Examining a sample of massive galaxies at 1:4 < z < 2:5 with K Vega < 22 from GOODS, we compare photometry from Spitzer at mid-and far-IR to submillimeter, radio, and rest-frame UV wavelengths, to test the agreement between different tracers of star formation rates (SFRs) and to explore the implications for galaxy assembly. For z $ 2 galaxies with moderate luminosities (L 8 m < 10 11 L ), we find that the SFR can be estimated consistently from the multiwavelength data based on local luminosity correlations. However, 20%Y30% of massive galaxies, and nearly all those with L 8 m > 10 11 L , show a mid-IR excess that is likely due to the presence of obscured active nuclei, as shown in a companion paper. There is a tight and roughly linear correlation between stellar mass and SFR for 24 mYdetected galaxies. For a given mass, the SFR at z ¼ 2 was larger by a factor of $4 and $30 relative to that in star-forming galaxies at z ¼ 1 and 0, respectively. Typical ultraluminous infrared galaxies (ULIRGs) at z ¼ 2 are relatively ''transparent'' to ultraviolet light, and their activity is long lived (k400 Myr), unlike that in local ULIRGs and high-redshift submillimeter-selected galaxies. ULIRGs are the common mode of star formation in massive galaxies at z ¼ 2, and the high duty cycle suggests that major mergers are not the dominant trigger for this activity. Current galaxy formation models underpredict the normalization of the mass-SFR correlation by about a factor of 4 and the space density of ULIRGs by an order of magnitude but give better agreement for z > 1:4 quiescent galaxies.

Aims. We study the relationship between the local environment of galaxies and their star formation rate (SFR) in the Great Observatories Origins Deep Survey, GOODS, at z ∼ 1. Methods. We use ultradeep imaging at 24 µm with the MIPS camera onboard Spitzer to determine the contribution of obscured light to the SFR of galaxies over the redshift range 0.8 ≤ z ≤ 1.2. Accurate galaxy densities are measured thanks to the large sample of ∼1200 spectroscopic redshifts with high (∼70%) spectroscopic completeness. Morphology and stellar masses are derived from deep HST-ACS imaging, supplemented by ground based imaging programs and photometry from the IRAC camera onboard Spitzer. Results. We show that the star formation-density relation observed locally was reversed at z ∼ 1: the average SFR of an individual galaxy increased with local galaxy density when the universe was less than half its present age. Hierarchical galaxy formation models (simulated lightcones from the Millennium model) predicted such a reversal to occur only at earlier epochs (z > 2) and at a lower level. We present a remarkable structure at z ∼ 1.016, containing X-ray traced galaxy concentrations, which will eventually merge into a Virgo-like cluster. This structure illustrates how the individual SFR of galaxies increases with density and shows that it is the ∼1−2 Mpc scale that affects most the star formation in galaxies at z ∼ 1. The SFR of z ∼ 1 galaxies is found to correlate with stellar mass suggesting that mass plays a role in the observed star formation-density trend. However the specific SFR (=SFR/M ) decreases with stellar mass while it increases with galaxy density, which implies that the environment does directly affect the star formation activity of galaxies. Major mergers do not appear to be the unique or even major cause for this effect since nearly half (46%) of the luminous infrared galaxies (LIRGs) at z ∼ 1 present the HST-ACS morphology of spirals, while only a third present a clear signature of major mergers. The remaining galaxies are divided into compact (9%) and irregular (14%) galaxies. Moreover, the specific SFR of major mergers is only marginally stronger than that of spirals. Conclusions. These findings constrain the influence of the growth of large-scale structures on the star formation history of galaxies. Reproducing the SFR-density relation at z ∼ 1 is a new challenge for models, requiring a correct balance between mass assembly through mergers and in-situ star formation at early epochs.

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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.

