Scientific Synergy between LSST and Euclid

Abstract

Euclid and the Large Synoptic Survey Telescope (LSST) are poised to dramatically change the astronomy landscape early in the next decade. The combination of high-cadence, deep, wide-field optical photometry from LSST with highresolution, wide-field optical photometry, and near-infrared photometry and spectroscopy from Euclid will be powerful for addressing a wide range of astrophysical questions. We explore Euclid/LSST synergy, ignoring the political issues associated with data access to focus on the scientific, technical, and financial benefits of coordination. We focus primarily on dark energy cosmology, but also discuss galaxy evolution, transient objects, solar system science, and galaxy cluster studies. We concentrate on synergies that require coordination in cadence or survey overlap, or would benefit from pixellevel co-processing that is beyond the scope of what is currently planned, rather than scientific programs that could be accomplished only at the catalog level without coordination in data processing or survey strategies. We provide two quantitative examples of scientific synergies: the decrease in photo-z errors (benefiting many science cases) when highresolution Euclid data are used for LSST photo-z determination, and the resulting increase in weak-lensing signal-to-noise ratio from smaller photo-z errors. We briefly discuss other areas of coordination, including high-performance computing resources and calibration data. Finally, we address concerns about the loss of independence and potential cross-checks between the two missions and the potential consequences of not collaborating.

Applying Information Theory to Design Optimal Filters for Photometric Redshifts

“…In fact, it seems that since the Balmer break is so important as we have shown that following it into the infrared regime is essential to improve LSST photometric redshifts. This is in line with previous work highlighting the potential photometric redshift improvement from combining LSST observations with infrared data from future space telescope missions ( Rhodes et al 2017;Graham et al 2019). In a future paper we will look at how much information is gained from adding infrared and UV filters as well as what optimal filters in these wavelengths designed to complement LSST would look like.…”

Optimizing the LSST Observing Strategy for Dark Energy Science: DESC Recommendations for the Deep Drilling Fields and other Special Programs

“…The most immediate impact of overlap between the LSST DDFs and WFIRST and Euclid Deep Fields will be improved photo-z scatter at the 20% (Graham et al 2018 in prep.) to 50% level [11] from 0.3 < z < 3.0. The improved DDF photo-z estimates will be critical for training photoz estimates in the WFD survey, where the wide Euclid NIR coverage region is too shallow to significantly improve LSST photo-z on its own.…”

Gaussian mixture models for blended photometric redshifts

“…These two sets correspond to an LSST-like optical survey (Ivezić et al 2019), and the same survey with additional Euclid-like infrared observations (Laureijs et al 2011). The complementarity of LSST and Euclid has been investigated previously (e.g., Rhodes et al 2017); additional filter bands will help to break colour-redshift degeneracies and therefore enable more accurate photometric redshifts. Simulated observations are generated by redshifting a template, integrating over the relevant filter response curves, scaling the results to a given i-band magnitude, adding observational noise and imposing selection criteria.…”