The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) is designed to document the first third of galactic evolution, over the approximate redshift (z) range 8-1.5. It will image >250,000 distant galaxies using three separate cameras on the Hubble Space Telescope, from the mid-ultraviolet to the near-infrared, and will find and measure Type Ia supernovae at z > 1.5 to test their accuracy as standardizable candles for cosmology. Five premier multi-wavelength sky regions are selected, each with extensive ancillary data. The use of five widely separated fields mitigates cosmic variance and yields statistically robust and complete samples of galaxies down to a stellar mass of 10 9 M to z ≈ 2, reaching the knee of the ultraviolet luminosity function of galaxies to z ≈ 8. The survey covers approximately 800 arcmin 2 and is divided into two parts. The CANDELS/Deep survey (5σ point-source limit H = 27.7 mag) covers ∼125 arcmin 2 within Great Observatories Origins Deep Survey (GOODS)-N and GOODS-S. The CANDELS/Wide survey includes GOODS and three additional fields (Extended Groth Strip, COSMOS, and Ultra-deep Survey) and covers the full area to a 5σ pointsource limit of H 27.0 mag. Together with the Hubble Ultra Deep Fields, the strategy creates a three-tiered "wedding-cake" approach that has proven efficient for extragalactic surveys. Data from the survey are nonproprietary and are useful for a wide variety of science investigations. In this paper, we describe the basic motivations for the survey, the CANDELS team science goals and the resulting observational requirements, the field selection and geometry, and the observing design. The Hubble data processing and products are described in a companion paper.

A major problem in extragalactic astronomy is the inability to distinguish in a robust, physical, and model independent way how galaxy populations are related to each other and to their formation histories. A similar, but distinct, and also long standing question is whether the structural appearances of galaxies, as seen through their stellar light distributions, contain enough physical information to offer this classification. We argue through the use of 240 images of nearby galaxies that three model independent parameters measured on a single galaxy image reveal its major ongoing and past formation modes, and can be used as a robust classification system. These parameters quantitatively measure: the concentration (C), asymmetry (A) and clumpiness (S) of a galaxy's stellar light distribution. When combined into a three dimensional 'CAS' volume all major classes of galaxies in various phases of evolution are cleanly distinguished. We argue that these three parameters correlate with important modes of galaxy evolution: star formation and major merging activity. This is argued through the strong correlation of Hα equivalent width and broad band colors with the clumpiness parameter, the uniquely large asymmetries of 66 galaxies undergoing mergers, and the correlation of bulge to total light ratios, and stellar masses, with the concentration index. As an obvious goal is to use this system at high redshifts to trace evolution, we demonstrate that these parameters can be measured, within a reasonable and quantifiable uncertainty, with available data out to z ∼ 3 using the Hubble Space Telescope GOODS ACS and Hubble Deep Field images.

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.

We analyze star formation (SF) as a function of stellar mass (M ⋆ ) and redshift z in the All Wavelength Extended Groth Strip International Survey (AEGIS). For 2905 field galaxies, complete to 10 10 (10 10.8 )M ⊙ at z < 0.7(1), with Keck spectroscopic redshifts out to z = 1.1, we compile SF rates (SFR) from emission lines, GALEX, and Spitzer MIPS 24µm photometry, optical-NIR M ⋆ measurements, and HST morphologies. Galaxies with reliable signs of SF form a distinct "main sequence (MS)", with a limited range of SFR at a given M ⋆ and z (1σ ±0.3 dex), and log(SFR) approximately proportional to log(M ⋆ ). The range of log(SFR) remains constant to z > 1, while the MS as a whole moves to higher SFR as z increases. The range of SFR along the MS constrains the amplitude of episodic variations of SF, and the effect of mergers on SFR. Typical galaxies spend ∼ 67(95)% of their lifetime since z = 1 within a factor of 2(4) of their average SFR at a given M ⋆ and z. The dominant mode of the evolution of SF since z ∼ 1 is apparently a gradual decline of the average SFR in most individual galaxies, not a decreasing frequency of starburst episodes, or a decreasing factor by which SFR are enhanced in starbursts. LIRGs at z ∼ 1 seem to mostly reflect the high SFR typical for massive galaxies at that epoch. The smooth MS may reflect that the same set of few physical processes governs star formation prior to additional quenching processes. A gradual process like gas exhaustion may play a dominant role.