We present an analysis of the deepest Herschel images in four major extragalactic fields GOODS-North, GOODS-South, UDS, and COSMOS obtained within the GOODS-Herschel and CANDELS-Herschel key programs. The star formation picture provided by a total of 10 497 individual far-infrared detections is supplemented by the stacking analysis of a mass complete sample of 62 361 starforming galaxies from the Hubble Space Telescope (HST) H band-selected catalogs of the CANDELS survey and from two deep ground-based K s band-selected catalogs in the GOODS-North and the COSMOS-wide field to obtain one of the most accurate and unbiased understanding to date of the stellar mass growth over the cosmic history. We show, for the first time, that stacking also provides a powerful tool to determine the dispersion of a physical correlation and describe our method called "scatter stacking", which may be easily generalized to other experiments. The combination of direct UV and far-infrared UV-reprocessed light provides a complete census on the star formation rates (SFRs), allowing us to demonstrate that galaxies at z = 4 to 0 of all stellar masses (M * ) follow a universal scaling law, the so-called main sequence of star-forming galaxies. We find a universal close-to-linear slope of the log 10 (SFR)-log 10 (M * ) relation, with evidence for a flattening of the main sequence at high masses (log 10 (M * /M ) > 10.5) that becomes less prominent with increasing redshift and almost vanishes by z 2. This flattening may be due to the parallel stellar growth of quiescent bulges in star-forming galaxies, which mostly happens over the same redshift range. Within the main sequence, we measure a nonvarying SFR dispersion of 0.3 dex: at a fixed redshift and stellar mass, about 68% of star-forming galaxies form stars at a universal rate within a factor 2. The specific SFR (sSFR = SFR/M * ) of star-forming galaxies is found to continuously increase from z = 0 to 4. Finally we discuss the implications of our findings on the cosmic SFR history and on the origin of present-day stars: more than two-thirds of present-day stars must have formed in a regime dominated by the "main sequence" mode. As a consequence we conclude that, although omnipresent in the distant Universe, galaxy mergers had little impact in shaping the global star formation history over the last 12.5 billion years.

We present evidence for very high gas fractions and extended molecular gas reservoirs in normal, near-infrared selected (BzK) galaxies at z∼1.5. Our results are based on multi-configuration CO[2-1] observations obtained at the IRAM Plateau de Bure Interferometer. All six star forming galaxies observed were detected at high significance. High spatial resolution observations resolve the CO emission in four of them, implying sizes of the gas reservoirs of order of 6-11 kpc and suggesting the presence of ordered rotation. The galaxies have UV morphologies consistent with clumpy, unstable disks, and UV sizes that are consistent with those measured in CO. The star formation efficiencies are homogeneously low within the sample and similar to those of local spirals -the resulting gas depletion times are ∼ 0.5 Gyr, much higher than what is seen in high-z submm galaxies and quasars. The CO luminosities can be predicted to within 0.15 dex from the observed star formation rates and stellar masses, implying a tight correlation of the gas mass with these quantities. We use new dynamical models of clumpy disk galaxies to derive dynamical masses for our sample. These models are able to reproduce the peculiar spectral line shapes of the CO emission. After accounting for the stellar and dark matter masses we derive molecular gas reservoirs with masses of 0.4-1.2×10 11 M ⊙ . The implied conversion (CO luminosity-to-gas mass) factor is very high: α CO = 3.6 ± 0.8, consistent with a Galactic conversion factor but four times higher than that of local ultra-luminous IR galaxies that is typically used for high-redshift objects. The gas mass in these galaxies is comparable to or larger than the stellar mass, and the gas accounts for an impressive 50-65% of the baryons within the galaxies' half light radii. We are thus witnessing truly gasdominated galaxies at z ∼ 1.5, a finding that explains the high specific SFRs observed for z > 1 galaxies. The BzK galaxies can be viewed as scaled-up versions of local disk galaxies, with low efficiency star formation taking place inside extended, low excitation gas disks. These galaxies are markedly different than local ULIRGs and high-z submm galaxies and quasars, where higher excitation and more compact gas is found.

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 present evidence that bona fide disks and starburst systems occupy distinct regions in the gas mass versus star formation (SF) rate plane, both for the integrated quantities and for the respective surface densities. This result is based on CO observations of galaxy populations at low and high redshifts, and on the current consensus for the CO luminosity to gas mass conversion factors. The data suggest the existence of two different star formation regimes: a long-lasting mode for disks and a more rapid mode for starbursts, the latter probably occurring during major mergers or in dense nuclear SF regions. Both modes are observable over a large range of SF rates. The detection of CO emission from distant near-IR selected galaxies reveals such bimodal behavior for the first time, as they allow us to probe gas in disk galaxies with much higher SF rates than are seen locally. The different regimes can potentially be interpreted as the effect of a top-heavy IMF in starbursts. However, we favor a different physical origin related to the fraction of molecular gas in dense clouds. The IR luminosity to gas mass ratio (i.e., the SF efficiency) appears to be inversely proportional to the dynamical (rotation) timescale. Only when accounting for the dynamical timescale, a universal SF law is obtained, suggesting a direct link between global galaxy properties and the local SF rate.