Time delay cosmography

Abstract

Gravitational time delays, observed in strong lens systems where the variable background source is multiply-imaged by a massive galaxy in the foreground, provide direct measurements of cosmological distance that are very complementary to other cosmographic probes. The success of the technique depends on the availability and size of a suitable sample of lensed quasars or supernovae, precise measurements of the time delays, accurate modeling of the gravitational potential of the main deflector, and our ability to characterize the distribution of mass along the line of sight to the source. We review the progress made during the last 15 years, during which the first competitive cosmological inferences with time delays were made, and look ahead to the potential of significantly larger lens samples in the near future.

Discovery of the Lensed Quasar System DES J0408-5354

“…Our modeling paper, Agnello et al (2017), presents a detailed model of the system, from which we expect the longest time delay in the system to be about 80 days, making DES J0408-5354 well suited (Treu & Marshall 2016) for time delay measurements, which we are undertaking via a monitoring campaign within the STRIDES Collaboration. The lensing model also constrains the mass of the possible perturbing galaxy and thus provides information about substructure in the lensing mass distribution.…”

“…Further spectroscopic follow-up and high-resolution imaging data should provide more needed details about the main lensing galaxy and its environment. We thus expect this quad system to be particularly useful for the application of time delay cosmography (e.g., Treu & Marshall 2016) and substructure studies (e.g., Dalal & Kochanek 2002). DES J0408-5354 heralds a much larger sample of some 20 of these very rare and valuable quad lensed quasar systems anticipated to be discovered by the DES.…”

“…In particular, lensed quasar systems can provide information on, for example, intervening absorption line systems (e.g., Smette et al 1995), properties of quasar host galaxies (e.g., Peng et al 2006), dark matter substructure in the lens (e.g., Dalal & Kochanek 2002), and the stellar content of the lensing galaxy (e.g., Schechter et al 2014). Moreover, when combined with time delay measurements and careful lens modeling, lensed quasar systems can provide powerful cosmological constraints that are complementary to those of other techniques (e.g., Suyu et al 2016;Treu & Marshall 2016;Bonvin et al 2017).…”

First Lensed Quasar Systems from the VST-ATLAS Survey: One Quad, Two Doubles, and Two Pairs of Lensless Twins*

“…Treu & Marshall (2016) present a current survey of the use of time-delay measurements for cosmography. The microlensing of lensed quasars can be used to determine sizes for the emitting regions of quasars (Rauch & Blandford 1991;Agol & Krolik 1999;Pooley et al 2007;Blackburne et al 2011) and to measure the dark matter fraction in lensing galaxies (Schechter & Wambsganss 2004;Pooley et al 2012;Jiménez-Vicente et al 2015).…”

Quasar lenses and pairs in the VST-ATLAS and Gaia

“…Through the lensing effect, one can ob-tain 1 : (i) a purely gravitational measurement of the properties of the deflector(s), including their invisible components like dark matter halos and individual stars (e.g. Oguri et al 2014); (ii) a magnified view of the background quasar, accretion disk, and host galaxy (Peng et al 2006;Sluse et al 2015;Motta et al 2017;Ding et al 2017); (iii) information about distances and thus cosmological parameters (Refsdal 1964;Cárdenas et al 2013;Treu & Marshall 2016;Suyu et al 2017). Spectroscopy of close sightlines, be they to multiple quasar images or to pairs of physically distinct quasars, is a probe of: (i) kinematics of the cosmic web at high redshift (Rauch et al 2005); (ii) small-scale structure of Lyα absorbers (Smette et al 1992;Dinshaw et al 1998;Rorai et al 2017); (iii) physical conditions of the cool ISM/CGM of galaxies and quasars (Farina et al 2014;Zahedy et al 2016).…”