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July 2011  •  Volume 4 Number 2

Strong Gravitational Lensing – LSST Investigates Cosmology, Distant Galaxies and Dark Matter

This E-News article is based on Chapter 12 of the LSST Science Book: Strong Gravitational Lensing. The Authors of Chapter 12 are:

  • Phil Marshall
  • Maruša Bradač
  • George Chartas
  • Gregory Dubler
  • Árdis Elíadóttir
  • Emilio Falco
  • Chris Fassnacht
  • James Jee
  • Charles Keeton
  • Masamune Oguri
  • Anthony Tyson

A simple recipe: Take a lot of mass – a galaxy or cluster of galaxies – and place it in the line of sight to a distant object to create a powerful tool for studying cosmology and galaxy structure. This technique, gravitational lensing, provides information not only about the magnified source but also about the foreground lens. LSST will contain more strong gravitational lensing events – those where multiple images of the background source form – than any other survey before it, opening up a wealth of possibilities to answer some of the most fundamental questions about the Universe. The survey’s large volume, high accuracy photometry and multi-filter time series will produce some big advances in strong lensing science.

A Primer on Lensing

Strong gravitational lensing bends light from a distant source to create distorted or multiple images. Credit: Tony Tyson, Greg Kochanski and Ian Dell’Antonio; Frank O’Connell and Jim McManus, adopted from The New York Times

The geometric configuration of the lensing setup and its result are simple: light leaves a distant source (1) and then passes through a massive foreground object – a galaxy, group or cluster and its associated dark matter. The gravity of this object acts as a lens (2), bending and focusing the light toward observers on Earth (3): the LSST will see distorted images of the background source. Strong gravitational lensing occurs when the density of the intermediate object is so high that it creates multiple images of the background object.

A good lens model fits the positions of the multiple images, but it also predicts the time delay of the ray in comparison to a light ray traveling in a direct path in empty space. Although researchers cannot calculate the time delay of single ray, they can calculate the difference between two images for a multiply imaged source. So a time variable background object, such as an active galactic nucleus (AGN), not only constrains the mass distribution of the lensing galaxy, but allows the distance to the lens to be backed out as well. As Phil Marshall, Chair of the LSST Strong Lensing Science Collaboration puts it, “such well-calibrated lenses provide a high precision optical bench for cosmology experiments.” LSST will open up the time domain in a way no previous optical telescope has.

Sometimes the image of the light sources forms a ring. Using the lens equation, scientists can determine the direct relationship between this Einstein Ring and the mass of the lensing object. This allows precise measurement of the mass of distant objects to further refine their characteristics.

“Weighing galaxies is difficult,” says Marshall. “Strong lensing allows us to measure the mass of galaxies and clusters to an accuracy of a few percent.”

Strong Gravitational Lenses in the LSST Survey

LSST will discover an immense number of strong gravitational lenses, thanks to its high spatial resolution and outstanding image quality. The sample will be big enough to reveal many rare, exotic strong lensing events as well. The typical objects’ lensing cross-section is a strong function of its mass, but because galaxies are far more numerous than the more massive groups and clusters, most strong lenses are produced by massive elliptical galaxies. However, researchers expect to find thousands of group and cluster-scale lenses too.

Galaxy lenses. Researchers expect massive elliptical galaxies at redshift 0.5-1.0 to dominate the galaxy-scale lens population. For this reason, the typical gravitational lens will look like a bright red galaxy with some residual blue flux around it. Because the detection of the systems relies on scientists’ abilities to distinguish the lens light from the source light, edge-on spirals may be easier to use as lenses in some cases. The Sloan Lens ACS Survey leads the way in detecting galaxy-sized lenses. Marshall predicts: “LSST will increase the number of galaxy-scale strong lens detections by an order of magnitude over Dark Energy Survey, and by a factor of 100 over the current sample. And we expect to find nearly 10,000 lensed quasars during the survey – and measure the time delays in several thousand of them.” The LSST lensed quasar sample will be around two orders of magnitude larger than the current largest survey of lensed quasars. Because we understand well the light curves of supernovae (SNe), scientists can use strongly lensed supernovae to provide accurate estimates of time delays between images. Redshifts of imaged SNe will typically be around z ~ 0.8 while the redshifts of the giant elliptical galaxies acting as lenses at about z ~ 0.2. LSST should find about 330 lensed supernovae. “No-one has ever seen a strongly-lensed supernova,” says Marshall, “but we expect them to be pretty useful. LSST will enable their use on an industrial scale – and may even find the first one.”

