Sky subtraction that preserves low-surface-brightness flux

Project information

Project leads: Aaron Watkins (Hertfordshire), Sugata Kaviraj (Hertfordshire), Chris Collins (LJMU), Rubin Algorithms and Pipeline Team (Lee Kelvin, Yusra Al-Sayyad, Sophie Reed, Dan Taranu, Robert Lupton).

This work package is part of the LSST:UK in-kind contribution, with the Galaxies SC as a secondary recipient (the primary recipient is the Rubin Algorithms and Pipeline Team).

Point of contact: Aaron Watkins (a.watkins@herts.ac.uk)

Rubin project code: UKD-UKD-S6

Relevant working groups: Low-surface-brightness Science

Project status: Active

Rationale

Our statistical understanding of how the Universe evolves is strongly determined by the objects and structures that are brighter than the surface-brightness limits of wide-area surveys. While huge strides have been made in understanding galaxy evolution over the last few decades, using surveys like the SDSS, our comprehension is naturally constrained by aspects of the Universe that are actually observable in such datasets. For example, the completeness of galaxies in surveys like the SDSS decreases rapidly, to below 10 per cent, for objects with effective surface brightnesses fainter than ~24.5 mag arcsec-2.

The low-surface-brightness (LSB) regime, defined as the domain that is invisible in past wide-area surveys like the SDSS, contains a wealth of information that is essential for understanding how the observable Universe evolves over cosmic time. For example, all dwarf galaxies at cosmological distances lie in this regime, as do LSB structures (e.g. merger-induced tidal features and intra-cluster light) that offer fundamental constraints on the evolution of the observable Universe.

Under ideal conditions, LSST is capable of reaching depths fainter than μ~30 - 31 mag arcsec-2 in most photometric bands over around 20,000 square degrees. The LSB Universe thus represents virtually all the extra-galactic discovery space of this transformational survey. However, LSB structures are acutely sensitive to sky over-subtraction. Preservation of LSB flux in LSST images is, therefore, a key requirement of the data-processing pipeline, without which the discovery space of LSST could be severely limited.

This work package has tested the performance of the DM pipeline in terms of the preservation of LSB flux (see results section below) and is now developing algorithms that will preserve LSB flux in LSST images.

Results

Exploration of the effect of pipeline sky subtraction on low-surface-brightness science (A. Watkins et al.)

This report uses injections of mock images into the Rubin pipeline to quantify the effect of sky subtraction on the detection and characterisation of low-surface-brightness structure (e.g. dwarf galaxies, intra-cluster light and faint tidal features).