2025A&A...697A..91R

Ultra-deep optical imaging surveys have reached unprecedented depths (≳30 mag arcsec^–2; 3σ, 10″ × 10″), thus facilitating the study of very faint galactic structures. However, the ultraviolet bands, which are key to the study of stellar populations, remain essentially unexplored at these depths. In this paper, we present a detailed surface brightness and color analysis of the outermost regions of 20 nearby galaxies in the LBT Imaging of Galactic Haloes and Tidal Structues (LIGHTS) fields observed by GALEX in the far-UV and near-UV. We adapted and applied a low surface brightness oriented methodology that has proven effective in ultra-deep optical surveys. We propose a novel approach to background subtraction for UV imaging. Instead of subtracting a constant value from the background, we subtract a Poisson distribution that transforms the background into a pseudo-Gaussian distribution centered at zero. Furthermore, the point spread function (PSF) deconvolution algorithms developed for optical data are applied to our sample, using a novel set of very extended (R = 750″) PSFs for the GALEX bands. This methodology allowed us to obtain depths ranging from 28.5 to 30 mag arcsec^–2 (3σ; 10″ × 10″) with reliable surface brightness profiles up to 31 mag arcsec^–2. This is about 1 mag deeper than with standard UV techniques. We used the surface brightness and color profiles to show that the application of PSF deconvolution, especially in the far-UV, effectively mitigates the excess of light present in the outer regions of certain galaxies compared to the standard GALEX pipeline. This finding is crucial for any accurate stellar population inference from the color profiles. Additionally, a qualitative analysis of the results is presented, with particular emphasis on the surface brightness and color properties of the galaxies beyond their optical edges. Our work highlights the importance of developing innovative low surface brightness methods for UV surveys.