.

Testing LMC Microlensing Scenarios: The Discrimination Power of the SuperMACHO Microlensing Survey

Characterizing the nature and spatial distribution of the lensing objects that produce the previously measured microlensing optical depth toward the Large Magellanic Cloud (LMC) remains an open problem. We present an appraisal of the ability of the SuperMACHO Project, a next-generation microlensing survey directed toward the LMC, to discriminate between various proposed lensing populations. We consider two scenarios: lensing by a uniform foreground screen of objects and self-lensing by LMC stars. The optical depth for screen lensing is essentially constant across the face of the LMC, whereas the optical depth for self-lensing shows a strong spatial dependence. We have carried out extensive simulations, based on data obtained during the first year of the project, to assess the SuperMACHO survey's ability to discriminate between these two scenarios. In our simulations we predict the expected number of observed microlensing events for various LMC models for each of our fields by adding artificial stars to the images and estimating the spatial and temporal efficiency of detecting microlensing events using Monte Carlo methods. We find that the event rate itself shows significant sensitivity to the choice of the LMC luminosity function, limiting the conclusions that can be drawn from the absolute rate. If instead we determine the differential event rate across the LMC, we will decrease the impact of these systematic biases and render our conclusions more robust. With this approach the SuperMACHO Project should be able to distinguish between the two categories of lens populations. This will provide important constraints on the nature of the lensing objects and their contributions to the Galactic dark matter halo.

The binary period and outburst behaviour of the Small Magellanic Cloud X-ray binary pulsar system SXP504

A probable binary period has been detected in the optical counterpart to the X-ray source CXOU J005455.6-724510 = RX J0054.9-7245 = AX J0054.8-7244 = SXP504 in the Small Magellanic Cloud. This source was detected by Chandra on 2002 July 4 and subsequently observed by XMM-Newton on 2003 December 18. The source is coincident with an Optical Gravitational Lensing object in the light curves of which several optical outburst peaks are visible at ∼268-d intervals. Timing analysis shows a period of 268.6 ± 0.1 d at >99 per cent significance. Archival Rossi X-ray Timing Explorer data for the 504-s pulse-period have revealed detections which correspond closely with predicted or actual peaks in the optical data. The relationship between this orbital period and the pulse period of 504 s is within the normal variance found in the Corbet diagram.

Possible arcmin-separation gravitational lensed quasi-stellar objects in the 2dF QSO survey

We report the possible discovery of multiple gravitationally-lensed images of QSOs with angular separations on arcminute scales. The QSOs were selected from the completed 2dF QSO survey as having redshifts and optical colours which are statistically consistent. In this paper we present higher-quality optical spectra of the candidates and discuss the likelihood of their genuinely being systems lensed by massive clusters of galaxies. From a comparison of the spectra it appears that up to six pairs of QSOs may be lensed multiple images, although the true number may be less than that and further observations should be undertaken to amass more evidence and to detect the lensing clusters. Two of the candidates may be associated with low redshift clusters of galaxies.

Performance of the Keck long wavelength spectrometer

The W.M. Keck Observatory’s Long Wavelength Spectrometer (LWS), is the mid-infrared facility instrument for imaging and low to moderate resolution spectroscopy for the wavelength range of 3–28 μm. At the heart of LWS is an 128 × 128 Si:As blocked impurity band (BIB) array. The detector is a moderate flux device with a 4-channel readout and well capacity of 1 × 10 7 e − making it properly suited for the thermal background environment for a ground based imaging spectrometer. The commissioned instrument, mounted on the Keck I 10-m telescope, delivers diffraction-limited images of 0.29″ FWHM at 11.7 μm. An overview of the performance characteristics of instrument and telescope are discussed including a detailed analysis of telescope emissivity. LWS scientific programs cover a broad range of topics that extend from the Solar System to extragalactic cosmology. This paper includes three examples that demonstrate the capabilities of Keck I and LWS: a high resolution mapping of the Titan atmosphere, a spectroscopic and imaging study of the compact nebula around the super massive binary RY Scuti, and high resolution imaging of a gravitational lensed quasi-stellar object QSO 2237+0305.

A gravitationally lensed quasar with quadruple images separated by 14.62 arcseconds.

Gravitational lensing is a powerful tool for the study of the distribution of dark matter in the Universe. The cold-dark-matter model of the formation of large-scale structures (that is, clusters of galaxies and even larger assemblies) predicts the existence of quasars gravitationally lensed by concentrations of dark matter so massive that the quasar images would be split by over 7 arcsec. Numerous searches for large-separation lensed quasars have, however, been unsuccessful. All of the roughly 70 lensed quasars known, including the first lensed quasar discovered, have smaller separations that can be explained in terms of galaxy-scale concentrations of baryonic matter. Although gravitationally lensed galaxies with large separations are known, quasars are more useful cosmological probes because of the simplicity of the resulting lens systems. Here we report the discovery of a lensed quasar, SDSS J1004 + 4112, which has a maximum separation between the components of 14.62 arcsec. Such a large separation means that the lensing object must be dominated by dark matter. Our results are fully consistent with theoretical expectations based on the cold-dark-matter model.

