[Paper Review] Active Galaxy Science in the LSST Deep-Drilling Fields: Footprints, Cadence Requirements, and Total-Depth Requirements

W. N. Brandt, Qingling Ni+26 more|arXiv (Cornell University)|Nov 15, 2018

Geophysics and Gravity MeasurementsEarth and Planetary Sciences20 citations

TL;DR

This white paper proposes optimized footprints, cadence, and depth requirements for the LSST Deep-Draining Fields (ELAIS-S1, XMM-LSS, CDF-S, COSMOS) to enable transformative active galactic nucleus (AGN) science. It advocates for a two-night cadence in grizy bands over 7–8.5-month seasons, total depths of i ≈ 28.5, z ≈ 27.9, and y ≈ 26.5, and high-quality data processing to support reverberation mapping, variability studies, and transient detection.

ABSTRACT

This white paper specifies the footprints, cadence requirements, and total-depth requirements needed to allow the most-successful AGN studies in the four currently selected LSST Deep-Drilling Fields (DDFs): ELAIS-S1, XMM-LSS, CDF-S, and COSMOS. The information provided on cadence and total-depth requirements will also likely be applicable to enabling effective AGN science in any additional DDFs that are chosen.

Motivation & Objective

• To define precise footprints for LSST Deep-Drilling Fields (DDFs) that maximize overlap with existing high-quality multiwavelength data for AGN studies.
• To establish a two-night cadence in grizy bands over extended observing seasons to enable high-precision photometric reverberation mapping of AGN broad-line regions.
• To achieve total depths of i ≈ 28.5, z ≈ 27.9, and y ≈ 26.5 to detect faint, high-redshift AGNs (z > 6–7) and improve photometric redshift accuracy.
• To support long-term AGN variability studies, including damped random walk modeling and detection of transient phenomena like tidal disruptions and changing-look AGNs.
• To ensure robust data processing pipelines, including nightly co-adds, difference imaging, and alert systems, for timely scientific exploitation of DDF data.

Proposed method

• Propose a two-night cadence in grizy bands across all four DDFs to sample AGN continuum and broad-line region variability with high temporal resolution.
• Require total depths of g ≈ 28.5, r ≈ 28.5, i ≈ 28.5, z ≈ 27.9, and y ≈ 26.5 to detect faint AGNs at z > 6–7 and improve photometric redshifts.
• Implement a 7–8.5 month observing season per year, including 1–1.5 months of precursor observations before spectroscopic campaigns to enable early sampling of the driving continuum.
• Enforce image quality requirements of <1.2" in r-band, scaled for other filters, to ensure high-fidelity photometry and host galaxy studies.
• Develop a dedicated data processing pipeline for DDFs, including nightly co-adds, difference imaging, and real-time alert generation from DIASource catalogs.
• Create annual co-adds at multiple signal-to-noise thresholds (10th to 90th percentiles of image quality) to enable deep host galaxy and variability studies.

Experimental results

Research questions

• RQ1 What footprint and cadence configuration maximizes the detection and characterization of AGNs across the LSST DDFs, especially in relation to existing multiwavelength data?
• RQ2 How frequently must LSST observe the DDFs to enable reliable photometric reverberation mapping of broad-line regions in distant AGNs?
• RQ3 What total depth is required in each filter to detect statistically significant samples of high-redshift AGNs (z > 6–7) and improve photometric redshift accuracy?
• RQ4 How can LSST data processing be optimized to support long-term AGN variability studies, including damped random walk modeling and transient detection?
• RQ5 What data processing and alert systems are necessary to enable real-time discovery and follow-up of AGN transients, such as tidal disruptions and changing-look AGNs?

Key findings

• A two-night cadence in grizy bands over 7–8.5-month seasons is required to achieve high recovery fractions of reverberation lags in AGN broad-line regions and accretion disks.
• Total depths of i ≈ 28.5, z ≈ 27.9, and y ≈ 26.5 are necessary to detect ~30–100 AGNs at z > 7 and z > 6, respectively, enabling studies of early black hole growth.
• Achieving u-band depth ≈ 28.3 improves photometric redshift quality, especially for AGNs at z < 4, reducing outlier rates in the main survey.
• Nightly co-adds, difference imaging, and real-time alert generation from DIASource catalogs are essential for detecting transient AGN phenomena and enabling follow-up.
• Annual co-adds at varying image quality percentiles (10th to 90th) enable optimal host galaxy studies and deep variability analysis at faint magnitudes.
• Special attention is required for bright stars like Mira in the XMM-LSS field to prevent optical artifacts and data processing issues in DDF science.