Extreme chemical abundance ratio suggesting an exotic origin for an
  ultra-diffuse galaxy
  Ultra diffuse galaxies are a population of extended galaxies but with
relatively low luminosities. The origin of these objects remains unclear,
largely due to the observational challenges of the low surface brightness
Universe. We present here a detailed stellar population analysis of a
relatively isolated UDG, DGSAT I, based on spectroscopic data from the Keck
Cosmic Web Imager integral field unit. The star formation history of DGSAT I
seems to be extended, with a mean luminosity-weighted age of ~3 Gyr, in
agreement with previous photometric studies. However, we find a very high
[Mg/Fe] abundance ratio, which is extreme even in the context of the highly
alpha-enhanced massive ellipticals and ultra-faint dwarfs. The
[Mg/Fe]-enhancement of DGSAT I appears to be 10 times higher than the most
magnesium-enhanced stellar systems discovered to date, and suggests that the
chemical enrichment of this object was dominated by core-collapse supernovae.
Intriguingly, this breaks the canonical relation between [Mg/Fe] and star
formation time-scale. With a measured velocity dispersion of 56 +/- 10 km/s,
DGSAT I also shows a high mass-to-light ratio, which indicates that it is
highly dark matter-dominated. The metal-poor conditions of DGSAT I may have
enhanced the formation of massive stars, while at the same time, additional
mechanisms are needed to prevent iron-rich yields from being recycled into
stars. These results suggest that some ultra-diffuse galaxies could have
experienced chemical enrichment episodes similar to the first building blocks
of galaxies.
