Quasar Images Research Articles

Gravitational Lensing in a Concordance ΛCDM Universe: The Importance of Secondary Matter along the Line of Sight

To date, in almost all strong gravitational lensing analyses for modeling giant arc systems and multiple quasar images, it has been assumed that all the deflecting matter is concentrated in one lens plane at a certain distance—the thin lens approximation. However, in a few observed cases, lenses at more than one redshift have been identified as contributing to the image splitting. Here we report on a quantitative investigation of the importance and frequency of significant multiple lensing agents. We use multilens plane simulations to evaluate how frequently the combination of two or more lens planes is essential for multiple imaging, as compared with the cases for which a single lens plane alone provides enough focusing to be supercritical. We find that the fraction of cases for which more than one lens plane contributes significantly to a multiimage lensing situation is a strong function of source redshift. For sources at redshift unity, 95% of lenses involve only a single mass concentration, but for a more typical scenario with, e.g., a source at a redshift of zs = 3.8, as many as 38% of the strongly lensed quasars/arcs occur because of a significant matter contribution from one or more additional lens planes. In the roughly 30% of cases when additional planes make a significant contribution, the surface mass density of the primary lens will be overestimated by about 10%-15%, if the additional contributions are not recognized.

Microlensing in the double quasar SBS 1520+530

We present the results of a monitoring campaign of the double quasar SBS 1520+530 at Maidanak observatory from April 2003 to August 2004. We obtained light curves in V and R filters that show small-amplitude ∆m ≈ 0.1 mag intrinsic variations of the quasar on time scales of about 100 days. The data set is consistent with the previously determined time delay of ∆t = (130 ± 3) days by Burud et al. (2002, A&A, 391, 481). We find that the time delay corrected magnitude difference between the quasar images is now larger by (0.14±0.03) mag than during the observations by Burud et al. (2002). This confirms the presence of gravitational microlensing variations in this system.

Discovery of a Fifth Image of the Large Separation Gravitationally Lensed Quasar SDSS J1004+4112

Abstract We report on the discovery of a fifth lensed image in the large separation lensed quasar system SDSS J1004$+$4112. A faint point source located ${0\rlap {.}{}^{\mathrm {\prime \prime }}2}$ from the center of the brightest galaxy in the lensing cluster was detected in images taken with the Advanced Camera for Surveys (ACS) and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space Telescope. The flux ratio between the point source and the brightest lensed component in the ACS image is similar to that in the NICMOS image. The location and brightness of the point source are consistent with lens model predictions for a lensed image. We therefore conclude that the point source is likely to be a fifth lensed image of the source quasar. In addition, the NICMOS image reveals the lensed host galaxy of the source quasar, which can strongly constrain the structure of the lensing critical curves, and thereby the mass distribution of the lensing cluster.

The Lens Galaxy in PG 1115+080 is an Ellipse

We use the structure of the Einstein ring image of the quasar host galaxy in the four-image quasar lens PG 1115+080 to determine the angular structure of the gravitational potential of the lens galaxy. We find that it is well described as an ellipsoid and that the best-fit nonellipsoidal models are consistent with the ellipsoidal model. We find upper limits on the standard parameters for the deviation from an ellipse of |a/a0| < 0.035 and |a/a0| < 0.064. We also find that the position of the center of mass is consistent with the center of light, with an upper limit of |Δ| < 0005 on the offset between them. Neither the ellipsoidal nor the nonellipsoidal models can reproduce the observed image flux ratios while simultaneously maintaining a reasonable fit to the Einstein ring, so the anomalous flux ratio of the A1 and A2 quasar images must be due to substructure in the gravitational potential such as compact satellite galaxies or stellar microlenses rather than odd angular structure in the lens galaxy.

ChandraObservations of the Gravitational Lenses B1600+434 and B1608+656

We observed B1600+434 and B1608+656 with Chandra ACIS, detecting both quasar images in B1600+434 and three of four images in B1608+656. We did not detect significant X-ray emission from nearby galaxy groups or clusters associated with each lens galaxy. The upper limits on the X-ray luminosity of any cluster within 4' of each lens and at each lens redshift are ~2 × 1042 and ~6 × 1042 ergs s-1 for B1600+434 and B1608+656, respectively. The radio-loud source quasars have power-law photon indices of Γ = 1.9 ± 0.2 and Γ = 1.4 ± 0.3 and X-ray luminosities of 1.4 × 1045 and 2.9 × 1044 ergs s-1 for B1600+434 and B1608+656, respectively, before correcting for the magnification. We detected a differential absorption column density of ΔNH ~ 3 × 1021 cm-2 between the two images of B1600+434, roughly consistent with expectations from differential extinction estimates of ΔE(B - V) = 0.1 mag and a standard dust-to-gas ratio. The differential absorption observed in gravitational lenses may serve as an important probe to study the gas content in high-redshift galaxies, since it can separate the absorbing column originating from the lens galaxy and those intrinsic to quasars. We also detected 157 serendipitous X-ray sources in the two Chandra fields and identified the brighter optical counterparts using the Sloan Digital Sky Survey and Palomar Digital Sky Survey.

