Abstract

We investigate the on-average properties for 28 star-forming ultra-diffuse galaxies (UDGs) located in low-density environments, by stacking their spectra from the Sloan Digital Sky Survey. These relatively isolated UDGs, with stellar masses of log(10) (M-*/M-circle dot) similar to 8.57 +/- 0.29, have the on-average total stellar metallicity [M/H] similar to -0.82 +/- 0.14, iron metallicity [Fe/H] similar to -1.00 +/- 0.16, stellar age t(*) similar to 5.2 +/- 0.5 Gyr, alpha-enhancement [alpha/Fe] similar to 0.24 +/- 0.10, and oxygen abundance 12+log(O/H) similar to 8.16 +/- 0.06, as well as central stellar velocity dispersion 54 +/- 12 km s(-1). On the star formation rate versus stellar mass diagram, these UDGs are located lower than the extrapolated star-forming main sequence from the massive spirals, but roughly follow the main sequence of low-surface-brightness dwarf galaxies. We find that these star-forming UDGs are not particularly metal-poor or metal-rich for their stellar masses, as compared with the metallicity-mass relations of the nearby typical dwarfs. With the UDG data of this work and previous studies, we also find a coarse correlation between [Fe/H] and magnesium-element enhancement [Mg/Fe] for UDGs: [Mg/Fe] similar or equal to -0.43(+/- 0.26) [Fe/H] -0.14(+/- 0.40).