A new study explores the conditions in an extremely distant galaxy to understand how early galaxies may evolve to form the objects we see now in the local universe.
Early Universe Galaxies
Over the past few years since JWST went online, astronomers have discovered some of the most distant galaxies in the universe. To be visible from over 13 billion light-years away, these galaxies must be extremely bright and thus powered by an intense energy source — typically either active massive central black holes known as active galactic nuclei (AGN) or very strong star formation that causes a burst in stellar light.
The universe’s earliest galaxies are astronomical fossils, providing clues to how nearby galaxies and objects have evolved to their current state. These high-redshift galaxies may be progenitors of globular clusters or beginnings of growing galaxies — objects whose origins are not well understood. Further characterizing the newly discovered galaxies may reveal how objects in the local universe came to be.
ALMA detection of the 88-micron emission line from doubly ionized oxygen. The left-hand panel plots the intensity map of GHZ2 as seen on the sky, where the yellow in the center corresponds to the galaxy. The right-hand panel shows the ALMA spectrum with the 88-micron emission line highlighted in yellow. Click to enlarge. [Zavala et al. 2024]
Star Formation or AGN?
One such high-redshift galaxy is GHZ2, which was recently discovered with JWST and lies at a redshift corresponding to ~400 million years after the Big Bang. To complement the JWST spectroscopic observations, Jorge A. Zavala (National Astronomical Observatory of Japan) and collaborators observed the galaxy using the Atacama Large Millimeter/submillimeter Array (ALMA). These observations aimed to measure far-infrared emission lines that are critical in determining if a galaxy is powered by star formation or an AGN. The authors targeted and successfully recovered the 88-micron (1 micron = 10-6 meter) emission line from doubly ionized oxygen — an emission line known to correlate with a galaxy’s star formation activity. From the detection, the authors derived a redshift that agrees with the redshift determined from JWST.
With the ALMA observations and prior measurements from JWST, the authors compare GHZ2 to other galaxies. They find that GHZ2 exhibits characteristics similar to other galaxies that are dominated by star formation. Additionally, this very high-redshift galaxy appears to share characteristics with giant star-forming regions, which further suggests that GHZ2 is likely driven by extreme star formation. The authors find that, though some AGN contribution fraction cannot be definitively ruled out, GHZ2 is most likely dominated by metal-poor and young stellar populations.
How Will GHZ2 Evolve?
The stellar mass–velocity dispersion relationship with data points plotted for different object types including globular clusters (light-purple circles), ultra-compact dwarf galaxies (solid purple circles), dwarf ellipticals (down-pointing triangles), compact ellipticals (blue solid circles), and elliptical and S0 galaxies (green squares). The target galaxy GHZ2 is shown with a yellow star, which falls in closest agreement with compact ellipticals and ultra-compact dwarf galaxies in this relationship. Click to enlarge.
[Zavala et al. 2024]
Though the exact evolution of GHZ2 is unclear, this study underscores the significance of ALMA and JWST as a powerful pair of instruments capable of characterizing the most distant galaxies in the universe. Future observations with both instruments will continue to clarify what the early universe looked like and how it has evolved over time.
Citation
“ALMA Detection of [O iii] 88 μm at z = 12.33: Exploring the Nature and Evolution of GHZ2 as a Massive Compact Stellar System,” Jorge A. Zavala et al 2024 ApJL 977 L9. doi:10.3847/2041-8213/ad8f38