Gravitational Lensing by Globular Clusters and Arp Objects

We try to explain quasar-galaxy associations by gravitational lensing by globular clusters, located in the halos of foreground galaxies. We propose observational test for verification of this hypothesis. We processed SUPERCOSMOS sky survey and found overdensities of star-like sources with zero proper motions in the vicinities of foreground galaxies from CfA3 catalog. We show mean effect for galaxies with different redshifts. Two effects can explain observational data - these are lensing by globular clusters and lensing by dwarf galaxies. We made CCD 3-color photometry with 2.0-1.2 meter telescopes to select extremely lensed objects around several galaxies for spectroscopic observations.

The Gravitational Scattering of the Cosmic Microwave Background by Dark Clusters of Galaxies

The spectrum distortion of the cosmic microwave background (CMB) due to the transverse peculiar motion of galaxy clusters was restudied. The fundamental equation for estimating the frequency shift of the photons that pass through the moving gravitational lensing object is derived from the geodesic equation. The fractional temperature fluctuation of the CMB induced by the transverse peculiar motion of a galaxy cluster was estimated to be a few times 10-7. The effect is distinguishable from the CMB primary temperature fluctuation and other secondary effects by its peculiar dipole pattern in the cluster scale. The effect provides a new complemental method of confirming the existence of the dark cluster; otherwise, the gravitational lensing effect is the only reliable effect revealing the existence of such an exotic object.

Strong Gravitational Lensing Time Delay Statistics and the Density Profile of Dark Halos

The distribution of differential time delays Δt between images produced by strong gravitational lensing contains information on the mass distributions in the lensing objects as well as on cosmological parameters such as H0. We derive an explicit expression for the conditional probability distribution function of time delays P(Δt | θ), given an image separation between multiple images θ and related statistics. We consider lensing halos described by the singular isothermal sphere (SIS) approximation and by its generalization as proposed by Navarro, Frenk, & White (NFW), which has a density profile ρ ∝ r-α in the innermost region. The time delay distribution is very sensitive to these profiles; steeper inner slopes tend to produce larger time delays. For example, if H0 = 70 km s-1 Mpc-1, a Λ-dominated cosmology and a source redshift zS = 1.27 are assumed, lenses with θ = 5'' produce a time delay of Δt = 1.5, 0.39, 0.15, and 0.071 yr (50% confidence interval) for SIS, generalized NFW with α = 1.5, α = 1.0, and α = 0.5, respectively. At a fixed image separation, the time delay is determined by the difference in the lensing potential between the position of the two images, which typically occur at different impact parameters. Although the values of Δt are proportional to the inverse of H0, P(Δt | θ) is rather insensitive to all other cosmological model parameters, source redshifts, magnification biases, and so on. A knowledge of P(Δt | θ) will also be useful in designing the observing program of future large-scale synoptic variability surveys and for evaluating possible selection biases operating against large splitting lens systems.

Interpreting the M22 Spike Events

Recently Sahu et al., using the Hubble Space Telescope to monitor stars in the direction of the old (~12 Gyr) globular cluster M22, detected six events in which otherwise constant stars brightened by ~50% during a time of 1 day. They tentatively interpret these unresolved events as being caused by microlensing of background bulge stars by free-floating planets in M22. Using simple analytic arguments, I show that if these spike events are caused by microlensing, the lensing objects are unlikely to be associated with M22 and unlikely to be part of a smoothly distributed Galactic population. Thus, either there happens to be a massive dark cluster of planets along our line of sight to M22 or the spike events are not caused by microlensing. The lensing planets cannot be bound to stars in the core of M22: if they were closer than ~8 AU, the lensing influence of the parent star would have been detectable. Moreover, in the core of M22, all planets with separations 1 AU would have been ionized by random stellar encounters. Most unbound planets would have escaped the core via evaporation, which preferentially affects such low-mass objects. Bound or free-floating planets can exist in the outer halo of M22; however, for reasonable assumptions, the maximum optical depth to such a population falls short of the observed optical depth, τ ~ 3 × 10-6, by a factor of 5-10. Therefore, if real, these events represent the detection of a significant free-floating Galactic planet population. The optical depth to these planets is comparable to and mutually exclusive from the optical depth to resolved events measured by microlensing survey collaborations toward the bulge and, thus, implies a similar additional mass of lensing objects. If the spatial and kinematic distributions of the two populations are the same, there are more than 103 planets per bulge microlens. Such a population is difficult to reconcile with both theory and observations.