Lessons on Star-forming Ultra-diffuse Galaxies from The Stacked Spectra of Sloan Digital Sky Survey

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 ${\log }_{10}(M_{\ast }/M_{\odot })\sim 8.57\pm 0.29$, 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 $t_{\ast }\sim 5.2\pm 0.5$ 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]$-0.14(\pm 0.40)$.

Accepted for publication in ApJL

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