Abstract
ABSTRACT We present the steps taken to produce a reliable and complete input galaxy catalogue for the Dark Energy Spectroscopic Instrument (DESI) Bright Galaxy Survey (BGS) using the photometric Legacy Survey DR8 DECam. We analyse some of the main issues faced in the selection of targets for the DESI BGS, such as star–galaxy separation, contamination by fragmented stars and bright galaxies. Our pipeline utilizes a new way to select BGS galaxies using Gaia photometry and we implement geometrical and photometric masks that reduce the number of spurious objects. The resulting catalogue is cross-matched with the Galaxy And Mass Assembly (GAMA) survey to assess the completeness of the galaxy catalogue and the performance of the target selection. We also validate the clustering of the sources in our BGS catalogue by comparing with mock catalogues and the Sloan Digital Sky Survey (SDSS) data. Finally, the robustness of the BGS selection criteria is assessed by quantifying the dependence of the target galaxy density on imaging and other properties. The largest systematic correlation we find is a 7 per cent suppression of the target density in regions of high stellar density.
Highlights
The Dark Energy Spectroscopic Instrument1 (DESI) (DESI Collaboration et al 2016) is a multi-fibre spectrograph that will be used1 http://desi.lbl.gov/to carry out a number of wide-field surveys of galaxies and quasars to map the large-scale structure of the Universe
To avoid this and to achieve more accurate photometry for large galaxies in the SGA2020 catalogue, TRACTOR is seeded with different priors, and within an elliptical mask centred on the large galaxy TRACTOR fits secondary detections using only the point spread functions (PSF) model
In order to reduce the number of image artefacts and fragments of ‘shredded’ galaxies that would otherwise be classified as Bright Galaxy Sample (BGS) targets we apply a cut on the fibre magnitude that is defined as a function of r-band magnitude as follows: 22.9 + (r − 17.8) for r < 17.8 rfibmag
Summary
The Dark Energy Spectroscopic Instrument (DESI) (DESI Collaboration et al 2016) is a multi-fibre spectrograph that will be used. The characterisation and definition of the target list for each DESI survey is a critical step for efficient survey execution and to allow reliable measurements of galaxy clustering We describe this process for the DESI bright galaxy survey (hereafter BGS), a flux limited sample of around 10 million galaxies, using photometry from a new imaging survey, the Legacy Surveys (LS). To make predictions for BGS target sample we make use of the mock galaxy catalogue created from the Millennium-XXL (MXXL) N-body simulation of Angulo et al (2012) by Smith et al (2017) This mock is tuned match the luminosity function, colour distribution, and clustering properties of the SDSS MGS at low redshift, and the evolution of these statistics to redshift z ≈ 0.5 as measured from the GAMA survey (Driver et al 2012; Liske et al 2015; Baldry et al 2017).
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