Two most metal-poor galaxies identified in this work. The left, middle and right panels display the observed images, models, and residuals of the detected objects in the DESI legacy imaging surveys, respectively. Credit: arXiv (2023). DOI: 10.48550/arxiv.2312.00300
By analyzing the early data from the Dark Energy Spectroscopic Instrument
(DESI), an international team of astronomers has identified 95 new extremely
metal-poor galaxies at a low redshift. The finding is detailed in a paper
published December 1 on the
pre-print server arXiv.
Extremely metal-poor galaxies (XMPGs) are the ones with metallicity below
0.1 of the solar metallicity. Given that these galaxies are chemically
unevolved, they can serve as excellent laboratories for investigating the
chemical evolution theories of galaxies and studying the physical processes
in the early stages of their evolution.
Although XMPGs are assumed to be quite common at high redshift, they are
difficult to observe due to their low masses. Therefore, astronomers are
interested in the observations of local XMPGs at low redshift, as they are
perceived as possible analogs of primeval high-redshift young galaxies of
this type in terms of mass and metallicity.
Now, a group of astronomers led by Hu Zou of the University of Chinese
Academy of Sciences in Beijing, China, has obtained a large sample of XMPGs
based on the early data from DESI.
“This work focuses on the analysis of DESI early data, including the Early
Data Release (EDR; DESI Collaboration et al. 2023b), the detection of XMPGs
and exploration of their mass-metallicity relation,” the researchers wrote
in the paper.
All in all, the team first selected 1,623 star-forming galaxies with
significant oxygen emission lines detection. From this sample, they
identified 223 extremely metal-poor galaxies at redshifts below 1.0.
Finally, they confirmed 95 of them as bona fide XMPGs, while the 128
galaxies remain XMPG candidates.
Most of the XMPGs reported in the study were found at a low redshift of
below 0.3 and turned out to be dwarf galaxies—with stellar masses not
exceeding 1 billion solar masses. The most metal-poor galaxy in the sample,
designated DESIJ150535.89+314639.4, has an oxygen abundance at a level of
only 1/34 of that of the sun, has a stellar mass of about 15 million solar
masses, and its star-forming rate was calculated to be 0.22 solar masses per
year.
The astronomers noted that preliminary imaging examination of the two most
metal-poor galaxies discovered from their sample revealed two different
morphologies. This suggests different evolution and physical origins;
however, further studies are required in order to confirm this hypothesis.
Summing up the results, the authors of the paper underlined that the
extremely metal-poor galaxies detected by them are possibly low-redshift
analogs of galaxies at high redshifts reaching 6.0 or even beyond. Therefore,
they could be excellent objects for exploring the universe at its early
stages of evolution.
More information: Hu Zou et al, A Large Sample of Extremely Metal-poor
Galaxies at z<1 Identified from the DESI Early Data,
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