Naked Cusp Research Articles

SDSS J0909+4449: A large-separation strongly lensed quasar at z ∼ 2.8 with three images

ABSTRACT We report the discovery of SDSS J0909+4449, an exceptional system consisting of a quasar at z = 2.788 strongly lensed by a group of galaxies at z ∼ 0.9 into three images separated by up to 14 arcsec based on archival data collected by the Sloan Digital Sky Survey, extended Baryon Oscillation Spectroscopic Survey, Beijing–Arizona Sky Survey, the Mayall z-band Legacy Survey, and the Gemini Telescope. We discuss two hypotheses on the nature of SDSS J0909+4449, i.e. a rare triply imaged quasar in the naked cusp configuration and a typical quadruply imaged quasar with the fourth image undetected in this data. We find that simple lens models can provide excellent fits to the observed image positions and the non-detection under either hypothesis. Deeper imaging data, spectroscopic observations, and follow-up light-curve measurements will be helpful in determining which hypothesis is correct and provide better constraints on the lens mass distribution. Nevertheless, given its unusually large image separations, SDSS J0909+4449 will be a unique probe for the mass structure and the underlying cooling and stellar feedback processes on group or cluster scales.

ANALYTICAL SOLUTIONS OF SINGULAR ISOTHERMAL QUADRUPOLE LENS

Using analytical method, we study the Singular Isothermal Quadrupole (SIQ) lens system, which is the simplest lens model that can produce four images. In this case, the radial mass distribution is in accord with the profile of the Singular Isothermal Sphere (SIS) lens, and the tangential distribution is given by adding a quadrupole on the monopole component. The basic properties of the SIQ lens have been studied in this paper, including deflection potential, deflection angle, magnification, critical curve, caustic, pseudo-caustic and transition locus. Analytical solutions of the image positions and magnifications for the source on axes are derived. As have been found, naked cusps will appear when the relative intensity $k$ of quadrupole to monopole is larger than 0.6. According to the magnification invariant theory of the SIQ lens, the sum of the signed magnifications of the four images should be equal to unity \citep{dal98}. However, if a source lies in the naked cusp, the summed magnification of the left three images is smaller than the invariant 1. With this simple lens system, we study the situations that a point source infinitely approaches a cusp or a fold. The sum of magnifications of cusp image triplet is usually not equal to 0, and it is usually positive for major cusp while negative for minor cusp. Similarly, the sum of magnifications of fold image pair is usually neither equal to 0. Nevertheless, the cusp and fold relations are still equal to 0, in that the sum values are divided by infinite absolute magnifications by definition.

Effects of galaxy-halo alignment and adiabatic contraction on gravitational lens statistics

We study the strong gravitational lens statistics of triaxial cold dark matter (CDM) halos occupied by central early-type galaxies. We calculate the image separation distribution for double, cusp and quad configurations. The ratios of image multiplicities at large separations are consistent with the triaxial NFW model, and at small separations are consistent with the singular isothermal ellipsoid (SIE) model. At all separations, the total lensing probability is enhanced by adiabatic contraction. If no adiabatic contraction is assumed, naked cusp configurations become dominant at approximately 2.5'', which is inconsistent with the data. We also show that at small-to-moderate separations, the image multiplicities depend sensitively on the alignment of the shapes of the luminous and dark matter projected density profiles. In constrast to other properties that affect these ratios, the degree of alignment does not have a significant effect on the total lensing probability. These correlations may therefore be constrained by comparing the theoretical image separation distribution to a sufficiently large lens sample from future wide and deep sky surveys such as Pan-Starrs, LSST and JDEM. Understanding the correlations in the shapes of galaxies and their dark matter halo is important for future weak lensing surveys.

The Third Image of the Large-Separation Lensed Quasar SDSS J1029+2623

We identify a third image in the unique quasar lens SDSS J1029+2623, the second known quasar lens produced by a massive cluster of galaxies. The spectrum of the third image shows similar emission and absorption features but has a redder continuum than the other two images, which can be explained by differential extinction or microlensing. We also identify several lensed arcs. Our observations suggest a complicated structure of the lens cluster at z ≈ 0.6. We argue that the three lensed images are produced by a naked cusp on the basis of successful mass models, the distribution of cluster member galaxies, and the shapes and locations of the lensed arcs. Lensing by a naked cusp is quite rare among galaxy-scale lenses but is predicted to be common among large-separation lensed quasars. Thus the discovery can be viewed as support for an important theoretical prediction of the standard cold dark matter model.

Effects of Triaxiality on the Statistics of Large‐Separation Gravitational Lenses

We study the statistics of large-separation gravitational lens systems produced by nonspherical halos in the cold dark matter (CDM) model. Specifically, we examine how the triaxiality of CDM halos affects the overall lensing probabilities and the relative numbers of different image configurations (double, quadruple, and naked cusp lenses). We find that triaxiality significantly enhances lensing probabilities by a factor of ~2-4, so it cannot be ignored. If CDM halos have central density slopes ? 1.5, we predict that a significant fraction (20%) of large-separation lenses should have naked cusp image configurations; this contrasts with lensing by isothermal (? ? 2) galaxies where naked cusp configurations are rare. The image multiplicities depend strongly on the inner density slope ?: for ? = 1, the naked cusp fraction is 60%, while for ? = 1.5, quadruple lenses are actually the most probable. Thus, the image multiplicities in large-separation lenses offer a simple new probe of the inner density profiles of dark matter halos. We also compute the expected probabilities and image multiplicities for lensed quasars in the Sloan Digital Sky Survey and argue that the recent discovery of the large-separation quadruple lens SDSS J1004+4112 is consistent with expectations for CDM.

Resolved nuclear CO(1-0) emission in APM 08279+5255: gravitational lensing by a naked cusp?

The ultraluminous broad absorption line quasar APM 08279+5255 is one of the most luminous systems known. Here, we present an analysis of its nuclear CO(1–0) emission. Its extended distribution suggests that the gravitational lens in this system is highly elliptical, probably a highly inclined disc. The quasar core, however, lies in the vicinity of a naked cusp, indicating that APM 08279+5255 is truly the only odd-image gravitational lens. This source is the second system for which the gravitational lens can be used to study structure on sub-kiloparsec scales in the molecular gas associated with the AGN host galaxy. The observations and lens model require CO distributed on a scale of ∼400 pc. Using this scale, we find that the molecular gas mass makes a significant, and perhaps dominant, contribution to the total mass within a couple of hundred parsecs of the nucleus of APM 08279+5255.