UV LIGHTS

New tools for revealing the low surface brightness regime in the ultraviolet

¹ Instituto de Astrofísica de Canarias, c/ Vía Láctea s/n, E-38205 – La Laguna, Tenerife, Spain
² Departamento de Astrofísica, Universidad de La Laguna, E-38205 – La Laguna, Tenerife, Spain
³ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193 08193, Cerdanyola del Valles, Spain
⁴ Department of Physics and Astronomy, University of Padua, Vicolo Osservatorio 3, 35122 Padova, Italy
⁵ Steward Observatory and Department of Astronomy, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA
⁶ Departamento de Física Teórica, Atómica y Óptica, Universidad de Valladolid, 47011, Valladolid, Spain
⁷ NASA Ames Research Center, Moffett Field, CA 94035, USA
⁸ Centro de Estudios de Física del Cosmos de Aragón (CEFCA), Plaza San Juan, 1, E-44001, Teruel, Spain
⁹ School of Physics and Astronomy, University of Nottingham, University Park, Nottingham NG7 2RD, UK
¹⁰ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
¹¹ Departamento de Física de la Tierra y Astrofísica, Universidad Complutense de Madrid, E-28040 Madrid, Spain

^⋆ Corresponding author.

Received: 31 October 2024
Accepted: 17 March 2025

Abstract

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.