[Paper Review] LSST Observing Strategy White Paper: LSST Observations of WFIRST Deep Fields

R. J. Foley, Anton M. Koekemoer+39 more|arXiv (Cornell University)|Nov 30, 2018

Gamma-ray bursts and supernovae4 citations

TL;DR

This white paper proposes that LSST conduct coordinated mini-surveys of WFIRST Supernova Survey deep fields over a 2-year period, matching WFIRST's 5-day cadence and achieving co-added depths of ~28–29 mag in $ugrizy$ filters. The strategy enables joint optical/NIR light curves for Type Ia supernovae, significantly improving distance precision, photo-z calibration, and transient science across cosmology and extragalactic astronomy.

ABSTRACT

The Wide-Field Infrared Survey Telescope (WFIRST) is expected to launch in the mid-2020s. With its wide-field near-infrared (NIR) camera, it will survey the sky to unprecedented detail. As part of normal operations and as the result of multiple expected dedicated surveys, WFIRST will produce several relatively wide-field (tens of square degrees) deep (limiting magnitude of 28 or fainter) fields. In particular, a planned supernova survey is expected to image 3 deep fields in the LSST footprint roughly every 5 days over 2 years. Stacking all data, this survey will produce, over all WFIRST supernova fields in the LSST footprint, ~12-25 deg^2 and ~5-15 deg^2 regions to depths of ~28 mag and ~29 mag, respectively. We suggest LSST undertake mini-surveys that will match the WFIRST cadence and simultaneously observe the supernova survey fields during the 2-year WFIRST supernova survey, achieving a stacked depth similar to that of the WFIRST data. We also suggest additional observations of these same regions throughout the LSST survey to get deep images earlier, have long-term monitoring in the fields, and produce deeper images overall. These fields will provide a legacy for cosmology, extragalactic, and transient/variable science.

Motivation & Objective

• To enable joint optical (LSST) and near-infrared (WFIRST) photometric light curves for Type Ia supernovae by matching LSST observations to the WFIRST Supernova Survey cadence.
• To achieve co-added image depths comparable to WFIRST’s deep fields (~28–29 mag) in $ugrizy$ filters, maximizing synergy for cosmological and extragalactic science.
• To improve photometric redshift accuracy by combining high-resolution WFIRST imaging with LSST’s deep optical coverage, reducing degeneracies and improving galaxy deblending.
• To support calibration of WFIRST’s HLS grism survey by providing deep, multi-epoch optical data in overlapping fields.
• To create a lasting legacy dataset for transient monitoring, cosmology, and multi-wavelength source characterization across the faint magnitude frontier.

Proposed method

• Propose a mini-survey strategy within LSST’s 10-year survey, focusing only on WFIRST Supernova Survey deep fields (5–15 deg²) over a 2-year window.
• Match LSST cadence to WFIRST’s planned ~5-day repeat visits, with observations ideally in all $ugrizy$ filters per visit.
• Implement deep-drilling-style observations with individual exposure depths and co-added depths matching WFIRST’s single-epoch and stacked limits.
• Prioritize simultaneous observations during the WFIRST SN survey to enable joint light-curve analysis and reduce time dilation effects on high-redshift SNe.
• Use nightly co-adds and difference imaging of co-added data to enable transient detection and long-term monitoring.
• Advocate for integrated data processing pipelines that combine LSST and WFIRST data in a shared software environment to maximize scientific access and synergy.

Experimental results

Research questions

• RQ1 How can LSST observations of WFIRST deep fields improve the precision of Type Ia supernova distance measurements through joint optical/NIR light curves?
• RQ2 What is the optimal LSST observing cadence and depth to maximize overlap and synergy with WFIRST’s 2-year Supernova Survey in terms of supernova detection and light-curve fidelity?
• RQ3 To what extent can combined LSST and WFIRST data improve photometric redshift accuracy for galaxies in deep extragalactic fields?
• RQ4 How does the inclusion of LSST’s $ugri$ filters enhance the characterization of transient sources and dust properties in high-redshift supernovae compared to WFIRST alone?
• RQ5 What is the impact of long-term, deep monitoring of these fields on the detection and analysis of faint, variable, and transient sources?

Key findings

• LSST observations matching WFIRST’s 5-day cadence and reaching co-added depths of ~28–29 mag in $ugrizy$ filters will enable joint light curves for ~20,000 Type Ia supernovae, significantly improving distance precision beyond what either telescope can achieve alone.
• The combined optical/NIR light curves will allow full rest-frame optical tracking from $z=0$ to $z=3$, enabling systematic tests of dust properties and intrinsic SN behavior.
• The coordinated survey will detect ~8,000 SNe Ia at $z<1$, with 0.05 mag distance modulus uncertainty for ~1,000 of them in the $0.45<z<0.55$ and $0.95<z<1.05$ redshift bins, meeting key cosmological goals.
• The fields will enable a 99.9% purity in photometric redshifts at the WFIRST HLS flux limit, with 22 deg² of deep grism spectroscopy planned to calibrate the survey.
• The strategy will produce a legacy dataset with 1.5% of total LSST survey time (474 hours) dedicated to these fields, yielding deeper, more uniform coverage than standalone LSST or WFIRST surveys.
• The proposed approach will reduce photo-$z$ degeneracies by combining high-resolution WFIRST imaging with deep LSST optical data, improving galaxy deblending and source characterization.