The ASAS-SN Low Surface Brightness Survey I: Proof-of-Concept and Potential Applications

Abstract: The ASAS-SN Low Surface Brightness Survey utilizes the $\sim7$ years of g-band CCD data from ASAS-SN (The All-Sky Automated Survey for Supernovae) to create stacked images of the entire sky. It is significantly deeper than previous photographic surveys. Our median/95th percentile cumulative exposure time per field is 58.1/86.8 hours, and our median $3{\sigma}$ g-band surface brightness limit off the Galactic plane ($|b| > 20{\deg}$) is 26.1 mag arcsec$^{-2}$. We image large-scale diffuse structures within the Milky Way, such as multiple degree-spanning supernova remnants and star-forming nebulae, and tidal features of nearby galaxies. To quantify how effective our deep images are, we compare with a catalog of known ultra-diffuse galaxies and find a recovery rate of 82$\%$. In the future, we intend to use this data set to perform an all-sky search for new nearby dwarf galaxies, create an all-sky Galactic cirrus map, create an all-sky low surface brightness mosaic for public use, and more.

• The paper presents a novel image stacking method using seven years of ASAS-SN data to achieve a 26.1 mag arcsec⁻² sensitivity for LSB detection.
• It demonstrates an 82% recovery rate of known ultra-diffuse galaxies, validating its methodology for uncovering faint cosmic structures.
• The survey’s extensive sky coverage and median 58.1-hour exposure per field enable detailed studies of Galactic structures and tidal features in nearby galaxies.

The ASAS-SN Low Surface Brightness Survey I: Proof-of-Concept and Potential Applications

The paper "The ASAS-SN Low Surface Brightness Survey I: Proof-of-Concept and Potential Applications" presents the methodology and applications of a new low surface brightness (LSB) survey conducted using data from The All-Sky Automated Survey for Supernovae (ASAS-SN). Utilizing approximately seven years of $g$-band CCD data, the authors have created a deep, stacked image survey of the entire sky, significantly surpassing the depth of previous photographic surveys, with a median surface brightness sensitivity of 26.1 mag arcsec$^{-2}$ at a 3$\sigma$ confidence level off the Galactic plane.

This survey addresses the challenge of observing LSB sources, which are generally fainter than the natural night sky glow, making their detection and analysis technically demanding. The ASAS-SN Low Surface Brightness Survey achieves this by coadding images to enhance depth, achieving a median cumulative exposure time per field of 58.1 hours. The resulting sky survey allows for the detection and study of extensive diffuse structures within the Milky Way, such as supernova remnants (SNRs) and star-forming nebulae, as well as tidal features in nearby galaxies.

The paper evaluates the effectiveness of the ASAS-SN survey by comparing its findings with a catalog of known ultra-diffuse galaxies (UDGs). The recovery rate of known ultra-diffuse galaxies using this survey is 82%, showing substantial potential for discovering new dwarf galaxies and other faint structures across the sky. The authors also highlight the survey's capacity to support future projects, such as generating an all-sky map of Galactic cirrus and a low surface brightness mosaic for public use.

Key Results

• Survey Depth and Coverage: The survey achieves a median $3\sigma$ sensitivity limit of 26.1 mag arcsec$^{-2}$, with the vast majority of the sky outside the Galactic plane reaching the formal LSB regime (sensitivity greater than 24 mag arcsec$^{-2}$).
• Image Stacking and Calibration: By stacking images from multiple CCD observations, the authors significantly extend the depth of sky coverage, achieving a single camera stack point-source sensitivity of $g \approx 21.75$ mag.
• Detection Efficacy: The survey successfully recovers known LSBG features, with 75% of off-Galactic plane fields reaching the 24 mag arcsec$^{-2}$ sensitivity threshold, indicating effective background suppression through image stacking.

Applications and Future Directions

The survey's capability to conduct large-scale LSB observations opens avenues for various astrophysical investigations:

1. Galaxy Evolution and Cosmology: The ASAS-SN provides a data set with which to study the low brightness outskirts of galaxies, galactic halos, and tidal interaction features, essential for understanding galaxy formation and evolution.
2. Milky Way Structure and Star Formation: With its wide-field view and depth, the survey assists in studying large Galactic structures, providing insights into the distribution of star-forming regions and the structure of the interstellar medium (ISM).
3. Public Data Products: A major goal is to create publicly available data products, including an all-sky LSB mosaic, thus supporting the broader scientific community in LSB research.

Conclusion

The ASAS-SN Low Surface Brightness Survey exemplifies the effective reuse of existing time-domain astronomy data for LSB research, providing a feasible model for future LSB observations across the sky. The survey's promising initial results affirm its potential as an essential tool for discovering new astrophysical phenomena, advancing both theoretical and observational astronomy. The paper sets the stage for further developments, including improvements in data processing, expanded scientific applications, and the enhancement of the ASAS-SN survey through multi-camera integrations. The work offers a valuable contribution to the understanding of LSB phenomena in both galactic and extragalactic contexts.