Hubble Image of Abell 2218 is an example of gravitational lensing. It shows the arc-like pattern spread across the picture – an illusion caused by the gravitational field of the cluster. The process magnifies, brightens and distorts images of source objects and creates a “zoom lens” to view objects. Using galaxy clusters as cosmic telescopes, LSST will help measure the luminosity function of z > 7 galaxies (the time of rapid reionization). Credits: W.Couch (University of New South Wales), R. Ellis (Cambridge University), and NASA.

When a galaxy lenses an image of a distant quasar, we gain information about the foreground lens galaxy and the background source on a number of different length scales: macrolensing ( ~ 1 arcsec) due to global mass distribution, millilensing (~ 1 milliarcsec) from dark matter substructure, and microlensing (~ 1 microarcsec) as stars in the lens galaxy cross in front of the source. All of these, and the size of the source as well, have effects on the image fluxes, which have to be disentangled. LSST’s well-sampled six-band light curves will make this possible.

Galaxy cluster lenses. In 1986 researchers discovered the first giant arc in Abell 370 and have discovered many more in the intervening years. Most of the high-mass galaxy clusters have core surface densities high enough to create conspicuous strong lensing features such as multiple images, arcs and arclets. The number of lensed arcs in a cluster is a strong function of cluster mass. The ability to detect and identify strongly lensed features depends on the source size, the image quality and the detailed properties of cluster mass distributions. Cluster strong lensing is very useful in exploring the mass distribution in clusters, which is critical to understanding properties of dark matter. Measurements of strong lensing combined with weak lensing measurements allow study of the density profile of clusters over a large range of radii leading to a test of structure formation models.

LSST’s high resolution will provide a strong advantage over other ground-based surveys in identifying systems of multiple images by their colors and morphologies. LSST will likely detect around 1,000 multiple image systems.

LSST will Probe Many Mysteries with Strong Lenses

LSST will provide opportunities to measure the gross mass structures of massive galaxies as they operate as strong gravitational lenses, allowing researchers to trace their evolution. LSST data alone will produce accurate measurements of image positions, fluxes, and time delays for several thousand lensed quasars, AGN and supernovae. It will also detect approximately 10,000 lensed galaxies, which can then be modeled. LSST’s statistically complete sample will provide the opportunity to measure the mass function and mass evolution of massive galaxies over a wide redshift range up to and including the era of their formation (z ~ 1-2, several billion years after the Big Bang). Marshall says, “We expect the LSST strong lenses to provide the best assessment of the distribution of massive galaxy density profiles out to z ~ 1.5.” Follow-up observations will supplement and enhance the LSST data. Spectroscopic redshifts will allow absolute mass measurements, which sharpen up when the lens strength and stellar dynamics are combined; high-resolution infra-red imaging will further expand the opportunities.

Galaxy groups, the most common galaxy environment in the local Universe, may be the sites of many of the changes in galaxy morphology and star formation rate between z ~ 2 and today. Although galaxy groups have been very well studied at low redshifts, little is known about those at moderate redshifts (0.3 < z < 1); they are difficult to detect and assumptions need to be made to determine mass estimates. Strong lensing provides the most precise method to measure object masses beyond the local Universe. The large sample size (about 1,000 groups), wide redshift range, and rigorous mass measurements will permit unprecedented study of mass in galaxy groups.

