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

TL;DR

The AGEL survey confirms that CNN-based methods effectively identify strong gravitational lenses at higher redshifts using deep imaging and spectroscopy, providing a valuable resource for future astrophysical and cosmological studies.

Contribution

This study demonstrates the high success rate of CNN-based searches in identifying strong gravitational lenses and provides spectroscopic redshifts for a significant sample, expanding the known lens population.

Findings

CNN search methods are 88% successful in identifying lenses.

Spectroscopic redshifts measured for 68 lenses, with sources at z>1.34.

The survey includes 41 deflectors at z>0.5 suitable for follow-up studies.

Abstract

We present spectroscopic confirmation of candidate strong gravitational lenses using the Keck Observatory and Very Large Telescope as part of our ASTRO 3D Galaxy Evolution with Lenses (AGEL) survey. We confirm that 1) search methods using Convolutional Neural Networks (CNN) with visual inspection successfully identify strong gravitational lenses and 2) the lenses are at higher redshifts relative to existing surveys due to the combination of deeper and higher resolution imaging from DECam and spectroscopy spanning optical to near-infrared wavelengths. We measure 104 redshifts in 77 systems selected from a catalog in the DES and DECaLS imaging fields (r<22 mag). Combining our results with published redshifts, we present redshifts for 68 lenses and establish that CNN-based searches are highly effective for use in future imaging surveys with a success rate of 88% (defined as 68/77). We report 53 strong lenses with spectroscopic redshifts for both the deflector and source (z_src>z_defl), and 15 lenses with a spectroscopic redshift for either the deflector (z_defl>0.21) or source (z_src>1.34). For the 68 lenses, the deflectors and sources have average redshifts and standard deviations of 0.58+/-0.14 and 1.92+/-0.59 respectively, and corresponding redshift ranges of (0.21<z_defl<0.89) and (0.88<z_src<3.55). The AGEL systems include 41 deflectors at zdefl>0.5 that are ideal for follow-up studies to track how mass density profiles evolve with redshift. Our goal with AGEL is to spectroscopically confirm ~100 strong gravitational lenses that can be observed from both hemispheres throughout the year. The AGEL survey is a resource for refining automated all-sky searches and addressing a range of questions in astrophysics and cosmology.