Abstract
ABSTRACT We present the Herschel Extragalactic Legacy Project (HELP). This project collates, curates, homogenizes, and creates derived data products for most of the premium multiwavelength extragalactic data sets. The sky boundaries for the first data release cover 1270 deg2 defined by the Herschel SPIRE extragalactic survey fields; notably the Herschel Multi-tiered Extragalactic Survey (HerMES) and the Herschel Atlas survey (H-ATLAS). Here, we describe the motivation and principal elements in the design of the project. Guiding principles are transparent or ‘open’ methodologies with care for reproducibility and identification of provenance. A key element of the design focuses around the homogenization of calibration, meta data, and the provision of information required to define the selection of the data for statistical analysis. We apply probabilistic methods that extract information directly from the images at long wavelengths, exploiting the prior information available at shorter wavelengths and providing full posterior distributions rather than maximum-likelihood estimates and associated uncertainties as in traditional catalogues. With this project definition paper, we provide full access to the first data release of HELP; Data Release 1 (DR1), including a monolithic map of the largest SPIRE extragalactic field at 385 deg2 and 18 million measurements of PACS and SPIRE fluxes. We also provide tools to access and analyse the full HELP data base. This new data set includes far-infrared photometry, photometric redshifts, and derived physical properties estimated from modelling the spectral energy distributions over the full HELP sky. All the software and data presented is publicly available.
Highlights
A fundamental requirement for rigorous testing of any theories of galaxy formation and evolution is a complete statistical audit or census of the stellar content and star-formation rates of galaxies in the Universe at different times and as a function of the mass of the dark matter halos that host them.This audit requires many elements
The master list number counts as a function of the optical and near infrared band magnitude on each field are summarised in Shirley et al (2019)
The effective far infrared flux depth is in turn dependent on the depths of all the bands contributing to the determination of the prior list
Summary
A fundamental requirement for rigorous testing of any theories of galaxy formation and evolution is a complete statistical audit or census of the stellar content and star-formation rates of galaxies in the Universe at different times and as a function of the mass of the dark matter halos that host them We need un-biased maps of large volumes of the Universe made with telescopes that probe different wavelengths at which different physical processes of interest manifest themselves. We need the machinery to bring together these various data sets and calculate the “value-added” physical data of primary interest, e.g. the distances, stellar masses, star-formation rates, active galactic nuclei (AGN) fractions and the intrinsic number densities of the different galaxy populations
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