The Herschel Multi‐tiered Extragalactic Survey (HerMES) is a legacy programme designed to map a set of nested fields totalling ∼380 deg2. Fields range in size from 0.01 to ∼20 deg2, 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 deg2 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.

Extragalactic background light (EBL) anisotropy traces variations in the total production of photons over cosmic history, and may contain faint, extended components missed in galaxy point source surveys. Infrared EBL fluctuations have been attributed to primordial galaxies and black holes at the epoch of reionization (EOR), or alternately, intra-halo light (IHL) from stars tidally stripped from their parent galaxies at low redshift. We report new EBL anisotropy measurements from a specialized sounding rocket experiment at 1.1 and 1.6 micrometers. The observed fluctuations exceed the amplitude from known galaxy populations, are inconsistent with EOR galaxies and black holes, and are largely explained by IHL emission. The measured fluctuations are associated with an EBL intensity that is comparable to the background from known galaxies measured through number counts, and therefore a substantial contribution to the energy contained in photons in the cosmos.At near-infrared wavelengths, where the large zodiacal light foreground complicates absolute photometry measurements, the extragalactic background light (EBL) may be best accessed by anisotropy measurements. On large angular scales, fluctuations are produced by the clustering of galaxies, which is driven by the underlying distribution of dark matter. EBL anisotropy measurements can probe emission from epoch of reionization (EOR) galaxies (1-3) and directcollapse black holes (4) that formed during the EOR before the universe was fully ionized by exploiting the distinctive Lyman cutoff feature in the rest-frame ultraviolet (UV), thus probing the UV luminosity density at high redshifts (5). However, large-scale fluctuations may also arise from the intrahalo light (IHL) created by stars stripped from their parent galaxies during tidal interactions (6) at redshift z < 3. A multi-wavelength fluctuation analysis can distinguish among these scenarios and constrain the EOR star formation rate.A search for such background components must carefully account for fluctuations produced 2 by known galaxy populations. Linear galaxy clustering is an important contribution to fluctuations on scales much larger than galaxies themselves. On fine scales, the variation in the number of galaxies produces predominantly Poissonian fluctuations, with an amplitude that depends on the luminosity distribution. Anisotropy measurements suppress foreground galaxy fluctuations by masking known galaxies from an external catalog.The first detections of infrared fluctuations in excess of the contribution from known galaxies with the Spitzer Space Telescope (7-9) were interpreted as arising from a population of faint first-light galaxies at z > 7. The Hubble Space Telescope was used at shorter wavelengths (10) to carry out a fluctuation study in a small deep field but did not report fluctuations in excess of known galaxy populations. Measurements with the AKARIsatellite (11) show excess fluctuations with a blue spectrum rapidly rising from 4.1μm to 2.4μm. Fluctuation measurements in a large survey fi...

We produce simulations of the atomic CII line emission in large sky fields in order to determine the current and future prospects for mapping this line during the high redshift epoch of reionization. We calculate the CII line intensity, redshift evolution and spatial fluctuations using observational relations between CII emission and the galaxy star formation rate (SFR) over the frequency range 200 -300 GHz. We estimate an averaged intensity of I CII = (4 ± 2) × 10 2 Jy sr −1 in the redshift range z ∼ 5.3 − 8.5. Observations of the CII emission in this frequency range will suffer contamination from emission lines at lower redshifts, in particular CO rotational lines. Using simulations, we estimated the CO contamination to be I CO ≈ 10 3 Jy sr −1 (originating from galaxies at z < 2.5). Using detailed simulations of the CII and CO emission across a range of redshifts, we generate maps as a function of angle and frequency, fully taking into account this resolution and light cone effects. In order to reduce the foreground contamination we find that we should mask galaxies below redshifts ∼ 2.5 with a CO(J:2-1) to CO(J:6-5) line flux density higher than 5 × 10 −22 W m −2 or a AB magnitude lower than m K = 22. We estimate that the additional continuum contamination originating in emission from stars and in dust, free-free, free-bound and two photon emission in the ISM is of the order of 10 5 Jy sr −1 however it can be removed from observation due to the smooth evolution of this foreground with frequency. We also consider the possibility of cross correlating foreground lines with galaxy surveys in order to probe the intensity of the foregrounds. Finally, we discuss the expected constraints from two experiments capable of measuring the expected CII power spectrum. Subject headings: cosmology: theory -diffuse radiation -intergalactic medium -large scale structure of universe