We investigate the stellar populations of Ly alpha emitters (LAEs) at z = 3.1 and 3.7 in 0.65 deg2 of the Subaru/XMM-Newton Deep Field, based on rest-frame ultraviolet-to-optical photometry obtained from the Subaru/XMM-Newton Deep Survey, the UKIRT Infrared Deep Sky Survey/Ultra Deep Survey (UKIDSS/UDS), and the Spitzer legacy survey of the UKIDSS/UDS. Among a total of 302 LAEs (224 for z = 3.1 and 78 for z = 3.7), only 11 are detected in the K band, i.e. brighter than K(3 Sigma) = 24.1 mag. Eight of the 11 K-detected LAEs are spectroscopically confirmed. In our stellar population analysis, we treat K-detected objects individually, while K-undetected objects are stacked at each redshift. We find that the K-undetected objects, which should closely represent the LAE population as a whole, have low stellar masses of similar to 108-108.5 M(circle dot), modest star formation rates (SFRs) of 1-100 M(circle dot) yr-1, and modest dust extinction of E(B - V)(star) < 0.2. The K-detected objects are massive, M(star) similar to 109-1010.5 M(circle dot), and have significant dust extinction with a median of E(B - V)(star) similar or equal to 0.3. Four K-detected objects with the reddest spectral energy distributions, two of which are spectroscopically confirmed, are heavily obscured with E(B - V)(star) similar to 0.65, and their continua resemble those of some local ultraluminous infrared galaxies (ULIRGs). Interestingly, they have large Ly alpha equivalent widths similar or equal to 70-250 A. If these four are excluded, our sample has a weak anticorrelation between Ly alpha equivalent width and M(star). We compare the stellar masses and the specific SFRs (sSFRs) of LAEs with those of Lyman-break galaxies (LBGs), distant red galaxies, submillimetre galaxies, and I- or K-selected galaxies with photometric redshifts of z(phot) similar to 3. We find that the LAE population is the least massive among all the galaxy populations in question, but with relatively high sSFRs, while near-infrared (NIR)-detected LAEs have M(star) and sSFR similar to LBGs. Our reddest four LAEs have very high sSFRs in spite of large M(star), thus occupying a unique region in the M(star) versus sSFR space.

Spitzer spectroscopy has revealed that similar or equal to 80% of submm galaxies (SMGs) are starburst (SB)-dominated in the mid-infrared. Here we focus on the remaining similar or equal to 20% that show signs of harboring powerful active galactic nuclei (AGNs). We have obtained Spitzer-InfraRed Spectrograph spectroscopy of a sample of eight SMGs that are candidates for harboring powerful AGNs on the basis of IRAC color selection (S(8 mu m)/S(4.5 mu m) > 2, i.e., likely power-law mid-infrared spectral energy distributions). SMGs with an AGN dominating (greater than or similar to 50%) their mid-infrared emission could represent the "missing link" sources in an evolutionary sequence involving a major merger. First of all, we detect polycyclic aromatic hydrocarbon (PAH) features in all of the SMGs, indicating redshifts from 2.5 to 3.4, demonstrating the power of the mid-infrared to determine redshifts for these optically faint dusty galaxies. Second, we see signs of both star formation (from the PAH features) and AGN activity (from continuum emission) in our sample: 62% of the sample are AGN-dominated in the mid-infrared with a median AGN content of 56%, compared with <30% on average for typical SMGs, revealing that our IRAC color selection has successfully singled out sources with proportionately more AGN emission than typical SB-dominated SMGs. However, we find that only about 10% of these AGNs dominate the bolometric emission of the SMG when the results are extrapolated to longer infrared wavelengths, implying that AGNs are not a significant power source to the SMG population overall, even when there is evidence in the mid-infrared for substantial AGN activity. When existing samples of mid-infrared AGN-dominated SMGs are considered, we find that S(8 mu m)/S(4.5 mu m) > 1.65 works well at selecting mid-infrared energetically dominant AGNs in SMGs, implying a duty cycle of similar to 15% if all SMGs go through a subsequent mid-infrared AGN-dominated phase in the proposed evolutionary sequence.

