along the line of sight behind the lens ( e . g ., SDSS J0946 + 1006 , which shows multiple concentric rings ) the relationship between the distances and the lens mass contains information about the dark energy density . However , these are rare : only 10 are expected in the entire DES footprint ( Gavazzi et al ., 2008 ). Also , time-delay measurements of variable-luminosity objects , like lensed quasars , can allow for measurements of the Hubble constant ( Refsdal et al ., 1964 ).
We see a variety of morphologies in the first galaxy- and galaxy-cluster-scale lenses discovered in early DES data sets , shown in Figure 1 ; the lensed sources range in redshift , 0.80 < z < 3.2 . The STRong-lensing Insights into Dark Energy Survey ( STRIDES ; Treu et al ., 2015 ) program aims to discover and follow up new time-delay lenses in DES data . Under these auspices , we have also discovered and confirmed two lensed quasars at z ~ 1.6 and ~ 2.4 ( Agnello et al ., 2015 ). Although these discoveries were made using Magellan / Baade , our Gemini Large and Long program is providing the capability for future confirmations .
Detective Work
DES is a deep-sky survey that covers 5,000 square degrees ( sq . deg .) of the southern Galactic Cap in five optical filter bands ( g , r , i , z , and Y ). The main instrument for DES is the Dark Energy Camera ( DECam ), a widefield ( 3 sq . deg .) camera mounted on the Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory in the Chilean Andes ( Flaugher et al ., 2015 ). The survey has finished three out of the planned five years . The Science Verification ( SV ) season took place after commissioning in late 2012 before the official science survey began . The SV data cover 250 sq . deg . (< 5 % of the full area ) and provide the imaging data for this work .
Searching through this area of sky is the first challenge in finding lenses . A team of ~ 20 DES scientists visually scanned the SV sky area , looking primarily for morphological features — multiple images , arcs , and full ( Einstein ) rings . We first performed a non-targeted search of the entire SV area , without focusing on any particular fields or objects . We then undertook a targeted search in the fields of galaxy clusters in the DES footprint . The redMaPPer cluster-finder ( Rykoff et al ., 2014 ) provided optically selected clusters . Overlapping fields of South Pole Telescope ( SPT ) data provided clusters selected with the Sunyaev-Zel ’ dovich effect ( Bleem et al ., 2015 ).
The resulting list of candidates was then refined by a group of three expert scanners , who reduced the total number of highly ranked candidates to 53 .
We also predicted the number of lenses we could find in DES by comparing our list to a different sample of highly ranked candidates / confirmed lenses found in the Canada-France-Hawai ‘ i Telescope Legacy Survey ( CFHTLS ) Strong Lensing Legacy Survey ( S2LS ; More et al ., 2012 ) — including source galaxies that survived a cut on the DES magnitude limit ( 24.5 magnitude in g-band ). There may be over 2,000 similar lenses in the full DES area , and about 100 in the SV region . While we accounted for the relative sky areas and depths of the two surveys , we had no mechanism to affirm the efficiency of human visual inspection .
Confirming Lenses with Spectroscopic Follow-up
The next puzzle piece we needed was confirmation that a source galaxy lies beyond the putative lensing galaxy . This requires a sufficiently precise spectroscopic measurement of the source galaxy ’ s redshift . Photometric
January 2017 | 2016 Year in Review GeminiFocus
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