Photometric Redshifts with the LSST II: The Impact of Near-Infrared and Near-Ultraviolet Photometry

A photograph and rendering mix of the exterior of the Vera C. Rubin Observatory building on Cerro Pachón in Chile. Image credit: LSST/NSF/AURA

The groundbreaking advances in cosmological astrophysics to be made by the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) all rely, at least in part, on accurate photometric redshift estimates for billions galaxies in our Universe. These photometric redshifts measure how much the light from a far off galaxy is shifted to lower frequencies by the accelerated expansion of the Universe, as the photons travel from their galaxy to ours. Astronomers use the photometric redshifts as a proxy for distance when they map out the locations of galaxies over the vast expanse of space, and backwards in time. These cosmological maps are essential to further our understanding the origin, evolution and future of the Universe.

The six filters in the Rubin Observatory LSST science camera will collect light at optical frequencies. Although it will be the most sensitive wide-field astronomical surveys to date, and will provide photometric redshifts for an unprecedented number of distant galaxies (approximately four billion!) — observations made in the optical frequencies alone impose a constraint on how well the photometric redshift can be measured. This recent paper by UW researchers, lead by staff scientist Melissa Graham, explores the benefits of incorporating measurements at ultraviolet and near-infrared frequencies. In particular, this team simulated ultraviolet data for the future Cosmological Advanced Survey Telescope for Optical and uv Research (CASTOR) mission and near-infrared data for the future Euclid Space Telescope and Nancy Grace Roman Space Telescope, and combined it with simulated optical data for the LSST. 

This research shows that including the ultraviolet and near-infrared data is the only way to significantly lower the fraction of galaxies for which the photometric redshift estimate is catastrophically incorrect — instances where faint red galaxies that are nearby are mistaken for bright blue galaxies that are at such great distances that they appear faint and red. In fact, these researchers found that incorporating data from multiple observatories can reduce such instances from 10% to less than 2%, which will allow astronomers to make much more precise cosmological maps. This research demonstrates why astronomers need many different types of telescopes operating at different frequencies  — and is a good example of how we all get by with a little help from our friends.


Cosmological Advanced Survey Telescope for Optical and uv Research (CASTOR)

Euclid Space Telescope:

Nancy Grace Roman Space Telescope:

Formally the Wide Field Infrared Survey Telescope (WFIRST)

Photometric Redshift:

Ultraviolet, Optical, and Near-Infrared: