CANDELS

CANDELS is a major extragalactic imaging survey conducted with the Hubble Space Telescope that mapped deep near-infrared and optical fields to study galaxy formation and evolution across cosmic time. The programme combined observations from the Wide Field Camera 3 and the Advanced Camera for Surveys to probe rest-frame ultraviolet to optical light for galaxies at redshifts up to z ≳ 8, enabling analyses of morphology, star-formation, and mass assembly. CANDELS targeted legacy survey fields coordinated with multiwavelength datasets from missions and facilities such as Spitzer Space Telescope, Chandra X-ray Observatory, Atacama Large Millimeter/submillimeter Array, and the Very Large Telescope.

Overview

CANDELS (Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey) was executed as a multi-cycle treasury programme on Hubble beginning in 2010, designed to produce deep near-infrared imaging across five well-studied extragalactic fields: GOODS-North, GOODS-South, COSMOS, EGS, and UDS. The survey strategy balanced depth and area to enable statistical studies comparable to legacy programmes like Hubble Ultra Deep Field while linking to wide-area efforts such as Sloan Digital Sky Survey and COSMOS ground-based campaigns. CANDELS observations were coordinated with spectroscopic surveys performed with Keck Observatory, Subaru Telescope, and Gemini Observatory to obtain redshifts from instruments including DEIMOS, MOSFIRE, and VIMOS.

Survey Design and Observations

The CANDELS observing plan combined "Wide" and "Deep" components: the Wide component covered the COSMOS, EGS, and UDS fields to intermediate depth, while the Deep component targeted the GOODS-North and GOODS-South regions with much longer exposures similar in ambition to the Hubble Ultra Deep Field and HUDF09. Observations used the Wide Field Camera 3 IR channel with filters F105W, F125W, and F160W and the Advanced Camera for Surveys with filters F606W and F814W to obtain multi-band photometry. Scheduling and pointing accounted for constraints from South Atlantic Anomaly passages and Earth occultation; exposures were dithered to mitigate detector artifacts, cosmic rays, and persistence effects from previous observations involving NICMOS and early WFC3 calibrations. CANDELS also exploited parallel observations to increase area, coordinating with Herschel Space Observatory far-infrared maps and deep radio imaging from the Karl G. Jansky Very Large Array.

Data Processing and Products

Raw CANDELS exposures were processed through pipelines at the Space Telescope Science Institute and reduced into mosaics using software such as Drizzle with careful astrometric alignment to reference catalogs from 2MASS, SDSS DR7, and ground-based imaging from Subaru and VISTA. Photometric catalogs were produced using SExtractor and template-fitting methods cross-calibrated with photometry from Spitzer/IRAC and spectrophotometry from VLT FORS2 and Keck LRIS. Data releases included science-ready images, weight maps, segmentation maps, point-spread-function models, and multiwavelength catalogs with photometric redshifts computed via codes like EAZY and stellar population parameters derived with FAST and spectral energy distribution fitting using libraries from Bruzual & Charlot. Public products were archived at the Mikulski Archive for Space Telescopes and adopted by teams using ALMA, Chandra, and XMM-Newton ancillary data.

Scientific Goals and Key Results

CANDELS aimed to constrain the emergence of galactic structure, the buildup of stellar mass, the evolution of star-formation rates, the role of mergers versus in-situ growth, and the demographics of high-redshift galaxies and active galactic nuclei. Key scientific outcomes included robust measurements of the luminosity function and stellar mass function at z ∼ 4–8, improved constraints on the cosmic star-formation history alongside results from GALEX and Spitzer, and morphological catalogs quantifying size evolution compared with predictions from simulations such as Illustris and EAGLE. CANDELS enabled discovery and characterization of candidate galaxies during the epoch of reionization, informed follow-up spectroscopy with Keck/MOSFIRE and VLT/X-shooter, and provided AGN host studies linked to Chandra Deep Field South X-ray sources. The survey produced influential results on compact quiescent galaxies at z ~ 2, major merger rates compared with theoretical models by Somerville-style semianalytic codes, and the role of dust-obscured star formation traced in conjunction with Herschel and ALMA.

Collaborations and Legacy Impact

CANDELS was executed by a large consortium including principal investigators and teams from institutions like University of California, Santa Cruz, Leiden University, University of Cambridge, Max Planck Institute for Astronomy, and the Space Telescope Science Institute. The programme fostered collaborations with spectroscopic consortia on Keck, VLT, Subaru, and Gemini, and its catalogs served as inputs for follow-up programs with James Webb Space Telescope early-release science, ALMA deep fields, and wide-field surveys such as Euclid and Roman Space Telescope. CANDELS legacy products remain heavily cited in studies linking deep imaging to theoretical frameworks from groups using Millennium Simulation outputs and inform target selection for time-domain programs with Vera C. Rubin Observatory and space-based probes of reionization. Category:Astronomy surveys