Dissecting ultra-diffuse galaxies in the field. Observational biases and spectroscopic determination of physical properties in low surface brightness systems

Abstract

Ultra-diffuse galaxies (UDGs) in the field, gravitationally isolated from galaxy clusters, are faint systems that remain poorly represented in the literature due to lack of spectroscopic confirmation and the difficulty of obtaining high signal-to-noise emission-line measurements. We present a spectroscopic study of 17 blue UDG candidates in the field using optical emission lines to confirm their ultra-diffuse nature and characterise their physical properties. Data were collected using the MODS instrument at the Large Binocular Telescope Observatory and the MISTRAL instrument at the Observatoire de Haute-Provence. Using the software we derived spectroscopic redshifts ( for our field UDG candidates. We then computed their effective radii (r_ ) and central surface brightnesses (μ_ ). We measured the Hα and Hβ emission line fluxes in the 17 spectra and derived star formation rates (SFRs) from the line-luminosity relation. We performed forced photometry on our sample to obtain photometric fluxes and applied an aperture correction on the Hα integrated fluxes, propagating the correction to the derived SFRs. Using colour relations and estimated dust attenuation, we then computed stellar masses ( eff 0,g and corrected the SFRs accordingly. Finally, we placed our sample on the star-forming main sequence, comparing its distribution with that of local star-forming galaxies. Two sources were further examined as potential giant low surface brightness galaxies (GLSBGs). In our samples, we identify nine confirmed UDGs, eight other low surface-brightness galaxies (LSBGs), including one GLSBG. The $ as an upper limit and 7.20 łeq łMstar łeq 8.08 _⊙. These values place them along the star-forming main sequence. of our field UDGs span a range of 0.015 - 0.037. Their r_ , and their μ_0,g range from 24.05 - 24.98 . Consistent with previous studies, the galaxies exhibit low to moderate dust content, with an average V-band attenuation of 0.29 mag. The spectroscopically confirmed UDGs presented in this work, after aperture correction, exhibit -2.95łeq łSFR $ łeq-1.60 _⊙, ^ ranges from 1.69 - 4.99 kpc mag -2 M yr -1 M Our results indicate that blue field UDGs are characterised by heterogeneous dust attenuation and occupy the same region of the star formation–stellar mass plane as dwarf LSBGs. This is consistent with the broader population of local star-forming galaxies and indicates no distinct global star-forming behaviour.