Extremely Metal-Poor Galaxies Discovered by Chinese Astronomers Provide New Insights on Evolution Theories of Star Clusters

Astronomers have unveiled a group of galaxies with characteristics that likely resemble the star clusters from the distant, early universe.

Extremely Metal-Poor Galaxies

In astronomy, the word "metal" is used to describe all elements that are heavier than hydrogen and helium. It is believed that the very early universe was almost entirely composed of primordial hydrogen and helium, which forged the Big Bang.

Meanwhile, the heavier elements were forged by stars throughout history, causing the formation of the metal-rich universe that we currently find ourselves in. Life, as we know it, consists of heavier elements such as calcium, carbon, iron, and oxygen. However, metal-poor galaxies have also been found in the local universe.

Extremely metal-poor galaxies (XMPGs) refer to star clusters with metallicity below 0.1 of the solar metallicity. Since these galaxies are chemically unevolved, they can serve as an ideal platform for investigating the chemical evolution theories of star clusters. They also aid in studying the physical processes involved in their early stages of evolution.

It is believed that XMPGs are quite common at high redshift. However, they are difficult to observe due to their low masses. Because of this, astronomers are interested in observing local XMPGs at low redshift since they are perceived as possible analogs of primeval high-redshift young galaxies of this type in terms of metallicity and mass.

Galaxy Identification from DESI Early Data

A team of astronomers conducted the study "A Large Sample of Extremely Metal-poor Galaxies at z<1 Identified from the DESI Early Data" on a group of galaxies, focusing on a large sample of XMPGs. Their investigation was done by analyzing the early Dark Energy Spectroscopic Instrument (DESI) data. These include XMPG detection, Early Data Release, and exploration of their mass-metallicity relation.

Led by Hu Zhou of the University of Chinese Academy of Sciences in Beijing, China, the researchers first chose 1,623 star-forming galaxies with significant oxygen emission line detection. From this group, they identified 223 XMPGs at redshifts below 10. The result has led them to confirm that 95 of the star clusters are bona fide XMPGS, while 128 remain XMPG candidates.

The majority of the XMPGs reported in the study were discovered at a low redshift below 0.3. They also turn out to be dwarf galaxies which do not exceed 1 billion solar masses. The most metal-poor galaxy in the sample, officially called DESIJ150535.89+314639.4, has an oxygen abundance of about 15 million solar masses. Its star-forming rate was measured to be 0.22 solar masses per year.

The initial imaging examination of the two most metal-poor galaxies unveiled from the sample revealed two different morphologies. The findings suggest different evolution and physical origins, but further studies are still needed to confirm this hypothesis.

Based on the study's result, the authors underline that the XMPGs they detected are likely low-redshift analogs of star clusters at high redshifts reaching 6.0 or more. This means that they can be excellent cosmic objects for exploring the universe at its early stages of evolution.

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