The Radio Variability of the Gravitational Lens PMN J1838-3427

We present the results of a radio variability study of the gravitational lens PMN J1838-3427. Our motivation was to determine the Hubble constant by measuring the time delay between variations of the two quasar images. We monitored the system for 4 months (approximately 5 times longer than the expected delay) using the Australia Telescope Compact Array at 9 GHz. Although both images were variable on a timescale of a few days, no correlated intrinsic variability could be identified, and therefore no time delay could be measured. Notably, the fractional variation of the fainter image (8%) was greater than that of the brighter image (4%), whereas lensed images of a point source would have the same fractional variation. This effect can be explained, at least in part, as the refractive scintillation of both images due to the turbulent interstellar medium of the Galaxy.

HE 0047-1756: A new gravitationally lensed double QSO

The quasar HE 0047-1756, at z=1.67, is found to be split into two images 1.44" apart by an intervening galaxy acting as a gravitational lens. The flux ratio for the two components is roughly 3.5:1, depending slightly upon wavelength. The lensing galaxy is seen on images obtained at 800 nm and 2.1 \mu; there is also a nearby faint object which may be responsible for some shear. The spectra of the two quasar images are nearly identical, but the emission line ratio between the two components scale differently from the continuum. Moreover, the fainter component has a bluer continuum slope than the brighter one. We argue that these small differences are probably due to microlensing. There are hints of an Einstein ring emanating from the brighter image toward the fainter one.

SDSS J1335+0118: A New Two-Image Gravitational Lens

Abstract We report on the discovery of the two-image gravitationally lensed quasar SDSS J1335$+$0118. The object was selected as a lens candidate from the Sloan Digital Sky Survey. The imaging and spectroscopic follow-up observations confirm that the system exhibits two gravitationally lensed images of a quasar at $z=1.57$. The image separation is ${1\rlap {.}{}^{\mathrm {\prime \prime }}56}$. We also detect an extended component between the two quasar images, likely to be a lensing galaxy. Preliminary mass modeling predicts a differential time delay, $\Delta t$ of $\sim30 \, h^{-1} \mathrm{day}$, assuming the redshift of the lens galaxy to be $0.5$.

Detection of X‐Rays from Galaxy Groups Associated with the Gravitationally Lensed Systems PG 1115+080 and B1422+231

Gravitational lenses that produce multiple images of background quasars can be an invaluable cosmological tool. Deriving cosmological parameters, however, requires modeling the potential of the lens itself. It has been estimated that up to a quarter of lensing galaxies are associated with a group or cluster that perturbs the gravitational potential. Detection of X-ray emission from the group or cluster can be used to better model the lens. We report on the first detection in X-rays of the group associated with the lensing system PG 1115+080 and the first X-ray image of the group associated with the system B1422+231. We find a temperature and rest-frame luminosity of 0.8 keV and 7 × 1042 ergs s-1, respectively, for PG 1115+080 and 1.0 keV and 8 × 1042 ergs s-1, respectively, for B1422+231. We compare the spatial and spectral characteristics of the X-ray emission with the properties of the group galaxies, with lens models, and with the general properties of groups at lower redshift.

Sub‐Milliarcsecond Imaging of Quasars and Active Galactic Nuclei. III. Kinematics of Parsec‐scale Radio Jets

