The Population of Galaxy–galaxy Strong Lenses in Forthcoming Optical Imaging Surveys

Abstract

Ongoing and future imaging surveys represent significant improvements in depth, area and seeing compared to current data-sets. These improvements offer the opportunity to discover up to three orders of magnitude more galaxy-galaxy strong lenses than are currently known. In this work we forecast the number of lenses discoverable in forthcoming surveys and simulate their properties. We generate a population of statistically realistic strong lenses and simulate observations of this population for the Dark Energy Survey (DES), Large Synoptic Survey Telescope (LSST) and Euclid surveys. We verify our model against the galaxy-scale lens search of the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS), predicting 250 discoverable lenses compared to 220 found by Gavazzi et al. (2014). The predicted Einstein radius distribution is also remarkably similar to that found by . For future surveys we find that, assuming Poisson limited lens galaxy subtraction, searches in DES, LSST and Euclid datasets should discover 2400, 120000, and 170000 galaxy-galaxy strong lenses respectively. Finders using blue minus red (g − i) difference imaging for lens subtraction can discover 1300 and 62000 lenses in DES and LSST. The uncertainties on the model are dominated by the high redshift source population which typically gives fractional errors on the discoverable lens number at the tens of percent level. We find that doubling the signal-to-noise ratio required for a lens to be detectable, approximately halves the number of detectable lenses in each survey, indicating the importance of understanding the selection function and sensitivity of future lens finders in interpreting strong lens statistics. We make our population forecasting and simulated observation codes publicly available so that the selection function of strong lens finders can easily be calibrated.

Observational selection biases in time-delay strong lensing and their impact on cosmography

“…to two effects: firstly changing q affects the magnification and separation of images, and secondly the more massive deflectors in our model are more spherical than less massive deflectors (Collett 2015). We find that P(q) given formation of a double is comparable to the prior.…”

“…We use these velocity dispersions to infer the lens Einstein radius. The lenses are assumed to be uniformly distributed in co-moving volume and the ellipticities of the lenses, q, are drawn from P(q|σV ) as fit to SDSS light profiles by Collett (2015). We assume the density slope of the deflectors, η is drawn from a Gaussian of width 0.15, centred at 1.08 2 , as observed by Auger et al Figure 1.…”

Finding strong gravitational lenses in the Kilo Degree Survey with Convolutional Neural Networks

“…Upcoming telescopes, such as Euclid (Laureijs et al 2011) and the Large Synoptic Survey Telescope (LSST; LSST Science Collaboration et al 2009), will increase the rate of discovery of new lenses, reaching the number of ∼ 10 5 new strong lensing systems (Oguri & Marshall 2010;Pawase et al 2012;Collett 2015). Also, the number of lenses that will be observed by the Square Kilometer Array is expected to be of the same of order of magnitude (McKean et al 2015).…”

“…The ongoing optical wide surveys, such as the Kilo Degree Survey (KiDS; see Sec. 2), the Dark Energy Survey (DES; The Dark Energy Survey Collaboration 2005) and the Subaru Hyper Suprime-Cam Survey (Miyazaki et al 2012) are expected to find samples of lenses of the order of ∼ 10 3 (see, e.g, Collett 2015). Sub-mm observations from Herschel (Negrello et al 2010) and the South Pole Telescope (Carlstrom et al 2011), together with deeper, high resolution observations from the the Atacama Large Millimeter/sub-millimeter Array, are expected to provide several hundred new lenses as well.…”

The DES Bright Arcs Survey: Hundreds of Candidate Strongly Lensed Galaxy Systems from the Dark Energy Survey Science Verification and Year 1 Observations

“…Ongoing and upcoming surveys will discover many more. Predictions for the number of lenses depend on the depth and area of the survey and range from a few thousand for the full Dark Energy Survey to more than a hundred thousand for near-future surveys (Oguri & Marshall 2010;Collett 2015).…”