Tuesday

The `low-surface-brightness’ (LSB) regime (i.e., the domain that is inaccessible in past large surveys) defines the discovery space for LSST, which can theoretically achieve unprecedented imaging depths of 30--31 magnitudes/arcsec^2. However, achieving this depth requires a carefully tailored data reduction pipeline. Specifically, if the night sky is even slightly over-subtracted, this can degrade LSB structures (e.g. dwarf galaxies, tidal streams, or intra-cluster light) greatly limiting the potential science cases LSST can address. Working with the Rubin Algorithms and Pipeline Team at Princeton, via an STFC-funded LSST:UK work package (which has been accepted as an UK in-kind contribution), we have led a year-long effort to measure the impact of the current pipeline's sky subtraction on the flux of astrophysical objects, using injections of model sources. This effort has demonstrated a systematic pipeline-induced over-subtraction of flux, beginning at 10% around 26 magnitudes/arcsec^2 and becoming increasingly more severe with decreasing surface brightness. Magnitudes of LSB objects are significantly impacted, with a non-negligible effect on high-surface-brightness objects as well, making much of extragalactic discovery science unattainable with the current pipeline. Given the severity of this problem, we describe our plan to mitigate these issues, to make new extragalactic science more feasible using LSST.