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 _ { \odot } ) \sim 8.57 \pm 0.29 , have the on-average total-stellar-metallicity [ M/H ] \sim - 0.82 \pm 0.14 , iron-metallicity [ Fe/H ] \sim - 1.00 \pm 0.16 , stellar age t _ { * } \sim 5.2 \pm 0.5 Gyr , \alpha -enhancement [ \alpha /Fe ] \sim 0.24 \pm 0.10 , and oxygen abundance 12+ \log ( O/H ) \sim 8.16 \pm 0.06 , as well as central stellar velocity dispersion 54 \pm 12 km/s .
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 ] \simeq - 0.43 ( \pm 0.26 ) [ Fe/H ] -0.14 ( \pm 0.40 ) .