Cosmography is the measurement of the distance scale of the Universe and its associated fundamental parameters. The large number of strong lenses observed with LSST will allow statistical approaches to cosmography. By combining the constraints from time delays with those from galaxy clustering and supernovae, researchers will be able to achieve higher accuracy on the dark energy equation of state parameters. “A few hundred well-measured LSST time delay lenses will give us a very interesting, complementary measurement of the accelerating expansion of the Universe,” says Marshall.

Chromatic variability is observed in lensed Active Galactic Nuclei (AGN), compact regions at the centre of a galaxy which display a much higher than normal luminosity over a broad spectral region. This variability implies that the effective size of the emission region, the accretion disk, varies with wavelength. With LSST’s larger sample size, scientists will be able to probe disk structure as a function of AGN luminosity, black hole mass and host galaxy properties.

The interstellar medium (ISM) in galaxies causes extinction of light passing through it. Because gravitationally lensed multiply imaged background sources provide two or four sight-lines through the deflecting galaxy, scientists can determine the differential extinction curve of the intervening galaxy. LSST’s combined sample of lensed quasars and supernovae will permit statistical studies of extinction properties of high redshift galaxies and the evolution of those dust distributions with redshift and their difference among galaxy types – for the first time outside the Local Group.

Additional Strong Lensing Science Team Members

  • Tim Axelrod
  • Ted Baltz
  • Roger Blandford
  • Ross Fadely
  • Veronica Motta
  • Aaron Roodman
  • Chris Stubbs
  • Tommaso Treu
  • Michael Strauss

Large amounts of dark matter exist in galaxy clusters. LSST data will provide opportunities for examination of dark matter distributions and its interaction with the baryonic mass component. With the LSST sample of over about 1,000 clusters capable of strong lensing, scientists will be able to probe and place limits on the self-interaction cross-section of dark matter, follow the growth of dark matter structure through cosmic time, and use well-calibrated clusters as cosmic telescopes to study hard-to-observe lower luminosity galaxies.

The cluster mass function is one of the four most promising dark energy probes. LSST will offer a unique opportunity to study the cluster dynamics in unprecedented detail and thereby construct a well-calibrated mass function that will allow quantification of the effects of dark energy. Marshall says, “The mass function is most sensitive to cosmology at the massive end – and it’s these massive clusters that make the most striking strong lenses. Combining weak and strong lensing for cluster mass measurement on a grand scale with LSST is an enticing prospect.”

From the masses of the heaviest galaxies, to the nature of dark matter and even dark energy, strong lensing science with the LSST will provide unprecedented opportunities for investigations into fundamental mysteries of the Universe.

Article written by Anna H. Spitz and Phil Marshall


LSST is a public-private partnership. Funding for design and development activity comes from the National Science Foundation, private donations, grants to universities, and in-kind support at Department of Energy laboratories and other LSSTC Institutional Members:

Adler Planetarium; Brookhaven National Laboratory (BNL); California Institute of Technology; Carnegie Mellon University; Chile; Cornell University; Drexel University; Fermi National Accelerator Laboratory; George Mason University; Google, Inc.; Harvard-Smithsonian Center for Astrophysics; Institut de Physique Nucléaire et de Physique des Particules (IN2P3); Johns Hopkins University; Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) – Stanford University; Las Cumbres Observatory Global Telescope Network, Inc.; Lawrence Livermore National Laboratory (LLNL); Los Alamos National Laboratory (LANL); National Optical Astronomy Observatory; Princeton University; Purdue University; Research Corporation for Science Advancement; Rutgers University; SLAC National Accelerator Laboratory; Space Telescope Science Institute; Texas A & M University; The Pennsylvania State University; The University of Arizona; University of California at Davis; University of California at Irvine; University of Illinois at Urbana-Champaign; University of Michigan; University of Pennsylvania; University of Pittsburgh; University of Washington; Vanderbilt University

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LSST E-News is a free email publication of the Large Synoptic Survey Telescope Project. It is for informational purposes only, and the information is subject to change without notice.

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