The low-surface-brightness (LSB) Universe - broadly defined as the regime that is undetectable in past wide-area surveys - offers a vast untapped discovery space that will be unlocked by LSST's unique combination of depth and area (e.g. Kaviraj 2020, Brough et al. 2020). Statistical studies of galaxies and structures in this regime are likely to produce a step change in our understanding of galaxy evolution, making LSB studies and the associated infrastructure required to exploit LSST's deep-wide data a particular priority over the next few years.
The LSST Galaxies SC LSB WG meets as a whole every other month with a short science presentation and opportunity for discussions. The meeting times alternate between those suitable for Europe- or US- and Australia-based members. Please feel free to email WG co-chairs Mireia Montes (mireia.montes@iac.es) and Aaron Watkins (a.watkins@herts.ac.uk) if you would like to join the WG and/or give a science presentation at our next meeting. We advertise meetings and share additional information via email and via the LSSTC slack channels #galaxies and #galaxies-lsb.
LSB WG Members are listed in this spreadsheet along with short descriptions of their LSST-related research.
The LSST Galaxies SC LSB WG is currently focussed on four Challenges to prepare for LSB science with LSST. Each challenge group meets independently to the main LSB WG and has its own leader, LSST slack channel (#lsb-challenge1 /2 /3 /4) and google doc where meeting minutes are captured (linked below). WG members are welcome to contribute to the Challenges and should contact the Challenge leaders in the first instance. The four Challenges are:
Challenge#1: How do LSST algorithms do at detecting LSB sources? Led by Aaron Watkins (originally with Dan Prole)
Challenge#2: What are the best ways to calculate distance for LSB galaxies? Led by Harry Ferguson
Challenge#3: Do observers and simulators measure the same quantity of ICL? Led by Sarah Brough (originally Mireia Montes)
Challenge#4. What are the most critical observables/measurables in LSB tidal features that will constrain theory? Led by Garreth Martin
Our statistical understanding of galaxy evolution is strongly influenced by the objects and structures that are brighter than the surface-brightness limits of wide-area surveys. Past wide-area surveys, like the SDSS, have enabled us to study the statistical properties of the galaxy population, based on hundreds of thousands of objects. Subsequent comparison of these datasets with cosmological simulations has allowed us to quantify the principal drivers of galaxy evolution over cosmic time.
While much progress has been made, our understanding of galaxy evolution is fundamentally constrained by aspects of the Universe that are actually observable in past surveys. The completeness of objects in surveys like the SDSS decreases rapidly for surface brightnesses fainter than ~24.5 mag arcsec-2 (e.g. Driver et al. 2005), rendering much of the low-surface-brightness (LSB) regime (e.g. van Dokkum et al. 2014; Greco et al. 2018), which includes the dwarf galaxy population, inaccessible at cosmological distances (e.g. Martin et al 2019).
Nevertheless, both theory (Martin et al. 2019) and observation (e.g. Dalcanton et al. 1997) indicate that the majority of galaxies actually resides in the LSB/dwarf regime. For example, ~50 (~85) per cent of galaxies down to 108 (107) MSun inhabit this regime (Table 2 in Martin et al. 2019). This has two important consequences. First, our empirical knowledge of galaxy evolution is based on the 'tip of the iceberg' i.e. a small subset of high surface-brightness systems. Second, and more importantly, our understanding of the physics of galaxy evolution is predicated on a small subset of the galaxy population. Both of these facts render our understanding of how the Universe evolves highly incomplete. Not surprisingly, many of the well-known tensions between theory and observation are in the LSB/dwarf regime, e.g. the apparent underproduction of dwarfs seen in simulations (e.g. Moore et al. 1999) and the core-cusp problem (e.g. Navarro et al. 1996).
Apart from incompleteness in the galaxy population itself, there are key LSB components of even high-surface-brightness galaxies that are largely invisible in past wide-area surveys (at least outside the local Universe), but which offer strong constraints on our theoretical paradigm. Two examples are merger-induced low-surface-brightness tidal features and intra-cluster light (ICL).
Tidal features encode galaxy assembly histories, making them excellent tracers of our structure-formation model. However, the surface-brightness of tidal features is inversely correlated with merger mass ratio. Given that low-mass galaxies far outnumber their massive counterparts, most mergers involve low mass ratios (i.e. are 'minor' mergers). These typically produce very faint tidal features that are undetectable in past wide-area surveys (e.g. Kaviraj et al. 2010; Duc 2015). Nevertheless, both theory (e.g. Oser et al. 2012; Martin et al. 2018) and observation (e.g. Kaviraj 2014) suggest that minor mergers are key drivers of galaxy evolution, making the analysis of LSB tidal features vital to our understanding of galaxy evolution.
In a similar vein, ICL is a significant component of galaxy clusters, which are important tests of our cosmological model. Since the ICL contributes a significant (and sometimes a dominant fraction) of the baryonic mass budget of clusters (e.g. Burke et al. 2012, Montes et al. 2021), the utility of clusters is closely linked to our ability to detect and characterize the ICL over cosmic time.