Introduction
Those with ideas for future newsletter items should contact the LSST:UK Project Managers (George Beckett and Terry Sloan: lusc_pm@mlist.is.ed.ac.uk), while everyone is encouraged to subscribe to the Rubin Observatory Digest for more general news from the US observatory team.
The low-surface-brightness Universe: unlocking LSST’s extragalactic discovery space
The low-surface-brightness (LSB) Universe, defined as the domain that is undetectable in past wide-area surveys, represents the entirety of the extra-galactic discovery space in LSST’s deep-wide images. LSB galaxies (which include all dwarf galaxies at cosmological distances and massive diffuse disks) dominate the galaxy number density. In addition, LSB structures such as intra-cluster light and merger-induced tidal features, offer strong constraints on our theoretical paradigm, particularly the build up of stellar mass with cosmic time. However, while LSST’s unique combination of area and depth is capable of opening up the LSB Universe, its ability to do so depends on the preservation of LSB flux by the data processing pipeline. Developing precise sky subtraction algorithms that preserve LSB flux is key to the delivery of LSST’s extragalactic discovery space and is the purpose of this project (WP 3.7).
To date, we have developed metrics to quantify the data management (DM) pipeline's impact on LSB flux: specifically, the over-subtraction of astrophysical LSB objects in terms of both their total magnitudes and radial surface brightness profiles. Using flat model injections, we have established a baseline for this impact and demonstrated a serious short-coming in the DM pipeline: over-subtraction occurs consistently for objects with surface brightnesses fainter than 26 magnitudes/arcsec^2 in all wavebands, far short of LSST's theoretical ability to reach limits of 30--31 magnitudes/arcsec^2. This compromises vast swathes of extra-galactic science and prevents LSST from achieving its full discovery space. The results were presented at the Rubin Project and Community Workshop in August 2020.
Our flat models have only provided a baseline, however, hence more testing is needed to uncover the specific nature of the problem. In the future, we will do this by injecting more realistic galaxies, spanning a wider range of profile shapes (including very extended objects such as intra-cluster light), as well as stamps from cosmological simulations containing LSB tidal features. Using these models to measure our metrics will enable us to explore specific aspects of the DM pipeline that might be revised to preserve LSB flux. Once known, we can then develop sky-subtraction algorithms that preserve LSB flux down to LSST's theoretical surface brightness limits. Given the conflicting needs of the extra-galactic and weak-lensing communities, however, this could require a bifurcation of the pipeline, which we will work towards delivering over the duration of this project.
The First Rubin Operations Bootcamp
The Observatory held its first Operations Bootcamp on 13-15 October 2020. As the name suggests, this was primarily aimed at staff starting work in the Rubin operations team - most of whom are starting to transition from the Construction project, as that draws to a close - but an invitation to participate was extended to some collaborators in the UK and France, and the materials from the Bootcamp are now available on a public wiki page. The topics covered over the three days ranged widely, as did the level of detail, but several are likely to be of interest to many readers of this newsletter, as they providing high-level updates. In particular, I recommend Part 1 of Day 1, which presents an overview of the project structure in operations and the Moving towards operations session that took up most of Day 3; the latter contains an update on Commissioning plans (although still not a revised schedule, as the “rebaselining” of the final stages of the construction project schedule due Covid delays is still being discussed with the US agencies), plus a discussion of possibilities for Early Science (i.e. that which can appear before Data Release 1) and of plans for the work of the Community Engagement Team.
Of particular interest in the Community Engagement Team (CET) section is the discussion of planning for Data Preview 0 (DP0). DP0 will be the first opportunity for some test users to access LSST-like data (TBC, but probably Hyper Suprime-Cam PDR2 and DESC DR2 simulations) through the Rubin Science Platform, which will be the system through which access to data releases will be provided. The CET is planning to have ~300 test users - to be called DP0 Delegates - who will have that access in return for some sort of contribution of benefit to the Observatory. This is still being discussed, but it could involve reporting back on running some test science case, or completing a feedback service, or helping to disseminate information about the Rubin Science Platform by running some satellite training event.
I am sure that a number of LSST:UK Consortium members would be interested in being DP0 Delegates, and we will keep you posted as we hear more about how this will work.