We characterize the mass-dependent evolution in a large sample of more than 8,000 galaxies using spectroscopic redshifts drawn from the DEEP2 Galaxy Redshift Survey in the range 0.4 < z < 1.4 and stellar masses calculated from K-band photometry obtained at Palomar Observatory. This sample spans more than 1.5 square degrees in four independent fields. Using restframe (U − B) color and [OII] equivalent widths, we distinguish star-forming from passive populations in order to explore the nature of "downsizing"-a pattern in which the sites of active star formation shift from high mass galaxies at early times to lower mass systems at later epochs. Over the redshift range probed, we identify a mass limit, M Q , above which star formation appears to be quenched. The physical mechanisms responsible for downsizing can thus be empirically quantified by charting the evolution in this threshold mass. We find that M Q decreases with time by a factor of ≈3 across the redshift range sampled according to M Q ∝ (1 + z) 3.5 . We demonstrate that this behavior is quite robust to the use of various indicators of star formation activity, including morphological type. To further constrain possible quenching mechanisms, we investigate how this downsizing signal depends on local galaxy environment using the projected 3 rd -nearest-neighbor statistic D p,3 which is particularly well-suited for large spectroscopic samples. For the majority of galaxies in regions near the median density, there is no significant correlation between downsizing and environment. However, a trend is observed in the comparison between more extreme environments that are more than 3 times overdense or underdense relative to the median. Here, we find that downsizing is accelerated in overdense regions which host higher numbers of massive, early-type galaxies and fewer late-types as compared to the underdense regions. Our results significantly constrain recent suggestions for the origin of downsizing and indicate that the process for quenching star formation must, primarily, be internally driven. By quantifying both the time and density dependence of downsizing, our survey provides a valuable benchmark for galaxy models incorporating baryon physics.

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This paper presents direct evidence for hierarchical galaxy assembly out to redshifts z ∼ 3. We identify major mergers using the model-independent CAS (concentration, asymmetry, clumpiness) physical morphological system on galaxies detected, and photometrically selected, in the WFPC2 and NICMOS Hubble Deep Field North. We specifically use the asymmetric distributions of rest-frame optical light measured through the asymmetry parameter (A) to determine the fraction of galaxies undergoing major mergers as a function of redshift (z), stellar mass (M ⋆ ), and absolute magnitude (M B ). We find that the fraction of galaxies consistent with undergoing a major merger increases with redshift for all galaxies, but most significantly, at 5 -10 σ confidence, for the most luminous and massive systems. The highest merger fractions we find are 40% -50% for galaxies with M B < −21, or M ⋆ > 10 10 M ⊙ at z > 2.5, i.e., objects identified as Lyman-break galaxies. Using these results, we model the merger fraction evolution in the form: f m (A,M ⋆ ,M B , z) = f 0 × (1 + z) mA . We find m A values ∼ 4 − 6 for the most luminous and massive galaxies, while lower mass and less luminous galaxies have smaller m A values. We use these merger fractions, combined with merger time scales calculated from N-body simulations, to derive galaxy merger rates to z ∼ 3. We also use stellar masses of HDF-N galaxies to determine the mass accretion rate of field galaxies involved in major mergers. We find an average stellar mass accretion rate ofṀ G ∼ 4 × 10 8 M ⊙ Gyr −1 galaxy −1 at z ∼ 1 for galaxies with stellar masses M ⋆ > 10 9 M ⊙ . This accretion rate changes with redshift as:Ṁ G = 1.6 × 10 8 (1 + z) 0.99±0.32 M ⊙ Gyr −1 galaxy −1 . We also find that the fraction of stellar mass density in galaxies involved in major mergers increases with redshift, with a peak mass fraction ∼ 0.5 for the brightest, M B < −21, and most massive, M ⋆ > 10 10 M ⊙ , systems near z ∼ 2.5. By comparing merger fractions predicted in Cold Dark Matter semi-analytic models with our results we find a reasonably good agreement for the largest and brightest systems, although we find more low-mass galaxy mergers at lower redshifts than what these models predict.

The extragalactic background light (EBL) is of fundamental importance both for understanding the entire process of galaxy evolution and for γ‐ray astronomy, but the overall spectrum of the EBL between 0.1 and 1000 μm has never been determined directly from galaxy spectral energy distribution (SED) observations over a wide redshift range. The evolving, overall spectrum of the EBL is derived here utilizing a novel method based on observations only. This is achieved from the observed evolution of the rest‐frame K‐band galaxy luminosity function up to redshift 4, combined with a determination of galaxy‐SED‐type fractions. These are based on fitting Spitzer Wide‐Area Infrared Extragalactic Survey (SWIRE) templates to a multiwavelength sample of about 6000 galaxies in the redshift range from 0.2 to 1 from the All‐wavelength Extended Groth Strip International Survey (AEGIS). The changing fractions of quiescent galaxies, star‐forming galaxies, starburst galaxies and active galactic nucleus (AGN) galaxies in that redshift range are estimated, and two alternative extrapolations of SED types to higher redshifts are considered. This allows calculation of the evolution of the luminosity densities from the ultraviolet (UV) to the infrared (IR), the evolving star formation rate density of the Universe, the evolving contribution to the bolometric EBL from the different galaxy populations including AGN galaxies and the buildup of the EBL. Our EBL calculations are compared with those from a semi‐analytic model, another observationally based model and observational data. The EBL uncertainties in our modelling based directly on the data are quantified, and their consequences for attenuation of very‐high‐energy γ‐rays due to pair production on the EBL are discussed. It is concluded that the EBL is well constrained from the UV to the mid‐IR, but independent efforts from IR and γ‐ray astronomy are needed in order to reduce the uncertainties in the far‐IR.