Dissecting ultra-diffuse galaxies in the field

Abstract

Context. Ultra-diffuse galaxies (UDGs) in the field are faint, diffuse systems that remain poorly represented in the literature due to the need for spectroscopic confirmation and the difficulty of obtaining high S/N emission line measurements. Aims. We present a spectroscopic study of 17 blue UDG candidates in the field using optical emission lines to confirm their diffuse nature and properties. Methods. We derived spectroscopic redshifts($z_{\rm spec}$) for our field UDG candidates. We then computed their effective radii ($r_{\rm eff}$) and central surface brightnesses ($μ_{0,g}$). We measured the H$α$ and H$β$ emission line fluxes in the 17 spectra and derived star-formation rates (SFR) from the line-luminosity relation. We performed forced photometry on our sample to obtain photometric fluxes and applied an aperture correction on the H$α$ integrated fluxes, propagating the correction to the derived SFRs. We then computed stellar masses ($M_*$) using colour relations and estimated dust attenuation and corrected the SFRs accordingly. Two sources were further examined as potential giant low surface-brightness galaxies(GLSBGs). Results. We identify nine confirmed UDGs, eight other low surface-brightness galaxies (LSBGs), including one GLSBG. The $z_{\rm spec}$ of our field UDGs span a range of $0.015-0.037$, their $r_{\rm eff}$ spans $1.69-4.99$ kpc and $μ_{0,g}$ between $24.05-24.98~{\rm mag~arcsec}^{-2}$. 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 the aperture correction performed, lie along the star-forming main sequence. Conclusions. 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.