Photometric stellar masses for galaxies in DESI Legacy Imaging Surveys

In many areas of extragalactic astrophysics, we need to convert the luminosity of a galaxy into its stellar mass. In this work, we aim to find a simple and effective formula to estimate the stellar mass from the images of galaxies delivered by the currently popular DESI Legacy Imaging Surveys. This survey provides an unsurpassed combination of deep imaging and extensive sky coverage in up to four photometric bands. We calibrated the desired formula using a sample of local galaxies observed in the Spitzer Survey of Stellar Structure in Galaxies (S4G), which was specifically dedicated to measuring stellar masses. For the absolute magnitudes, Mg and Mr, of a galaxy in the g and r bands of the Legacy Surveys, we can estimate the stellar masses as 0.673Mg − 1.108Mr + 0.996 with a scatter of 25%. Employing more complex functions does not improve the estimate appreciably, even after including the galaxy ellipticity, Sérsic index, or the magnitudes in different Legacy surveys bands. Generally, measurements in the r band were the most helpful, while adding z-band measurements did not improve the mass estimate much. We provide a Python-based script, <ns0:monospace>photomass_ls.py</ns0:monospace>, to automatically download images of any galaxy from the Legacy surveys database, create image masks, generate GALFIT input files with well-assessed initial values, perform GALFIT photometry, and calculate stellar mass estimates. Additionally, we tuned another version of the formula to the magnitudes provided by the Siena Galaxy Atlas 2020 (SGA-2020) with a scatter of 29%. For both our default and SGA-2020 formula, we offer two alternatives derived from different calibrations of S4G masses based on different methods and assumptions.