HELP: The Herschel Extragalactic Legacy Project

TL;DR

HELP is a comprehensive project that homogenizes and releases multi-wavelength extragalactic data, enabling advanced statistical analysis and providing extensive photometric and physical property data for large sky areas.

Contribution

The project introduces a standardized, open methodology for homogenizing and analyzing Herschel extragalactic data, including probabilistic extraction methods and full data access tools.

Findings

First data release covers 1270 deg2 with 18 million flux measurements.

Probabilistic methods provide full posterior distributions for source properties.

Includes extensive photometry, redshifts, and physical parameters for extragalactic sources.

Abstract

We present the Herschel Extragalactic Legacy Project (HELP). This project collates, curates, homogenises, and creates derived data products for most of the premium multi-wavelength 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 homogenisation 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 database. This new data set includes far-infrared photometry, photometric redshifts, and derived physical properties estimated from modelling the spectral energy distributions.