Lessons on Star-forming Ultra-diffuse Galaxies from The Stacked Spectra of Sloan Digital Sky Survey. (arXiv:2007.12712v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Rong_Y/0/1/0/all/0/1">Yu Rong</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zhu_K/0/1/0/all/0/1">Kai Zhu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Johnston_E/0/1/0/all/0/1">Evelyn J. Johnston</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zhang_H/0/1/0/all/0/1">Hong-Xin Zhang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cao_T/0/1/0/all/0/1">Tianwen Cao</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Puzia_T/0/1/0/all/0/1">Thomas H. Puzia</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Galaz_G/0/1/0/all/0/1">Gaspar Galaz</a>
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 $log_{10}(M_*/M_{odot})sim 8.57pm0.29$, have the on-average
total-stellar-metallicity [M/H]$sim -0.82pm0.14$, iron-metallicity
[Fe/H]$sim -1.00pm0.16$, stellar age $t_*sim5.2pm0.5$ Gyr,
$alpha$-enhancement [$alpha$/Fe]$sim 0.24pm0.10$, and oxygen abundance
12+log(O/H)$sim 8.16pm0.06$, as well as central stellar velocity dispersion
$54pm12$ 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]$simeq-0.43(pm0.26)$[Fe/H]$-0.14(pm0.40)$.
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 $log_{10}(M_*/M_{odot})sim 8.57pm0.29$, have the on-average
total-stellar-metallicity [M/H]$sim -0.82pm0.14$, iron-metallicity
[Fe/H]$sim -1.00pm0.16$, stellar age $t_*sim5.2pm0.5$ Gyr,
$alpha$-enhancement [$alpha$/Fe]$sim 0.24pm0.10$, and oxygen abundance
12+log(O/H)$sim 8.16pm0.06$, as well as central stellar velocity dispersion
$54pm12$ 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]$simeq-0.43(pm0.26)$[Fe/H]$-0.14(pm0.40)$.
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