LIGHTS

A robust technique to identify galaxy edges

¹ Instituto de Astrofísica de Canarias,c/ Vía Láctea s/n, E38205 La Laguna, Tenerife, Spain
² Departamento de Astrofísica, Universidad de La Laguna, E-38205 La Laguna, Tenerife, Spain
³ Steward Observatory and Department of Astronomy, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA
⁴ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, 08193 Barcelona, Spain
⁵ Centro de Estudios de Física del Cosmos de Aragon (CEFCA), Plaza San Juan, 1, E-44001 Teruel, Spain
⁶ School of Physics and Astronomy, University of Nottingham, University Park, Nottingham NG7 2RD, UK
⁷ NASA Ames Research Center, Space Science and Astrobiology Division, M.S. 245-6 Moffett Field CA 94035, USA
⁸ Department of Physics and Astronomy, University of Padova, Vicolo Osservatorio 3 I-35122, Italy
⁹ Departamento de Física Teórica, Atómica y Óptica, Universidad de Valladolid, 47011 Valladolid, Spain
¹⁰ Departamento de Física de la Tierra y Astrofísica, Universidad Complutense de Madrid, E-28040 Madrid, Spain

^⋆ Corresponding author.

Received: 24 April 2025
Accepted: 30 June 2025

Abstract

The LIGHTS survey is imaging galaxies at a depth and spatial resolution comparable to what the Legacy Survey of Space and Time (LSST) will produce in 10 years (i.e., ∼31 mag/arcsec²; 3σ in areas equivalent to 10″×10″). This opens up the possibility of probing the edges of galaxies, as the farthest location of in situ star formation, with a precision that we have been unable to achieve in the past. Traditionally, galaxy edges have been analyzed in one dimension through ellipse averaging or visual inspection. Our approach allows for a two-dimensional exploration of galaxy edges, which is crucial for understanding deviations from disk symmetry and the environmental effects on galaxy growth. In this paper, we propose a novel method using the second derivative of the surface mass density map of a galaxy to determine its edges. This offers a robust quantitative alternative to traditional edge-detection methods when deep imaging is available. Our technique incorporates Wiener-Hunt deconvolution to remove the effect of the point spread function from the galaxy itself. By applying our methodology to the LIGHTS galaxy NGC 3486, we identify the edge at 205″ ± 5″. At this radius, the stellar surface mass density is ∼1 M_⊙/pc², supporting a potential connection between galaxy edges and a threshold for in situ star formation. Our two-dimensional analysis of NGC 3486 reveals an edge asymmetry of ∼5%. These techniques will be of paramount importance for a physically motivated determination of the sizes of galaxies in ultra-deep surveys such as LSST, Euclid, and Roman.