This paper describes the Hubble Space Telescope imaging data products and data reduction procedures for the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS). This survey is designed to document the evolution of galaxies and black holes at z ∼ 1.5 − 8, and to study Type Ia SNe beyond z > 1.5. Five premier multi-wavelength sky regions are selected, each with extensive multiwavelength observations. The primary CANDELS data consist of imaging obtained in the Wide Field Camera 3 / infrared channel (WFC3/IR) and UVIS channel, along with the Advanced Camera for Surveys (ACS). The CANDELS/Deep survey covers ∼ 125 square arcminutes within GOODS-N and GOODS-S, while the remainder consists of the CANDELS/Wide survey, achieving a total of ∼ 800 square arcminutes across GOODS and three additional fields (EGS, COSMOS, and UDS). We summarize the observational aspects of the survey as motivated by the scientific goals and present a detailed description of the data reduction procedures and products from the survey. Our data reduction methods utilize the most up to date calibration files and image combination procedures. We have paid special attention to correcting a range of instrumental effects, including CTE degradation for ACS, removal of electronic bias-striping present in ACS data after SM4, and persistence effects and other artifacts in WFC3/IR. For each field, we release mosaics for individual epochs and eventual mosaics containing data from all epochs combined, to facilitate photometric variability studies and the deepest possible photometry. A more detailed overview of the science goals and observational design of the survey are presented in a companion paper.

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We present and explore deep narrow-and medium-band data obtained with the Subaru and the Isaac Newton telescopes in the ∼ 2 deg 2 COSMOS field. We use these data as an extremely wide, low-resolution (R ∼ 20 − 80) IFU survey to slice through the COSMOS field and obtain a large sample of ∼ 4000 Lyα emitters (LAEs) from z ∼ 2 to z ∼ 6 in 16 redshift slices (SC4K). We present new Lyα luminosity functions (LFs) covering a co-moving volume of ∼ 10 8 Mpc 3 . SC4K extensively complements ultradeep surveys, jointly covering over 4 dex in Lyα luminosity and revealing a global (2.5 < z < 6) synergy LF with α = −1.93 +0.12 −0.12 , log 10 Φ * Lyα = −3.45 +0.22 −0.29 Mpc −3 and log 10 L * Lyα = 42.93 +0.15 −0.11 erg s −1 . The Schechter component of the Lyα LF reveals a factor ∼ 5 rise in L * Lyα and a ∼ 7× decline in Φ * Lyα from z ∼ 2 to z ∼ 6. The data reveal an extra power-law (or Schechter) component above L Lyα ≈ 10 43.3 erg s −1 at z ∼ 2.2 − 3.5 and we show that it is partially driven by X-ray and radio AGN, as their Lyα LF resembles the excess. The power-law component vanishes and/or is below our detection limits above z > 3.5, likely linked with the evolution of the AGN population. The Lyα luminosity density rises by a factor ∼ 2 from z ∼ 2 to z ∼ 3 but is then found to be roughly constant (1.1 +0.2 −0.2 × 10 40 erg s −1 Mpc −3 ) to z ∼ 6, despite the ∼ 0.7 dex drop in UV luminosity density. The Lyα/UV luminosity density ratio rises from 4±1% to 30 ± 6% from z ∼ 2.2 to z ∼ 6. Our results imply a rise of a factor of ≈ 2 in the global ionisation efficiency (ξ ion ) and a factor ≈ 4 ± 1 in the Lyα escape fraction from z ∼ 2 to z ∼ 6, hinting for evolution in both the typical burstiness/stellar populations and even more so in the typical ISM conditions allowing Lyα photons to escape.

The extragalactic background light at far-infrared wavelengths 1-3 originates from opticallyfaint, dusty, star-forming galaxies in the universe with star-formation rates at the level of a few hundred solar masses per year 4 . Due to the relatively poor spatial resolution of farinfrared telescopes 5, 6 , the faint sub-millimetre galaxies are challenging to study individually. Instead, their average properties can be studied using statistics such as the angular power spectrum of the background intensity variations 7-10 . A previous attempt 11 at measuring this power spectrum resulted in the suggestion that the clustering amplitude is below the level computed with a simple ansatz based on a halo model 12 . Here we report a clear detection of the excess clustering over the linear prediction at arcminute angular scales in the power spectrum of brightness fluctuations at 250, 350, and 500 µm. From this excess, we find that submillimetre galaxies are located in dark matter halos with a minimum mass of log[M min /M ⊙ ] = 11.5 +0.7 −0.2 at 350 µm. This minimum dark matter halo mass corresponds to the most efficient mass scale for star formation in the universe 13 , and is lower than that predicted by semi-analytical models for galaxy formation 14 .Despite recent successes in attributing most of the extragalactic background light at submillimetre wavelengths to known galaxy populations through stacking analyses 15-17 , we have not individually detected the faint galaxies that are responsible for more than 85% of the total extragalactic intensity at these wavelengths 18 . The faint star-forming galaxies are expected to trace the large-scale structure of the Universe, especially in models where galaxy formation and evolution is closely connected to dark matter halos. While not individually detected in low resolution observations, the clustering of galaxies is expected to leave a distinct signature in the total intensity variations at sub-millimetre wavelengths. The amplitude of the power spectrum of intensity vari-2 ations as a function of the angular scale provides details on the redshift distribution and the dark matter halo mass scale of dusty, star-forming galaxies in the universe 7 .For this analysis, we used data from the Herschel Multi-tiered Extra-galactic survey (HerMES 18 ), taken with the Spectral and Photometric Imaging Receiver (SPIRE 19 ) onboard the Herschel Space Observatory 20 , during the Science Demonstration Phase (SDP) of Herschel. The data are composed of a wide 218 ′ by 218 ′ area in the Lockman Hole complemented by a narrow, but very deep (30 repeated scans), map of the Great Observatories Origins Deep Survey (GOODS) North field covering 30 ′ by 30 ′ . These fields have been very well studied at other wavelengths and they are known to have a low Galactic dust density, making it easier to distinguish the extragalactic component we wish to study. The observing time to complete each of the two fields was about 13.5 hours, observing simultaneously at 250, 350, and 500 µm.To limit the influence of a few ...

We study the production rate of ionizing photons of a sample of 588 Hα emitters (HAEs) and 160 Lyman-α emitters (LAEs) at z = 2.2 in the COSMOS field in order to assess the implied emissivity from galaxies, based on their UV luminosity. By exploring the rest-frame Lyman Continuum (LyC) with GALEX/N U V data, we find f esc < 2.8 (6.4)% through median (mean) stacking. By combining the Hα luminosity density with IGM emissivity measurements from absorption studies, we find a globally averaged f esc of 5.9 +14.5 −4.2 % at z = 2.2 if we assume HAEs are the only source of ionizing photons. We find similarly low values of the global f esc at z ≈ 3 − 5, also ruling out a high f esc at z < 5. These low escape fractions allow us to measure ξ ion , the number of produced ionizing photons per unit UV luminosity, and investigate how this depends on galaxy properties. We find a typical ξ ion ≈ 10 24.77±0.04 Hz erg −1for HAEs and ξ ion ≈ 10 25.14±0.09 Hz erg −1 for LAEs. LAEs and low mass HAEs at z = 2.2 show similar values of ξ ion as typically assumed in the reionization era, while the typical HAE is three times less ionizing. Due to an increasing ξ ion with increasing EW(Hα), ξ ion likely increases with redshift. This evolution alone is fully in line with the observed evolution of ξ ion between z ≈ 2 − 5, indicating a typical value of ξ ion ≈ 10 25.4 Hz erg −1 in the reionization era.