The SDSS-IV extended Baryon Oscillation Spectroscopic Survey: selecting emission line galaxies using the Fisher discriminant

Abstract

We present a new selection technique of producing spectroscopic target catalogues for massive spectroscopic surveys for cosmology. This work was conducted in the context of the extended Baryon Oscillation Spectroscopic Survey (eBOSS), which will use ∼200 000 emission line galaxies (ELGs) at 0.6 ≤ z spec ≤ 1.0 to obtain a precise baryon acoustic oscillation measurement. Our proposed selection technique is based on optical and near-infrared broad-band filter photometry. We used a training sample to define a quantity, the Fisher discriminant (linear combination of colours), which correlates best with the desired properties of the target: redshift and [Oii] flux. The proposed selections are simply done by applying a cut on magnitudes and this Fisher discriminant. We used public data and dedicated SDSS spectroscopy to quantify the redshift distribution and [Oii] flux of our ELG target selections. We demonstrate that two of our selections fulfil the initial eBOSS/ELG redshift requirements: for a target density of 180 deg −2 , ∼70% of the selected objects have 0.6 ≤ z spec ≤ 1.0 and only ∼1% of those galaxies in the range 0.6 ≤ z spec ≤ 1.0 are expected to have a catastrophic z spec estimate. Additionally, the stacked spectra and stacked deep images for those two selections show characteristic features of star-forming galaxies. The proposed approach using the Fisher discriminant could, however, be used to efficiently select other galaxy populations, based on multi-band photometry, providing that spectroscopic information is available. This technique could thus be useful for other future massive spectroscopic surveys such as PFS, DESI, and 4MOST.

Sloan Digital Sky Survey IV: Mapping the Milky Way, Nearby Galaxies, and the Distant Universe

“…In SEQUELS, Comparat et al (2015) drew ELG targets from the South Galactic Cap U-band Sky Survey (SCUSS; Zou et al 2015) and SDSS. In the last round of tests before the ELG program was finalized, Comparat et al (2016) and Raichoor et al (2016) combined WISE (Wright et al 2010), SCUSS, and SDSS to select ELG targets. The final selection functions are nearly as efficient as the DECaLS targeting but yielded a lower effective redshift.…”

The 13th Data Release of the Sloan Digital Sky Survey: First Spectroscopic Data from the SDSS-IV Survey Mapping Nearby Galaxies at Apache Point Observatory

“…Nine plates from SEQUELS, all released in DR12, included targets derived from an early test of the ELG selection algorithm (Alam et al 2015). These tests of ELG selection algorithms were part of a larger series of tests performed during SDSS-III and SDSS-IV Raichoor et al 2016;Delubac et al 2017. The spectra from these tests were also used in one of the first science results from eBOSS, a study of galactic-scale outflows traced by UV emission (hu et al 2015).…”

“…Raichoor et al (2016) describe the Fischer discriminant used to select ELG targets using photometry from SDSS, WISE, and SCUSS (Zou et al 2016) for the main ELG sample. Delubac et al (2017) produce the catalogs used for ELG targeting with SDSS+WISE or SDSS+WISE+SCUSS data.…”

SDSS-IV eBOSS emission-line galaxy pilot survey

“…For both ELGs and QSOs, schemes that select targets for spectroscopy exist using either color selection (Comparat et al 2013b;Schneider et al 2010) or higher dimensional algorithms (Raichoor et al 2016;Bovy et al 2012). To assess algorithms, eBOSS tested them on a 10 deg 2 sky patch covered by many photometric surveys around α ∼ 36…”

“…We mitigated the risk by investigating TS algorithms using existing wellknown photometry (SDSS and WISE) and more recent deeper ones (DES and SCUSS). Raichoor et al (2016) described in depth the optimization of the SDSS+WISE+SCUSS algorithms using the Fisher technique, while the further optimization of algorithms using DES photometry is described in this paper. The clustering properties and the homogeneity of targeting catalogs on large areas are discussed in Jouvel et al (2015) for the DESbased TS and in Delubac et al (in prep) for the SDSS+WISE -based TS.…”