[Paper Review] LSST's DC Bias Against Planets and Galactic-Plane Science

Andrew Gould|arXiv (Cornell University)|Apr 11, 2013

Stellar, planetary, and galactic studies3 references3 citations

TL;DR

This paper argues that excluding the Galactic plane from LSST's synoptic survey imposes a major scientific loss, particularly for microlensing-based planet detection and Galactic structure studies. It demonstrates that image subtraction techniques can overcome crowding issues, enabling recovery of nearly all lost science—especially planetary system distribution and rare transients—without compromising other LSST science objectives.

ABSTRACT

An LSST-like survey of the Galactic plane (deep images every 3-4 days) could probe the Galactic distribution of planets by two distinct methods: gravitational microlensing of planets beyond the snow line and transits by planets very close to their hosts. The survey would identify over 250 disk-lens/disk-source microlensing events per year that peak at r<19, including 10% reaching the high magnification A>100 that makes them especially sensitive to planets. Intensive followup of these events would be required to find planets, similar to what is done presently for Galactic bulge microlensing. The same data would enable a wealth of other science, including detection of isolated black holes, systematic study of brown-dwarf binaries, a pre-explosion lightcurve of the next Galactic supernova, pre-explosion lightcurves of stellar mergers, early nova lightcurves, proper motions of many more stars than can be reached by GAIA, and probably much more. As usual, the most exciting discoveries from probing the huge parameter space encompassed by Galactic-plane stellar populations might well be serendipitous. Unfortunately, the LSST collaboration plans to exclude the first and fourth quadrants of the Galactic plane from their "synoptic" observations because the DC image that resulted from repeated observations would be limited by crowding. I demonstrate that the majority of this science can be recovered by employing well-developed image subtraction analysis methods, and that the cost to other (high Galactic latitude) science would be negligible.

Motivation & Objective

• To demonstrate that LSST’s planned exclusion of the Galactic plane (first and fourth quadrants) severely limits key astrophysical science, particularly microlensing-based planet detection.
• To argue that the exclusion is based on a flawed assumption about crowding limiting dynamic co-add (DC) image quality, which can be overcome with advanced image subtraction.
• To show that excluding the Galactic plane sacrifices critical Galactic-plane science—such as planet distribution, isolated black holes, and pre-explosion lightcurves—without meaningful benefit to other LSST science.
• To quantify the scientific return of including the Galactic plane, especially for microlensing events and astrometric microlensing of compact objects.
• To advocate for a revised LSST observing strategy that includes the Galactic plane using proven image subtraction techniques to maximize scientific yield.

Proposed method

• Uses image subtraction analysis to mitigate crowding in dense Galactic plane fields, enabling detection of transient events like microlensing and novae in crowded regions.
• Applies microlensing lightcurve modeling to estimate detectability of planetary systems, particularly those beyond the snow line, via finite-source effects and magnification signatures.
• Estimates microlensing event rates in the Galactic disk: over 250 disk-lens/disk-source events per year with $r < 19$, including 10% with $A > 100$, which are highly sensitive to planets.
• Models the recovery of astrometric microlensing signals from compact objects (e.g., black holes) using LSST’s astrometric precision, achievable even in crowded fields with proper data reduction.
• Compares LSST’s planned observation strategy (excluding Galactic plane) with an alternative that includes it, showing negligible cost to high-latitude science but massive gain in Galactic science.
• Analyzes LSST’s observation logs and documentation (e.g., Abate et al. 2012, Ivezić 2013) to identify inconsistencies in the exclusion rationale, particularly the lack of technical justification for avoiding the plane.

Experimental results

Research questions

• RQ1 What is the scientific cost of excluding the Galactic plane from LSST’s synoptic survey, particularly for microlensing-based planet detection?
• RQ2 Can image subtraction techniques effectively recover transient signals (e.g., microlensing, novae, supernovae precursors) in crowded Galactic plane fields?
• RQ3 How many microlensing events with high magnification ($A > 100$) would LSST detect annually in the Galactic disk, and what fraction would be sensitive to planets?
• RQ4 What is the impact of excluding the Galactic plane on the detection of isolated black holes and other compact objects via astrometric microlensing?
• RQ5 Is the exclusion of the Galactic plane justified by technical constraints, or is it a scientifically avoidable limitation?

Key findings

• LSST’s planned exclusion of the Galactic plane would result in the loss of over 250 disk-lens/disk-source microlensing events per year with $r < 19$, including 10% with $A > 100$, which are highly sensitive to planets.
• Image subtraction techniques can effectively overcome crowding in the Galactic plane, enabling detection of microlensing, transits, and other transients without compromising data quality.
• The exclusion of the Galactic plane would severely limit the ability to study the Galactic distribution of planets, especially those beyond the snow line, which are otherwise accessible via microlensing.
• The same LSST data would enable detection of pre-explosion lightcurves of supernovae, stellar mergers, and novae, as well as proper motions of stars beyond GAIA’s reach.
• The scientific cost of excluding the Galactic plane is catastrophic for Galactic science, while the benefit to extragalactic science is minimal, as high-latitude regions are already better suited for such studies.
• Revising LSST’s strategy to include the Galactic plane would yield immense scientific returns—especially in exoplanet science, stellar astrophysics, and Galactic structure—without significant cost to other LSST science objectives.