We report the results of a 15 GHz (2 cm) multi-epoch VLBA program, begun in 1994 to study the outflow in radio jets ejected from quasars and active galaxies. The observed flow of 208 distinct features measured in 110 quasars, active galaxies, and BL Lac objects shows highly collimated relativistic motion with apparent transverse velocities typically between zero and about 15c, with a tail extending up to about 34c. Within individual jets, different features appear to move with a similar characteristic velocity which may represent an underlying continuous jet flow, but we also see some stationary and even apparently inward moving features which co-exist with the main features. Comparison of our observations with published data at other wavelengths suggests that there is a systematic decrease in apparent velocity with increasing wavelength, probably because the observations at different wavelengths sample different parts of the jet structure. The observed distribution of linear velocities is not consistent with any simple ballistic model. Either there is a rather broad range of Lorentz factors, a significant difference between the velocity of the bulk relativistic flow and the pattern speed of underlying shocks, or a combination of these options. Assuming a ballistic flow, comparison of observed apparent velocities and Doppler factors computed from the time scale of flux density variations is consistent with a steep power law distribution of intrinsic Lorentz factors, an isotropic distribution of orientations of the parent population, and intrinsic brightness temperatures about an order of magnitude below the canonical inverse Compton limit. It appears that the parent population of radio jets is not dominated by highly relativistic flows (abridged).

SDSS J115517.35+634622.0: A Newly Discovered Gravitationally Lensed Quasar

We report the discovery of SDSSJ115517.35+634622.0, a previously unknown gravitationally lensed quasar. The lens system exhibits two images of a $z = 2.89$ quasar, with an image separation of $1{\farcs}832 \pm 0.007$ . Near-IR imaging of the system reveals the presence of the lensing galaxy between the two quasar images. Based on absorption features seen in the Sloan Digital Sky Survey (SDSS) spectrum, we determine a lens galaxy redshift of $z = 0.1756$. The lens is rather unusual in that one of the quasar images is only $0{\farcs}22\pm0{\farcs}07$ ($\sim 0.1 R_{\rm eff}$) from the center of the lens galaxy and photometric modeling indicates that this image is significantly brighter than predicted by a SIS model. This system was discovered in the course of an ongoing search for strongly lensed quasars in the dataset from the SDSS.

The Effect of Central Baryonic Cores in Dark Halos on the Evaluation of Strong Lensing Probabilities

We present an estimate of the strong lensing probability by dark halos, with emphasis on the role of the baryonic matter arising purely from radiative cooling. We treat the contribution of the cooled baryons optimistically with all the cooled baryons confined within a central core, and including no feedback process from stellar evolution. Our two-component model provides a strong lensing probability that is in good agreement with the observed distribution of multiple images of quasars, provided that the cooled baryons are deposited within a spherical region of radius of 0.1 times the virial radius and follow an isothermal profile. It is pointed out that strong lensing may be used as an additional probe of baryon physics in dark halos though this may meanwhile complicate the test of the inner density profiles of dark matter in halos using the observed strong lensing probability.

Quasar Mesolensing—Direct Probe to Substructures around Galaxies

Abstract Recently, a “CDM crisis” is being discussed. The main point of this crisis is that the number of substructures presented by cosmological $N$-body simulations based on the CDM scenario for structure formation is much larger than the observed substructures. Therefore, it is crucial for this crisis to discriminate whether the expected number of CDM substructures really exist without detectable radiation, or do not exist. In this paper, we present a new idea to detect such invisible substructures by utilizing gravitational lensing. Here, we consider quasars that are gravitationally lensed by a foreground galaxy. A substructure around a lensing galaxy may superposed on one of the lensed images of such quasars. In this situation, additional image splitting should occur in the image behind the substructure, and additional multiple images are created. From our estimations, the separation and time delay between the additional multiple images are expected to be $1 \hbox{--} 30$ milli-arcsecond and $1 \hbox{--} 10^3$ second, respectively. Furthermore, we evaluate that the optical depth for such events is $\sim 0.1$. Consequently, future fine-resolution imaging and/or high-speed monitoring will unable us to find invisible substructures in one of a few multiple quasars.

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.

Five nights of intensiveR- andV-band photometry of QSO 0957+561A,B

We present R- and V-band photometry of the gravitational lens system QSO 0957+561 from five nights (one in 2000 January and four in 2001 March, corresponding to the approximate time delay for the system) of uninterrupted monitoring at the Nordic Optical Telescope. In the photometry scheme we have stressed careful magnitude calibration as well as corrections for the lens galaxy contamination and the crosstalk between the twin (A and B) quasar images. The resulting, very densely sampled, light curves are quite stable, in conflict with earlier claims derived from the same data material. We estimate high-precision timelag-corrected B/A flux ratios in both colour bands, as well as V–R colour indices for A and B, and discuss the short time-scale variability of the system.

