The Herschel Multi-tiered Extragalactic Survey (HerMES) is a legacy programme designed to map a set of nested fields totalling ∼380 deg^2. Fields range in size from 0.01 to ∼20 deg^2, using the Herschel-Spectral and Photometric Imaging Receiver (SPIRE) (at 250, 350 and 500 μm) and the Herschel-Photodetector Array Camera and Spectrometer (PACS) (at 100 and 160 μm), with an additional wider component of 270 deg^2 with SPIRE alone. These bands cover the peak of the redshifted thermal spectral energy distribution from interstellar dust and thus capture the reprocessed optical and ultraviolet radiation from star formation that has been absorbed by dust, and are critical for forming a complete multiwavelength understanding of galaxy formation and evolution.

The survey will detect of the order of 100 000 galaxies at 5σ in some of the best-studied fields in the sky. Additionally, HerMES is closely coordinated with the PACS Evolutionary Probe survey. Making maximum use of the full spectrum of ancillary data, from radio to X-ray wavelengths, it is designed to facilitate redshift determination, rapidly identify unusual objects and understand the relationships between thermal emission from dust and other processes. Scientific questions HerMES will be used to answer include the total infrared emission of galaxies, the evolution of the luminosity function, the clustering properties of dusty galaxies and the properties of populations of galaxies which lie below the confusion limit through lensing and statistical techniques.

This paper defines the survey observations and data products, outlines the primary scientific goals of the HerMES team, and reviews some of the early results.

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The Herschel Multi-tiered Extragalactic Survey: HerMES

The SIRTF Wide‐Area Infrared Extragalactic Survey (SWIRE), the largest SIRTF Legacy program, is a wide‐area imaging survey to trace the evolution of dusty, star‐forming galaxies, evolved stellar populations, and active galactic nuclei (AGNs) as a function of environment, from redshifts to the current epoch. SWIRE will survey seven high‐latitude fields, totaling 60–65 deg2 in all seven SIRTF bands: Infrared Array Camera (IRAC) 3.6, 4.5, 5.6, and 8 μm and Multiband Imaging Photometer for SIRTF (MIPS) 24, 70, and 160 μm. Extensive modeling suggests that the Legacy Extragalactic Catalog may contain in excess of 2 million IR‐selected galaxies, dominated by (1) ∼150,000 luminous infrared galaxies (LIRGs; LFIR > 1011 L⊙) detected by MIPS (and significantly more detected by IRAC), ∼7000 of these with ; (2) 1 million IRAC‐detected early‐type galaxies (∼ with and ∼10,000 with ); and (3) ∼20,000 classical AGNs detected with MIPS, plus significantly more dust‐obscured quasi‐stellar objects/AGNs among the LIRGs. SWIRE will provide an unprecedented view of the evolution of galaxies, structure, and AGNs.

The key scientific goals of SWIRE are (1) to determine the evolution of actively star forming and passively evolving galaxies in order to understand the history of galaxy formation in the context of cosmic structure formation; (2) to determine the evolution of the spatial distribution and clustering of evolved galaxies, starbursts, and AGNs in the key redshift range over which much of cosmic evolution has occurred; and (3) to determine the evolutionary relationship between “normal galaxies” and AGNs and the contribution of AGN accretion energy versus stellar nucleosynthesis to the cosmic backgrounds. The large area of SWIRE is important to establish statistically significant population samples over enough volume cells that we can resolve the star formation history as a function of epoch and environment, i.e., in the context of structure formation. The large volume is also optimized for finding rare objects.

The SWIRE fields are likely to become the next generation of large “cosmic windows” into the extragalactic sky. They have been uniquely selected to minimize Galactic cirrus emission over large scales. The Galaxy Evolution Explorer will observe them as part of its deep 100 deg2 survey, as will Herschel. SWIRE includes ∼9 deg2 of the unique large‐area XMM Large Scale Structure hard X‐ray imaging survey and is partly covered by the UKIDSS deep J and K survey. An extensive optical/near‐IR imaging program is underway from the ground.

The SWIRE data are nonproprietary; catalogs and images will be released twice yearly, beginning about 11 months after SIRTF launch. Details of the data products and release schedule are presented.

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SWIRE: The SIRTF Wide‐Area Infrared Extragalactic Survey

