The AGEL Survey: Spectroscopic Confirmation of Strong Gravitational\n Lenses in the DES and DECaLS Fields Selected Using Convolutional Neural\n Networks

Abstract

We present spectroscopic confirmation of candidate strong gravitational\nlenses using the Keck Observatory and Very Large Telescope as part of our ASTRO\n3D Galaxy Evolution with Lenses (AGEL) survey. We confirm that 1) search\nmethods using Convolutional Neural Networks (CNN) with visual inspection\nsuccessfully identify strong gravitational lenses and 2) the lenses are at\nhigher redshifts relative to existing surveys due to the combination of deeper\nand higher resolution imaging from DECam and spectroscopy spanning optical to\nnear-infrared wavelengths. We measure 104 redshifts in 77 systems selected from\na catalog in the DES and DECaLS imaging fields (r<22 mag). Combining our\nresults with published redshifts, we present redshifts for 68 lenses and\nestablish that CNN-based searches are highly effective for use in future\nimaging surveys with a success rate of 88% (defined as 68/77). We report 53\nstrong lenses with spectroscopic redshifts for both the deflector and source\n(z_src>z_defl), and 15 lenses with a spectroscopic redshift for either the\ndeflector (z_defl>0.21) or source (z_src>1.34). For the 68 lenses, the\ndeflectors and sources have average redshifts and standard deviations of\n0.58+/-0.14 and 1.92+/-0.59 respectively, and corresponding redshift ranges of\n(0.21<z_defl<0.89) and (0.88<z_src<3.55). The AGEL systems include 41\ndeflectors at zdefl>0.5 that are ideal for follow-up studies to track how mass\ndensity profiles evolve with redshift. Our goal with AGEL is to\nspectroscopically confirm ~100 strong gravitational lenses that can be observed\nfrom both hemispheres throughout the year. The AGEL survey is a resource for\nrefining automated all-sky searches and addressing a range of questions in\nastrophysics and cosmology.\n