Observations explore nature of massive ancient galaxy

By analyzing the data from the James Webb Space Telescope (JWST) and the Hubble Space Telescope (HST), astronomers from the University of Wisconsin-Madison and elsewhere have probed the properties of a massive and old galaxy designated SMILES-GS-191748. Results of the study, published August 7 on the pre-print server arXiv, shed more light on the nature of this galaxy.

SMILES-GS-191748 is a massive and quiescent galaxy at a redshift of 2.675. The galaxy most likely contains a very old stellar population that first formed when the universe was young.

Given that very little is known about the properties of SMILES-GS-191748, a team of astronomers led by University of Wisconsin-Madison’s Ian McConachie decided to inspect this galaxy using JWST and HST. They nicknamed SMILES-GS-191748 “Eridu,” after the ancient Bronze Age Sumerian city in Mesopotamia due to the galaxy’s suspected early formation time and apparent quiescent nature.

“Observations of Eridu in the GOODS-S [Great Observatories Origins Deep Survey-South] field form an exquisite dataset. With imaging from legacy HST programs, extensive JWST/NIRCam campaigns, and a deep JWST/MIRI survey, photometric coverage of Eridu extends from 4–25 µm,” the researchers wrote in the paper.

The study found that Eridu, taking into account its high redshift, is already extremely mature when the universe is only about 2.5 billion years old. Its mass was estimated to be approximately 900 billion solar masses.

The results indicate that Eridu had an average star formation rate of less than one solar mass per year over the last 100 million years. The galaxy most likely experienced early and fast star formation within about 300 million years after the Big Bang and quenching—some two billion years prior to observations.

Based on the collected data, the astronomers were able to infer abundances for iron and magnesium in Eridu. It turned out that the magnesium to iron abundance ratio is at a level of 0.65. This suggests that the galaxy is strongly alpha-element-enhanced.

According to the study, Eridu lies in a massive protocluster known as “Drishti,” which has a total mass of nearly one quadrillion solar masses. The researchers note that the high-density environment of Drishti provides additional mechanisms, like mergers, which could explain fast early mass assembly and rapid quenching of Eridu.

All in all, the new findings improve our knowledge about the nature and origin of Eridu; however, still more observations are required to gain a comprehensive understanding of its formation and evolution.

“Though the formation history of Eridu inferred by SED [spectral energy distribution] modeling is confounding, it is undeniably ancient, and possibly a descendant of the bright blue sources only recently detected at z > 10, or a more massive dust-obscured population. (…) Future studies of Eridu’s morphology, environment, dynamics, and spatially resolved properties will offer additional insights into its puzzling birth and evolution,” the authors of the paper conclude.

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