[Paper Review] Enhancing LSST Science with Euclid Synergy

P. Capak, Jean‐Charles Cuillandre+24 more|arXiv (Cornell University)|Apr 23, 2019

Astronomy and Astrophysical Research10 references13 citations

TL;DR

This paper proposes a coordinated survey strategy between the LSST and Euclid space telescope to significantly enhance dark energy and Milky Way science by overlapping LSST's ground-based optical imaging with Euclid's space-based optical and near-infrared imaging and spectroscopy. By dedicating just 0.3–3.8% of LSST’s 10-year survey to co-observing Euclid’s deep fields—including COSMOS, SXDS, CDFS, and the Akari Deep Field South—cosmological constraints on dark energy improve by 30–69%, and Milky Way halo mapping improves by an order of magnitude over 50% more area.

ABSTRACT

This white paper is the result of the Tri-Agency Working Group (TAG) appointed to develop synergies between missions and is intended to clarify what LSST observations are needed in order to maximally enhance the combined science output of LSST and Euclid. To facilitate LSST planning we provide a range of possible LSST surveys with clear metrics based on the improvement in the Dark Energy figure of merit (FOM). To provide a quantifiable metric we present five survey options using only between 0.3 and 3.8% of the LSST 10 year survey. We also provide information so that the LSST DDF cadence can possibly be matched to those of \emph{Euclid} in common deep fields, SXDS, COSMOS, CDFS, and a proposed new LSST deep field (near the Akari Deep Field South). Co-coordination of observations from the Large Synoptic Survey Telescope (LSST) and Euclid will lead to a significant number of synergies. The combination of optical multi-band imaging from LSST with high resolution optical and near-infrared photometry and spectroscopy from \emph{Euclid} will not only improve constraints on Dark Energy, but provide a wealth of science on the Milky Way, local group, local large scale structure, and even on first galaxies during the epoch of reionization. A detailed paper has been published on the Dark Energy science case (Rhodes et al.) by a joint LSST/Euclid working group as well as a white paper describing LSST/Euclid/WFIRST synergies (Jain et al.), and we will briefly describe other science cases here. A companion white paper argues the general science case for an extension of the LSST footprint to the north at airmass < 1.8, and we support the white papers for southern extensions of the LSST survey.

Motivation & Objective

• To define a coordinated survey strategy between LSST and Euclid that maximizes scientific return from their combined data.
• To identify specific LSST survey options—ranging from mini-surveys to Deep Drilling Fields—that optimally overlap with Euclid’s deep fields.
• To quantify the improvement in the Dark Energy figure of merit (FOM) from joint LSST-Euclid observations.
• To align LSST’s observing cadence with Euclid’s in key deep fields to enable high-fidelity photometric redshifts, weak lensing, and de-blending.
• To advocate for survey modifications, including extending LSST’s footprint northward and placing the Deep Drilling Field on the Akari Deep Field South, to maximize synergy.

Proposed method

• Proposes five ranked LSST survey options using 0.3–3.8% of the 10-year LSST survey time, with specific visit counts per filter and area coverage.
• Uses the Dark Energy figure of merit (FOM) as a quantitative metric to evaluate improvements from joint LSST-Euclid data.
• Focuses on co-observing Euclid’s deep fields—COSMOS, SXDS, CDFS, and Akari Deep Field South—by matching LSST cadence to Euclid’s observing schedule.
• Prioritizes uniformity of visits and co-added depth over single-visit depth, with a minimum of 5 visits per filter to control photometric systematics.
• Ranks survey constraints (e.g., image quality, sky brightness, co-added depth) on importance to guide survey design, with co-added depth ranked highest.
• Models performance based on area coverage and depth scaling, with FOM improving with increasing overlap and depth.

Experimental results

Research questions

• RQ1 How can LSST and Euclid jointly improve constraints on dark energy, and by how much?
• RQ2 What specific LSST survey configurations (footprint, depth, cadence) maximize synergy with Euclid’s deep fields?
• RQ3 How does co-observing in common deep fields like COSMOS and CDFS enhance photometric redshift accuracy and weak lensing measurements?
• RQ4 What is the optimal trade-off between survey area, co-added depth, and number of visits in LSST’s coordinated surveys?
• RQ5 How does extending LSST’s footprint northward and placing the Deep Drilling Field on the Akari Deep Field South improve overall scientific return?

Key findings

• Coordinating LSST with Euclid’s deep fields improves the Dark Energy figure of merit by 30–69%, depending on the survey configuration.
• Mapping the Milky Way halo improves by an order of magnitude over 50% more area when LSST surveys overlap with Euclid’s deep fields.
• The most effective LSST survey options use 0.3–3.8% of the 10-year survey time, with visit counts ranging from 25 to 1507 visits across multiple filters.
• The Akari Deep Field South is identified as a prime candidate for LSST’s Deep Drilling Field, with a proposed 10 deg² area and 1507 visits (738 in u, 96 in g, etc.).
• Uniformity of visits and co-added depth are more important than single-visit depth, with a minimum of 5 visits per filter recommended to control photometric systematics.
• Matching LSST cadence to Euclid’s in deep fields enables better de-blending of galaxy photometry and improved weak lensing shear measurements.