Spectroscopic Needs for Calibration of LSST Photometric Redshifts

TL;DR

This paper discusses the spectroscopic requirements and methods for calibrating LSST photometric redshifts, emphasizing the importance of cross-correlation techniques and existing surveys like eBOSS and DESI for accurate bias and uncertainty characterization.

Contribution

It highlights the limitations of current deep spectroscopic samples and advocates for cross-correlation methods and specific survey strategies to improve LSST photo-z calibration.

Findings

eBOSS survey enables basic calibration of LSST photo-z's

LSST-DESI overlap suffices for high-redshift calibration

Expanded DESI coverage improves cross-correlation calibration

Abstract

This white paper summarizes the conclusions of the Snowmass White Paper "Spectroscopic Needs for Imaging Dark Energy Experiments" (arXiv:1309.5384) which are relevant to the calibration of LSST photometric redshifts; i.e., the accurate characterization of biases and uncertainties in photo-z's. Any significant miscalibration will lead to systematic errors in photo-z's, impacting nearly all extragalactic science with LSST. As existing deep redshift samples have failed to yield highly-secure redshifts for a systematic 20%-60% of their targets, it is a strong possibility that future deep spectroscopic samples will not solve the calibration problem on their own. The best options in this scenario are provided by cross-correlation methods that utilize clustering with objects from spectroscopic surveys (which need not be fully representative) to trace the redshift distribution of the full sample. For spectroscopy, the eBOSS survey would enable a basic calibration of LSST photometric redshifts, while the expected LSST-DESI overlap would be more than sufficient for an accurate calibration at z>0.2. A DESI survey of nearby galaxies conducted in bright time would enable accurate calibration down to z~0. The expanded areal coverage provided by the transfer of the DESI instrument (or duplication of it at the Blanco Telescope) would enable the best possible calibration from cross-correlations, in addition to other science gains.