Below the iceberg: Low surface brightness astronomy with HST, Euclid, and Roman

Our current view of the universe is tightly limited by the light intensity ranges that we are able to detect. Like icebergs partially afloat, apparently well-defined and non-interacting galaxies such as M31 reveal a myriad of satellites, halos, and tidal tails when observed at surface brightness magnitudes lower than 30 mag arcsec⁻². Such structures are not always predicted by current Lambda-CDM models, making deep optical and NIR imaging the next frontier for galaxy evolution and cosmology. Extended low surface brightness sources like outer galactic discs, stellar halos, ultra-diffuse galaxies, or the intracluster light trace provide strong observational tests for cosmology and the structure of Dark Matter. Nevertheless, these features are thousands of times dimmer than the sky background, requiring an unprecedented advance in image processing to push the surface brightness limits to such faint levels. Upcoming observatories like NASA/Nancy Grace Roman or ESA/Euclid have the potential to become major cornerstones for the study of galactic formation and evolution, not only at high redshift but also at the dim and extended envelopes of the objects in the Local Universe.

In this contribution, we describe some of the most important technical advances of deep astronomical imaging from space, including source detection, in-orbit detector calibration, as well stray-light correction. By using specific techniques to minimize unwanted systematic effects, structures can be detected down to ~31 mag arcsec⁻² in Hubble Space Telescope images. These new techniques make it possible to reveal the large scale structures that surround the galaxies with the HST, which will be critical for the Euclid/VIS survey (Borlaff et al. 2021b) and the Wide Field Instrument aboard the Nancy Grace Roman Space Telescope. We demonstrate these methods in a new version of the WFC3/IR Hubble Ultra Deep Field (ABYSS HUDF, Borlaff et al. 2019), which improves the detection of extended stellar halos that were invisible until now, revealing that they were double their size at z=0.6-1.0. This new methodology will be fundamental for future space missions with an extremely wide field of view if we want to exploit their capabilities to their true limit, providing unique datasets to study the traces of cosmological evolution in the local Universe and beyond.