SDSS J090334.92+502819.2: A New Gravitational Lens

We report the discovery of a new gravitationally lensed quasar from the Sloan Digital Sky Survey, SDSS J090334.92+502819.2. This object was targeted for SDSS spectroscopy as a luminous red galaxy, but manual examination of the spectrum showed the presence of a quasar at z 3.6 in addition to a red galaxy at z = 0.388, and the SDSS image showed a second possible quasar image nearby. Follow-up imaging and spectroscopy confirmed the lensing hypothesis. In images taken at the Astrophysical Research Consortium 3.5 m telescope, two quasars are separated by 28; the lensing galaxy is clearly seen and is blended with one of the quasar images. Spectroscopy taken at the Keck II telescope shows that the quasars have identical redshifts of z 3.6, and both show the presence of the same broad absorption line–like troughs. We present simple lens models that account for the geometry and magnifications. The lens galaxy lies near two groups of galaxies and may be a part of them. The models suggest that the groups may contribute considerable shear that has a strong effect on the lens configuration.

Mass and Dust in the Disk of a Spiral Lens Galaxy

Gravitational lensing is a potentially important probe of spiral galaxy structure, but only a few cases of lensing by spiral galaxies are known. We present Hubble Space Telescope and Magellan observations of the two-image quasar PMN J2004-1349, revealing that the lens galaxy is a spiral galaxy. One of the quasar images passes through a spiral arm of the galaxy and suffers 3 mag of V-band extinction. Using simple lens models, we show that the mass quadrupole is well aligned with the observed galaxy disk. A more detailed model with components representing the bulge and disk gives a bulge-to-disk mass ratio of 0.16 ± 0.05. The addition of a spherical dark halo, tailored to produce an overall flat rotation curve, does not change this conclusion.

An optical imaging study of 0.4 $\leq $z≤0.8 quasar host galaxies

We have conducted an optical imaging study aimed at resolving the host galaxies of 79 radio-loud and radio-quiet quasars at , extending the number of investigated objects in this redshift range by ~45%. Observations were performed mainly in the R band but also in V and I band using the Nordic Optical Telescope on La Palma. In this paper we discuss the sample composition and observations and the reduction techniques used. The quasars were selected in pairs of radio-loud and radio-quiet objects matched in the z–V plane in order to facilitate a statistical comparison. The radio-loud part of the sample contains comparable numbers of flat and steep radio spectrum sources which also are matched in redshift and V magnitude. Point spread function subtraction was performed using one-dimensional luminosity profiles both on the quasar image and on a field star, and subtracted images and luminosity profiles are shown for each quasar field. The detection rate is 60% for the radio-quiet host galaxies and 80% for radio-loud hosts. The host galaxies have magnitudes which make them brighter than an L* galaxy by a factor of at the low end of the redshift range, which increases by times towards the higher end of the redshift range. Both radio-quiet and radio-loud hosts follow the radio galaxy R–z Hubble relation well. Analysis and discussion of colours and morphology is presented in (2003).

SDSS J1650+4251: A New Gravitational Lens

We report that the Sloan Digital Sky Survey quasar SDSS J165043.44+425149.3 is gravitationally lensed into two images, based on observations obtained with the WIYN 3.5 m telescope at the Kitt Peak National Observatory. The lensed quasar, at a redshift of z=1.54, appears as two images separated by 1.2" with B-band magnitudes of 17.8 and 20.0. The lensing galaxy is clearly detected in I-band images obtained in 0.3" seeing, after point spread function subtraction of the two quasar images. A strong metal-line absorption system is also identified in the unresolved SDSS spectrum of the double quasar, suggesting a plausible lens redshift of z=0.58. The UBRI flux ratios of the pair vary significantly from 8.5:1 in the blue to 5.4:1 in the red, a difference of 0.5 magnitudes, and may indicate the presence of microlensing in one or both quasar components. The predicted differential time delay between quasar images is on the order of one month, assuming the intervening absorption system is due to the lensing galaxy.

New aperture photometry of QSO 0957+561; application to time delay and microlensing

We present a re-reduction of archival CCD frames of the doubly imaged quasar 0957+561 using a new photometry code. Aperture photometry with corrections for both cross contamination between the quasar images and galaxy contamination is performed on about 2650 R-band images from a five year period (1992–1997). From the brightness data a time delay of days is derived using two different statistical techniques. The amount of gravitational microlensing in the quasar light curves is briefly investigated, and we find unambiguous evidence of both long term and short term microlensing. We also note the unusual circumstance regarding time delay estimates for this gravitational lens. Estimates by different observers from different data sets or even with the same data sets give lag estimates differing by typically 8 days, and error bars of only a day or two. This probably indicates several complexities where the result of each estimate depends upon the details of the calculation.