We report the discovery of a galaxy cluster at z = 1.62 located in the Spitzer Wide-Area Infrared Extragalactic survey XMM-LSS field. This structure was selected solely as an overdensity of galaxies with red Spitzer/Infrared Array Camera colors, satisfying ([3.6] - [4.5]) AB > -0.1 mag. Photometric redshifts derived from the Subaru XMM Deep Survey (BV iz bands), the UKIRT Infrared Deep Survey-Ultra-Deep Survey (UKIDSS-UDS, JK bands), and from the Spitzer Public UDS survey (3.6-8.0 mu m) show that this cluster corresponds to a surface density of galaxies at z approximate to 1.6 that is >20 sigma above the mean at this redshift. We obtained optical spectroscopic observations of galaxies in the cluster region using IMACS on the Magellan telescope. We measured redshifts for seven galaxies in the range z = 1.62-1.63 within 2.8 arcmin (<1.4 Mpc) of the astrometric center of the cluster. A posteriori analysis of the XMM data in this field reveal a weak (4 sigma) detection in the [0.5-2 keV] band compatible with the expected thermal emission from such a cluster. The color-magnitude diagram of the galaxies in this cluster shows a prominent red sequence, dominated by a population of red galaxies with (z - J) > 1.7 mag. The photometric-redshift probability distributions for the red galaxies are strongly peaked at z = 1.62, coincident with the spectroscopically confirmed galaxies. The rest-frame (U - B) color and scatter of galaxies on the red sequence are consistent with a mean luminosity-weighted age of 1.2 +/- 0.1 Gyr, yielding a formation redshift (z(f)) over bar = 2.35 +/- 0.10, and corresponding to the last significant star formation period in these galaxies.

We measure the rest-frame colors (dust-corrected), infrared luminosities, star formation rates, and stellar masses of 92 galaxies in a Spitzer-selected cluster at z = 1.62. By fitting spectral energy distributions (SEDs) to 10-band photometry (0.4 mu m < lambda(obs) < 8 mu m) and measuring 24 mu m fluxes for the 12 spectroscopically confirmed and 80 photometrically selected members, we discover an exceptionally high level of star formation in the cluster core of similar to 1700 M(circle dot) yr(-1) Mpc(-2). The cluster galaxies define a strong blue sequence in (U-V) color and span a range in color. We identify 17 members with L(IR) > 10(11) L(circle dot), and these IR luminous members follow the same trend of increasing star formation with stellar mass that is observed in the field at z similar to 2. Using rates derived from both the 24 mu m imaging and SED fitting, we find that the relative fraction of star-forming members triples from the lowest to highest galaxy density regions; e.g., the IR luminous fraction increases from similar to 8% at Sigma similar to 10 gal Mpc(-2) to similar to 25% at Sigma greater than or similar to 100 gal Mpc(-2). The observed increase is a reversal of the well-documented trend at z < 1 and signals that we have reached the epoch when massive cluster galaxies are still forming a substantial fraction of their stars.

Gravitational lensing is a powerful astrophysical and cosmological probe and is particularly valuable at submillimeter wavelengths for the study of the statistical and individual properties of dusty star-forming galaxies. However, the identification of gravitational lenses is often time-intensive, involving the sifting of large volumes of imaging or spectroscopic data to find few candidates. We used early data from the Herschel Astrophysical Terahertz Large Area Survey to demonstrate that wide-area submillimeter surveys can simply and easily detect strong gravitational lensing events, with close to 100% efficiency.

We investigate the stellar populations of Ly alpha emitters (LAEs) at z = 5.7 and 6.6 in a 0.65 deg(2) sky of the Subaru/XMM-Newton Deep Survey (SXDS) Field, using deep images taken with the Subaru/Suprime-Cam, United Kingdom Infrared Telescope/Wide Field Infrared Camera, and Spitzer/Infrared Array Camera (IRAC). We produce stacked multiband images at each redshift from 165 (z = 5.7) and 91 (z = 6.6) IRAC-undetected objects to derive typical spectral energy distributions (SEDs) of z similar to 6-7 LAEs for the first time. The stacked LAEs have as blue UV continua as the Hubble Space Telescope (HST)/Wide Field Camera 3 (WFC3) z-dropout galaxies of similar M-UV, with a spectral slope beta similar to -3, but at the same time they have red UV-to-optical colors with detection in the 3.6 mu m band. Using SED fitting we find that the stacked LAEs have low stellar masses of similar to(3-10) x 10(7) M-circle dot, very young ages of similar to 1-3 Myr, negligible dust extinction, and strong nebular emission from the ionized interstellar medium, although the z = 6.6 object is fitted similarly well with high-mass models without nebular emission; inclusion of nebular emission reproduces the red UV-to-optical colors while keeping the UV colors sufficiently blue. We infer that typical LAEs at z similar to 6-7 are building blocks of galaxies seen at lower redshifts. We find a tentative decrease in the Lya escape fraction from z = 5.7 to 6.6, which may imply an increase in the intergalactic medium neutral fraction. From the minimum contribution of nebular emission required to fit the observed SEDs, we place an upper limit on the escape fraction of ionizing photons of f(esc)(ion) similar to 0.6 at z = 5.7 and similar to 0.9 at z = 6.6. We also compare the stellar populations of our LAEs with those of stacked HST/WFC3 z-dropout galaxies.

