Anemic galaxy reveals shortcomings in the theory of ultra-diffuse galaxy formation

Using the Keck Cosmic Web Imager (KCWI), the team discovered a strange ultra-diffuse solitary galaxy (UDG).

This transparent and ghostly galaxy, called DGSAT I, contradicts the current theory of GOD formation. All previously studied UDGs were in clusters of galaxies, which served as a basis for the theory that they were once "normal" galaxies, but over time they became a mess because of the fact that they were "normal" galaxies. violent events in the cluster.

"There seemed to be a relatively neat image of the origins of galaxies, from spirals to ellipticals and giants to dwarves," said lead author Ignacio Martín-Navarro, a postdoctoral researcher at UCO. "However, the recent discovery of UDGs has raised new questions about the completeness of this table.All UDGs that have been studied in detail so far were in clusters of galaxies: dense regions of the world. violent interactions where the characteristics of galaxies at birth have been blurred by a difficult adolescence ".

The results of the team will be published in the edition of April 11, 2019 of the Monthly Notices from the Royal Astronomical Society and is now available online.

Because BDSD I is a rare exception in a key distribution group found outside a cluster, it provides a better understanding of the past. There has not been much activity around him that can alter its composition and evolution. In order to find out what made this galaxy so insensitive to stars, the team used KCWI to map the composition of the galaxy.

"The chemical composition of a galaxy provides a record of the environmental conditions at the time of its formation, as well as the way in which the trace elements of the human body can reveal eating habits and exposure to long-lasting pollutants", said co-author Aaron Romanowsky. UCO Astronomer and Associate Professor in the Department of Physics and Astronomy of the State University of San José.

DGSAT I surprised the researchers with its chemical composition. Current galaxies usually contain more heavy elements, such as iron and magnesium, compared to ancient galaxies born just after the Big Bang. But KCWI revealed that DGSAT I appears to be anemic; the iron content of the galaxy is remarkably low, as if it was formed from an almost virgin gas cloud that was not polluted by supernova death from previous stars. And yet, DGSAT I's magnesium levels are normal, which is what astronomers expect to find in modern galaxies. It's strange because these two elements are released during supernova events; you do not usually find one without the other.

"We do not understand this combination of pollutants, but one of our ideas is that extreme supernova explosions have made the galaxy grow gigantic during its teenage years, so as to retain magnesium preferentially at iron, "said Romanowsky.

UDGs are a relatively new class of galaxies that were discovered in 2015. They are as large as the Milky Way but have 100 to 1000 times fewer stars than our own galaxy, making them barely visible and difficult. to study.

KCWI is designed to overcome this obstacle with its extreme sensitivity and ability to capture high-resolution spectra from the farthest and farthest objects of our universe, such as UDGs.

"There is only one other instrument in the world with KCWI capabilities that allows us to measure the chemical composition of low-gloss surface galaxies," said co-author Jean Brodie, a professor of astronomy and microscopy. astrophysics at UCO. "But this one is in the southern hemisphere, where we do not have a good view of the SASB I, which is in the North."

KCWI performs a type of observation called full-field spectroscopy, which captures data in 3-D instead of 2-D. Traditionally, astronomers had two ways to study celestial objects, either by imaging or by spectroscopy. This instrument breaks the barrier between the two. In one observation, KCWI captures both the image, as well as the spectrum of each pixel in the image, which reveals the physical properties of the object, such as composition, temperature , speed, etc.

"That's what kind of observations for which we built KCWI; continue to push the boundaries to get the most information from the weakest objects," said John O. Meara, Chief scientist at the Keck Observatory. "We are very excited to see how many other objects, such as DGSAT I, we can study with Keck and continue to transform our understanding of the formation and evolution of galaxies over time."

The researchers plan to use the KCWI again, this time to make a closer observation of another GOD similar to DGSAT I; they plan to expand its composition in more detail in the hope of providing more data that could help astronomers identify the origin of the GMUs.

"Various ideas have been presented, from the most banal to the most exotic.An intriguing possibility is that some of these ghostly galaxies are living fossils from the dawn of the universe when stars and galaxies have emerged in a environment very different from today, "Romanowsky said. "Their birth is truly a fascinating mystery that our team seeks to solve."

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More information:
Ignacio Martín-Navarro et al. Ratio of extreme chemical abundance suggesting an exotic origin for an ultra-diffuse galaxy, Monthly Notices from the Royal Astronomical Society (2019). DOI: 10.1093 / mnras / stz252