We describe the details of the Hubble Space Telescope (HST ) Advanced Camera for Surveys (ACS) Wide Field Channel ( WFC) observations of the COSMOS field, including the data calibration and processing procedures. We obtained a total of 583 orbits of HST ACS/ WFC imaging in the F814W filter, covering a field that is 1.64 deg 2 in area, the largest contiguous field ever imaged with HST. The median exposure depth across the field is 2028 s (one HST orbit), achieving a limiting point-source depth AB( F814W ) ¼ 27:2 (5 ). We also present details of the astrometric image registration and distortion removal and image combination using MultiDrizzle, motivating the choice of our final pixel scale (30 mas pixel À1 ), based on the requirements for weak-lensing science. The final set of images are publicly available through the archive sites at IPAC and STScI, along with further documentation on how they were produced.

Ultraviolet and optical narrow and broad band images of NGC5253 obtained with the Hubble Space Telescope Wide Field and Planetary Camera 2 are used to derive the properties of the dust distribution and the recent star formation history of this metal-poor dwarf galaxy. Corrections for the effects of dust are important in the center of NGC5253: dust reddening is markedly inhomogeneous across the galaxy's central 20 ′′ region. One of the most obscured regions coincides with the region of highest star formation activity in the galaxy; clouds of more than 9 magnitudes of optical depth at V enshroud a 2.5 Myr old stellar cluster in this area. The ages of the bright clusters in the center of the galaxy are anti-correlated with the amount of dust obscuration the cluster suffers. This result agrees with the expectation that young stellar associations are located in heavily obscured regions, but after only 2-3 Myr they remove/emerge from the parental dust cloud and become almost extinction-free. On average, the

Classi cation of galaxy spectral energy distributions in terms of orthogonal basis functions provides an objective means of estimating the number of signi cant spectral components that comprise a particular galaxy type. We apply the Karhunen-Lo eve transform to derive a spectral eigensystem from a sample of ten galaxy spectral energy distributions. These spectra cover a wavelength range of 1200 A to 1 m and galaxy morphologies from elliptical to starburst. We nd that the distribution of spectral types can be fully described by the rst two eigenvectors (or eigenspectra). The derived eigenbasis is a ected by the normalization of the original spectral energy distributions. We investigate di erent normalization and weighting schemes, including weighting to the same bolometric magnitude and weighting by the observed distribution of morphological types. Projecting the spectral energy distributions on to their eigenspectra we nd that the coe cients de ne a simple spectral classi cation scheme. The galaxy spectral types can then be described in terms of a one parameter family (the angle in the plane of the rst two eigenvectors). We nd a strong correlation in the mean between our spectral classi cations and those determined from published morphological classi cations.

We present the first identification of large-scale structures ( LSSs) at z < 1:1 in the Cosmic Evolution Survey (COSMOS). The structures are identified from adaptive smoothing of galaxy counts in the pseudo-3D space ( , , z) using the COSMOS photometric redshift catalog. The technique is tested on a simulation including galaxies distributed in model clusters and a field galaxy population-recovering structures on all scales from 1 0 to 20 0 without a priori assumptions for the structure size or density profile. The COSMOS photometric redshift catalog yields a sample of 1:5 ; 10 5 galaxies with redshift accuracy, Áz FWHM /(1 þ z) 0:1 at z < 1:1 down to I AB 25 mag. Using this sample of galaxies, we identify 42 LSSs and clusters. Projected surface-density maps for the structures indicate multiple peaks and internal structure in many of the most massive LSSs. The stellar masses (determined from the galactic SEDs) for the LSSs range from M Ã $ 10 11 up to $3 ; 10 13 M . Five LSSs have total stellar masses exceeding 10 13 M . (Total masses including nonstellar baryons and dark matter are expected to be $50Y100 times greater.) The derived mass function for the LSSs is consistent (within the expected Poisson and cosmic variances) with those derived from optical and X-ray studies at lower redshift. To characterize structure evolution and for comparison with simulations, we compute a new statistic: the area filling factor as a function of the overdensity value compared to the mean at surface overdensity ( f A AE/AE(z) Â Ã ). The observationally determined f A has less than 1% of the surface area (in each redshift slice) with overdensities exceeding 10:1, and evolution to higher overdensities is seen at later epochs (lower z); both characteristics are in good agreement with what we find using similar processing on the Millennium Simulation. Although similar variations in the filling factors as a function of overdensity and redshift are seen in the observations and simulations, we do find that the observed distributions reach higher overdensities than the simulation, perhaps indicating overmerging in the simulation. All of the LSSs show a dramatic preference for earlier SED type galaxies in the denser regions of the structures, independent of redshift. The SED types in the central 1 and 1Y5 Mpc regions of each structure average about one SED type earlier than the mean type at the same redshift, corresponding to a stellar population age difference of $2Y4 Gyr at z ¼ 0:3Y1. We also investigate the evolution of key galactic propertiesmass, luminosity, SED, and star formation rate (SFR)-with redshift and environmental density as derived from overdensities in the full pseudo-3D cube. Both the maturity of the stellar populations and the ''downsizing'' of star formation in galaxies vary strongly with redshift (epoch) and environment. For a very broad mass range (10 10 Y10 12 M ), we find that galaxies in dense environments tend to be older; this is not just restricted to the most massive galaxies.1 Based on observations wi...

Our Hubble Space Telescope/Near‐Infrared Camera and Multi‐Object Spectrometer (HST/NICMOS) Paschen α survey of the Galactic Centre, first introduced by Wang et al., provides a uniform, panoramic, high‐resolution map of stars and an ionized diffuse gas in the central 416 arcmin2 of the Galaxy. This survey was carried out with 144 HST orbits using two narrow‐band filters at 1.87 and 1.90 μm in NICMOS Camera 3. In this paper, we describe in detail the data reduction and mosaicking procedures followed, including background level matching and astrometric corrections. We have detected ∼570 000 near‐infrared (near‐IR) sources using the ‘starfinder’ software and are able to quantify photometric uncertainties of the detections. The source detection limit varies across the survey field, but the typical 50 per cent completion limit is ∼17th magnitude (Vega system) in the 1.90 μm band. A comparison with the expected stellar magnitude distribution shows that these sources are primarily main‐sequence massive stars (≳7 M⊙) and evolved lower mass stars at the distance of the Galactic Centre. In particular, the observed source magnitude distribution exhibits a prominent peak, which could represent the red clump (RC) stars within the Galactic Centre. The observed magnitude and colour of these RC stars support a steep extinction curve in the near‐IR towards the Galactic Centre. The flux ratios of our detected sources in the two bands also allow for an adaptive and statistical estimate of extinction across the field. With the subtraction of the extinction‐corrected continuum, we construct a net Paschen α emission map and identify a set of Paschen α emitting sources, which should mostly be evolved massive stars with strong stellar winds. The majority of the identified Paschen α point sources are located within the three known massive Galactic Centre stellar clusters. However, a significant fraction of our Paschen α emitting sources are located outside the clusters and may represent a new class of ‘field’ massive stars, many of which may have formed in isolation and/or in small groups. The maps and source catalogues presented here are available electronically.