We measure the evolution of the stellar mass function (SMF) from z = 0−1 using multi-wavelength imaging and spectroscopic redshifts from the PRism MUlti-object Survey (PRIMUS) and the Sloan Digital Sky Survey (SDSS). From PRIMUS we construct an i < 23 flux-limited sample of ∼ 40,000 galaxies at z = 0.2 − 1.0 over five fields totaling ≈ 5.5 deg 2 , and from the SDSS we select ∼ 170,000 galaxies atz = 0.01−0.2 that we analyze consistently with respect to PRIMUS to minimize systematic errors in our evolutionary measurements. We find that the SMF of all galaxies evolves relatively little since z = 1, although we do find evidence for mass assembly downsizing; we measure a ≈ 30% increase in the number density of ∼ 10 10 M⊙ galaxies sincez ≈ 0.6, and a . 10% change in the number density of all & 10 11 M⊙ galaxies since z ≈ 1. Dividing the sample into star-forming and quiescent using an evolving cut in specific star-formation rate, we find that the number density of ∼ 10 10 M⊙ star-forming galaxies stays relatively constant since z ≈ 0.6, whereas the space-density of & 10 11 M⊙ star-forming galaxies decreases by ≈ 50% between z ≈ 1 and z ≈ 0. Meanwhile, the number density of ∼ 10 10 M⊙ quiescent galaxies increases steeply towards low redshift, by a factor of ∼ 2 − 3 since z ≈ 0.6, while the number of massive quiescent galaxies remains approximately constant since z ≈ 1. These results suggest that the rate at which star-forming galaxies are quenched increases with decreasing stellar mass, but that the bulk of the stellar mass buildup within the quiescent population occurs around ∼ 10 10.8 M⊙. In addition, we conclude that mergers do not appear to be a dominant channel for the stellar mass buildup of galaxies at z < 1, even among massive (& 10 11 M⊙) quiescent galaxies. Subject headings: Surveys – galaxies: evolution – galaxies: high-redshift – cosmology: large-scale structure of universe

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PRIMUS: Constraints on Star Formation Quenching and Galaxy Merging, and the Evolution of the Stellar Mass Function from z = 0-1

The largest of the SIRTF Legacy programs, SWIRE will survey 65 sq. deg. in seven high latitude fields selected to be the best wide low-extinction windows into the extragalactic sky. SWIRE will detect millions of spheroids, disks and starburst galaxies to z>3 and will map L* and brighter systems on scales up to 150 Mpc at z∼0.5–1. It will also detect ∼104 low extinction AGN and large numbers of obscured AGN. An extensive program of complementary observations is underway. The data are non-proprietary and will be made available beginning in Spring 2004.

SWIRE: The SIRTF Wide-area InfraRed Extragalactic Survey

We present maps, source lists and derived number counts from the largest, unbiased, extragalactic submillimetre (submm) survey so far undertaken with the SCUBA camera on the James Clerk Maxwell Telescope (JCMT). Our maps are located in two regions of sky (ELAIS N2 and Lockman-Hole E) and cover 260arcmin2 , to a typical rms noise level of sigma 850 ~=2.5mJybeam-1 . We have reduced the data using both the standard JCMT surf procedures, and our own IDL -based pipeline which produces zero-footprint maps and noise images. The uncorrelated noise maps produced by the latter approach have enabled us to apply a maximum likelihood method to measure the statistical significance of each peak in our maps, leading to properly quantified errors on the flux density of all potential sources. We detect 19 sources with signal-to-noise ratios (S/N)>4, and 38 with S/N>3.5. To assess both the completeness of this survey and the impact of source confusion as a function of flux density, we have applied our source-extraction algorithm to a series of simulated images. The result is a new estimate of the submm source counts over the flux-density range S 850 ~=5-15mJy, which we compare with estimates derived by other workers, and with the predictions of a number of models. Our best estimate of the cumulative source count at S 850 >8mJy is per square degree. Assuming that the majority of sources lie at z >1.5, this result implies that the comoving number density of high-redshift galaxies forming stars at a rate in excess of 1000Msolar yr-1 is ~=10-5 Mpc-3 , with only a weak dependence on the precise redshift distribution. This number density corresponds to the number density of massive ellipticals with L >3-4L * in the present-day Universe , and is also the same as the comoving number density of comparably massive, passively evolving objects in the redshift band 1<z <2 inferred from recent surveys of extremely red objects. Thus the bright submm sources uncovered by this survey can plausibly account for the formation of all present-day massive ellipticals. Improved redshift constraints, and ultimately an improved measure of submm source clustering can refine or refute this picture.

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The SCUBA 8-mJy survey - I. Submillimetre maps, sources and number counts

We describe the European Large Area ISO Survey (ELAIS). ELAIS was the largest single Open Time project conducted by ISO, mapping an area of 12deg^2 at 15μm with ISOCAM and at 90μm with ISOPHOT. Secondary surveys in other ISO bands were undertaken by the ELAIS team within the fields of the primary survey, with 6deg^2 being covered at 6.7μm and 1deg^2 at 175μm. This paper discusses the goals of the project and the techniques employed in its construction, as well as presenting details of the observations carried out, the data from which are now in the public domain. We outline the ELAIS `preliminary analysis' which led to the detection of over 1000 sources from the 15 and 90-μm surveys (the majority selected at 15μm with a flux limit of ~3mJy), to be fed into a ground-based follow-up campaign, as well as a programme of photometric observations of detected sources using both ISOCAM and ISOPHOT. We detail how the ELAIS survey complements other ISO surveys in terms of depth and areal coverage, and show that the extensive multi-wavelength coverage of the ELAIS fields resulting from our concerted and on-going follow-up programme has made these regions amongst the best studied areas of their size in the entire sky, and, therefore, natural targets for future surveys. This paper accompanies the release of extremely reliable subsets of the `preliminary analysis' products. Subsequent papers in this series will give further details of our data reduction techniques, reliability and completeness estimates and present the 15- and 90-μm number counts from the `preliminary analysis', while a further series of papers will discuss in detail the results from the ELAIS `final analysis', as well as from the follow-up programme.

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P. Papadopoulos

Papers

The European Large Area ISO Survey - I. Goals, definition and observations