Low Surface Brightness Science with Rubin: Unlocking LSST's Discovery Domain I

/meetings/rubin2022/sites/default/files/Low%20Surface%20Brightness.movThe co-chairs of the Galaxies SC Low Surface Brightness Working Group, Aaron Watkins and Sarah Brough, DM representative Lee Kelvin, and representatives from other Science Collaborations interested in low surface brightness astronomy, Ian Dell'Antonio, Annika Peter, and Yao-Yuan Mao, Peregrine McGehee, and Stephen Smartt have organized two breakout sessions on low surface brightness (LSB) astronomy with Rubin.

This, the first session, will be remotely accessible. The second session will be in-person only. All are welcome to attend one or both sessions.


    Agenda for Session 1

    • Lee Kelvin (remotely): "Reducing LSB Data with the LSST Science Pipelines"
    • Aaron Watkins: "Exploring alternative sky-subtraction algorithms for the LSST pipeline"
    • Garreth Martin: "Preparing for low surface brightness science with the Vera C. Rubin Observatory: Characterization of tidal features from mock images"
    • Nushkia Chamba (remotely): "The edges of galaxies"
    • Open discussion


    Chair & Helper Roles 

    • note taker: Aaron Watkins
    • time keeper: Sarah Brough
    • breakout summaries slide presenter: Aaron Watkins
    • virtual moderator: Sugata Kaviraj


    Abstracts for Session 1

    • Lee Kelvin
      • Title: Reducing LSB Data with the LSST Science Pipelines
      • Abstract: This talk will provide an overview of the data reduction process using the LSST Science Pipelines, with a focus on tasks which have the largest impact on LSB science. The various stages of background subtraction will be presented, and an overview given on future plans for stress testing these algorithms in a continuous integration framework. Finally, novel techniques to assist in background subtraction will be discussed, with preliminary results presented on their viability.
    • Aaron Watkins
      • Title: "Exploring alternative sky-subtraction algorithms for the LSST pipeline"
      • Abstract: Aggressive local sky-subtraction algorithms, like those typically employed in large-scale surveys, are excellent for separating point sources or sources with small angular size from the omnipresent diffuse astrophysical foreground. However, this diffuse astrophysical foreground itself contains a vast amount of information, from stellar tidal streams to intracluster light to Galactic cirrus. Preserving this foreground in LSST--even if only as a separate data product which can be removed as needed to aid in deblending--should therefore serve to greatly expand the survey's scientific reach beyond its four key goals. To this end, we have explored, using both fully synthetic data and real observations, alternative sky-subtraction algorithms, which target only telluric emission while avoiding accidental over-modeling and removal of low-surface-brightness astrophysical flux. We discuss three possible options, from the most simple to the most complex, including the expected benefits and drawbacks of each.
    • Garreth Martin
      • Title: "Preparing for low surface brightness science with the Vera C. Rubin Observatory: Characterization of tidal features from mock images"
      • Abstract: A natural consequence of the hierarchical structure formation paradigm is that an overwhelming majority of galaxies are expected to have undergone mergers or other types of tidal interaction. Such interactions leave behind long lived, but extremely faint and extended tidal features including tails, streams, loops and plumes which act as a fossil record for the host galaxy’s past interactions. Until now, surveys have generally compromised between depth and area and so suffer from shallow (and therefore biased) imaging or small homogeneous samples – resulting in weak observational constraints on the role of galaxy mergers and interactions. In the coming years Rubin Observatory will observe the entire Southern sky with sufficient depth to uncover undergoing tidal interactions in potentially millions of galaxies. Using realistic mock images produced with state-of-the-art cosmological simulations we have performed a comprehensive investigation of the expected nature, frequency and visibility of tidal features and debris across a range of environments and stellar masses as well as their reliability as an indicator of galaxy accretion histories, including how observational biases such as projection effects, the point-spread-function and survey depth may affect the proper characterisation and measurement of tidal features. In our simulated sample, tidal features are ubiquitous around MW mass galaxies but are also relatively common even at significantly lower masses. In fact, at sufficient depth, almost 100% of galaxies (M*/M⊙<10^9.5) possess tidal features and Rubin Observatory is expected to recover much of this flux – At a depth of 30.5 mag arcsec^-2, 80% of MW mass galaxies host detectable tidal features, falling to 60% with a more conservative 29.5 mag arcsec^-2 cut. When observed at sufficient depth, such objects frequently exhibit multiple (and sometimes very many) distinct tidal features often with complex shapes. When attempting to characterise such features, surface brightness limits, galaxy orientation and redshift all have a clear effect on the ability of expert classifiers to visually identify and characterise tidal features. Concurrence between classifiers generally improves with deeper imaging but morphologies can become more complex, introducing uncertainty in their precise characterisation. Building on this work, I will detail our current efforts to make reliable automated measurements of simulated tidal features, allowing us to make direct comparisons with human classifications as well as make predictions of the expected distribution of tidal feature properties (length, colour etc) and their expected surface brightness distribution.
      • Nushkia Chamba
        • Title: "The edges of galaxies"
        • Abstract: Galaxies form new stars across their entire structure. However, after reaching a certain radius in the outskirts, their star formation drops dramatically. The radial location of where such an abrupt drop in star formation occurs marks the edge of the stellar component in galaxies. In this contribution, we will present our analysis of the edges of almost 1000 nearby galaxies using ultra-deep optical imaging. We will present how the radial location of the edges of galaxies depends on their morphology and global stellar mass. In fact, we will show that the location of the edges of galaxies is strongly connected with their global stellar mass, producing one of the tightest scaling relationships (<0.1 dex) known for galaxies over nearly four orders of magnitude in stellar mass. We will discuss the implication of our findings in the context of galaxy formation and evolution.


      Background Information: the LSB Regime

      This is an important regime, holding much of LSST’s discovery space, and is a key area of current research that will be uniquely expanded upon by LSST’s combination of depth and area. For instance, galaxy evolution is traceable at a granular level through study of intracluster light and tidal streams (both Galactic and extragalactic), while faint dwarf galaxy populations (which are undetectable in past surveys at cosmological distances), via their shallow potential wells, are sensitive tracers of key processes such as tidal perturbation, ram-pressure stripping, and baryonic feedback. The end-stages of high-mass stellar evolution in the Milky Way are also visible through light echoes in supernova remnants, while those of lower mass stars can be seen in the extended diffuse emission of planetary nebulae. The detailed properties of dust grains can be studied via comparative optical and infrared observations of Galactic cirrus at varying elevations above the disk plane, and, on a more local level, comet tails and Zodiacal light contain clues to the history of the Solar System. LSB science therefore spans a broad range of topics accessible by LSST.

      In order to best detect and analyse the low surface brightness light in LSST images there are a number of key preparatory tasks we need to undertake due to the importance of sky foreground subtraction in preserving this light.  To this end, in-kind contributors are working with DM to implement continual tracking of surface brightness preservation metrics into the pipeline, and to revise the pipeline's sky-subtraction algorithm to ensure that all of LSST's potential discovery space is available. With LSST commissioning just around the corner and much effort underway, this is the ideal time to discuss how efforts made towards these preparatory tasks and strategies adopted can solve common problems across different Science Collaborations, to ensure that we are prepared for when data arrives. 

      Aaron E. Watkins, Sarah Brough, Lee Kelvin
      Suggested Audience: 
      Anyone from any astronomical sub-field interested in LSB work, including, broadly, those worried about the pipeline's potential impact on their science
      Data Management
      Applicable to: 
      Tuesday 08/09
      1:30pm - 3:00pm
      Dove Mountain Ballroom

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