Featureless stars: flux calibration for extremely large telescopes

ABSTRACT The spectrophotometric flux calibration of recent spectroscopic surveys has reached a limiting systematic precision of approximately $1-3$ per cent, and is often biased near the wavelengths associated with H i Balmer absorption. As we prepare for the next generation of imaging and spectroscopic surveys, and high-precision cosmology experiments, we must find a way to address this systematic. Towards this goal, we have identified a global network of 29 bright ($G< 17.5$) featureless white dwarf stars that have a spectral energy distribution consistent with an almost pure blackbody form over the entire optical and near-infrared wavelength range. Based on this sample, we have computed the systematic uncertainty and AB magnitude offsets associated with Gaia, SDSS, SMSS, Pan-STARRS, DES, and 2MASS, and we have also checked the consistency of our objects with both GALEX and WISE. The magnitude range of the featureless stars reported here are ideally suited to observations taken with the forthcoming generation of extremely large telescopes, as well as calibrating the survey data acquired by the Rubin, Euclid, and Roman observatories. Finally, all of the high-precision spectrophotometric standard stars reported here have been included in the latest release of the pypeit data reduction pipeline.