Ultra-diffuse galaxies are a relatively new class of galaxies that were first discovered in 2015. They are as large as our Milky Way but have between 100 to 1,000 times fewer stars than our own Galaxy, making them barely visible and difficult to study. All previously studied ultra-diffuse galaxies have been in galaxy clusters, which informed the basis for the theory that they were once ‘normal’ galaxies, but with time have been blasted into a fluffy mess due to violent events within the cluster. Now Dr. Ignacio Martín-Navarro from the University of California Observatories and colleagues have discovered a bizarre, solitary ultra-diffuse galaxy. Named DGSAT I, this galaxy contradicts the current theory on the formation of these transparent, ghost-like galaxies.
The ultra-diffuse galaxy DGSAT I. Image credit: Aaron Romanowsky / University of California Observatories / D. Martinez-Delgado / ARI.
“There seemed to be a relatively tidy picture of the origins of galaxies, from spirals to ellipticals, and from giants to dwarfs,” Dr. Martín-Navarro said.
“However, the recent discovery of ultra-diffuse galaxies raised new questions about how complete this picture is.”
“All of the ultra-diffuse galaxies that have been studied in detail so far were within galaxy clusters: dense regions of violent interaction where the galaxies’ characteristics at birth have been scrambled up by a difficult adolescence.”
Because DGSAT I is a rare exception of an ultra-diffuse galaxy found away from a cluster, it can provide a clearer window into the past.
In order to find out what caused this galaxy to be so sparse in starlight, Dr. Martín-Navarro and co-authors used the Keck Cosmic Web Imager at the Keck Observatory to map the composition of the galaxy.
“The chemical composition of a galaxy provides a record of the ambient conditions when it was formed, like the way that trace elements in the human body can reveal a lifetime of eating habits and exposure to pollutants,” said co-author Dr. Aaron Romanowsky, from the University of California Observatories and San José State University.
DGSAT I surprised the astronomers with its chemical makeup.
Today’s galaxies typically have more heavy elements in them, like iron and magnesium, compared to ancient galaxies born just after the Big Bang.
But the team revealed that DGSAT I appears to be anemic; the galaxy’s iron content is remarkably low, as if it formed from a nearly pristine gas cloud that was unpolluted by the supernova death of previous stars.
And yet DGSAT I’s magnesium levels are normal, consistent with what astronomers expect to find in modern galaxies. This is strange because both of these elements are released in supernova events; you typically don’t find one without the other.
“We don’t understand this combination of pollutants, but one of our ideas is that extreme blasts of supernovae caused the galaxy to pulsate in size during its adolescence, in a way that retains magnesium preferentially to iron,” Dr. Romanowsky said.
The team’s results appear in the Monthly Notices of the Royal Astronomical Society.
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Ignacio Martín-Navarro et al. 2019. Extreme chemical abundance ratio suggesting an exotic origin for an ultradiffuse galaxy. MNRAS 484 (3): 3425-3433; doi: 10.1093/mnras/stz252