We present a technique to identify optical counterparts of 250-mu m-selected sources from the Herschel-ATLAS survey. Of the 6621 250 mu m > 32-mJy sources in our science demonstration catalogue we find that similar to 60 per cent have counterparts brighter than r = 22.4 mag in the Sloan Digital Sky Survey. Applying a likelihood ratio technique we are able to identify 2423 of the counterparts with a reliability R > 0.8. This is approximately 37 per cent of the full 250-mu m catalogue. We have estimated photometric redshifts for each of these 2423 reliable counterparts, while 1099 also have spectroscopic redshifts collated from several different sources, including the GAMA survey. We estimate the completeness of identifying counterparts as a function of redshift, and present evidence that 250-mu m-selected Herschel-ATLAS galaxies have a bimodal redshift distribution. Those with reliable optical identifications have a redshift distribution peaking at z approximate to 0.25 +/- 0.05, while submillimetre colours suggest that a significant fraction with no counterpart above the r-band limit have z > 1. We also suggest a method for selecting populations of strongly lensed high-redshift galaxies. Our identifications are matched to UV-NIR photometry from the GAMA survey, and these data are available as part of the Herschel-ATLAS public data release.

We identify an abundant population of extreme emission-line galaxies (EELGs) at redshift z similar to 1.7 in the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey imaging from Hubble Space Telescope/Wide Field Camera 3 (HST/WFC3). Sixty-nine EELG candidates are selected by the large contribution of exceptionally bright emission lines to their near-infrared broadband magnitudes. Supported by spectroscopic confirmation of strong [OIII] emission lines-with rest-frame equivalent widths similar to 1000 angstrom-in the four candidates that have HST/WFC3 grism observations, we conclude that these objects are galaxies with similar to 10(8) M-circle dot in stellar mass, undergoing an enormous starburst phase with M-*/(M) over dot(*) of only similar to 15 Myr. These bursts may cause outflows that are strong enough to produce cored dark matter profiles in low-mass galaxies. The individual star formation rates and the comoving number density (3.7 x 10(-4) Mpc(-3)) can produce in similar to 4 Gyr much of the stellar mass density that is presently contained in 10(8)-10(9) M-circle dot dwarf galaxies. Therefore, our observations provide a strong indication that many or even most of the stars in present-day dwarf galaxies formed in strong, short-lived bursts, mostly at z > 1.

This paper describes the Hubble Space Telescope imaging data products and data reduction procedures for the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). This survey is designed to document the evolution of galaxies and black holes at z approximate to 1.5-8, and to study Type Ia supernovae at z > 1.5. Five premier multi-wavelength sky regions are selected, each with extensive multi-wavelength observations. The primary CANDELS data consist of imaging obtained in the Wide Field Camera 3 infrared channel (WFC3/IR) and the WFC3 ultraviolet/optical channel, along with the Advanced Camera for Surveys (ACS). The CANDELS/Deep survey covers similar to 125 arcmin(2) within GOODS-N and GOODS-S, while the remainder consists of the CANDELS/Wide survey, achieving a total of similar to 800 arcmin(2) across GOODS and three additional fields (Extended Groth Strip, COSMOS, and Ultra-Deep Survey). 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 charge transfer efficiency degradation for ACS, removal of electronic bias-striping present in ACS data after Servicing Mission 4, 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.

Using Hubble Space Telescope/WFC3 imaging taken as part of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey, we examine the role that major galaxy mergers play in triggering active galactic nucleus (AGN) activity at z similar to 2. Our sample consists of 72 moderate-luminosity (L-X similar to 10(42-44) erg s(-1)) AGNs at 1.5 < z < 2.5 that are selected using the 4 Ms Chandra observations in the Chandra Deep Field South, the deepest X-ray observations to date. Employing visual classifications, we have analyzed the rest-frame optical morphologies of the AGN host galaxies and compared them to a mass-matched control sample of 216 non-active galaxies at the same redshift. We find that most of the AGNs reside in disk galaxies (51.4(-5.9)(+5.8)%), while a smaller percentage are found in spheroids (27.8(-4.6)(+5.8)%). Roughly 16.7(-3.5)(+5.3)% of the AGN hosts have highly disturbed morphologies and appear to be involved in a major merger or interaction, while most of the hosts (55.6(-5.9)(+5.6)%) appear relatively relaxed and undisturbed. These fractions are statistically consistent with the fraction of control galaxies that show similar morphological disturbances. These results suggest that the hosts of moderate-luminosity AGNs are no more likely to be involved in an ongoing merger or interaction relative to non-active galaxies of similar mass at z similar to 2. The high disk fraction observed among the AGN hosts also appears to be at odds with predictions that merger-driven accretion should be the dominant AGN fueling mode at z similar to 2, even at moderate X-ray luminosities. Although we cannot rule out that minor mergers are responsible for triggering these systems, the presence of a large population of relatively undisturbed disk-like hosts suggests that the stochastic accretion of gas plays a greater role in fueling AGN activity at z similar to 2 than previously thought.