NASA’s James Webb Space Telescope (JWST) has made another groundbreaking discovery that expands our understanding of the early universe and the process of cosmic reionization.
New findings have revealed the detection of the Lyman-alpha (Lyα) hydrogen emission line in an early galaxy, JADES-GS-z13-1-LA, believed to have formed only 326 million years after the Big Bang. The discovery is providing astronomers with new insights into the process of cosmic reionization, and also suggests how it might have occurred much earlier than previously thought.
Cosmic reionization
Cosmic reionization is the process that began as neutral atoms throughout the cosmos began to reionize, marking the second significant phase transition of gas in the universe recognized by cosmologists. Generally, this refers to the reionization of hydrogen, although helium in the early universe is also believed to have undergone similar processes long ago. The process occurred as ultraviolet radiation generated by some of the earliest galaxies in the universe illuminated these cold neutral gases.
Now, in a new astronomical breakthrough made possible by Webb, new insights from the JWST’s Advanced Deep Extragalactic Survey (JADES) have helped reveal a galaxy that displays a redshift of z=13.0, and whose Lyα emission line can be discerned, even despite the neutral hydrogen that astronomers would have expected to block it.
Previously, this emission line had only been observed in galaxies formed after 550 million years. The unexpected new discovery is helping to shed light on the characteristics of some of the universe’s very early galaxies, as well as their fundamental part in the reionization of the Universe.
Right now, scientists are unsure of how Lyα light is able to escape from this galaxy, given that it should be blocked by the neutral intergalactic medium (IGM) that would have dominated the universe at the time corresponding to the light now being observed by Webb. This makes the discovery significant, since it seems to imply that the galaxy in question, JADES-GS-z13-1-LA, could reside in a region where reionization may have began earlier than previously expected, which could have helped light travel freely through the IGM.
Exceptional Brightness
Another significant aspect of the discovery involves the brightness of the Lyα line in JADES-GS-z13-1-LA, which appears to possess an equivalent width of over 40Å (denoting 40 angstrom, which involves the metric unit of length for measuring wavelengths of electromagnetic radiation). Add to this the deep blue UV continuum also noted during the observations, which points to the galaxy’s role in producing ionizing photons, which also contributes to the reionization process.
Ultimately, the presence of such a discernible Lyman-alpha (Lyα) hydrogen emission line at such an early time in cosmic history potentially upends our understanding of current models that describe how and at what time reionization began in the universe.
One key component of the discoveries involved the sensitivity of the JWST’s NIRSpec instrument, without which the discovery of the Lyα line likely could not have been made. This, in addition to the NIRSpec instrument’s ability to detect other spectral features, particularly those associated with the elements carbon, helium, and oxygen, all of which provide a rich view of the structure and behavior of JADES-GS-z13-1-LA.
Unanswered Questions
Presently, astronomers still aren’t certain what the exact mechanism may be that is allowing the Lyα light to escape. However, one possibility involves thermal or kinetic effects, which they believe may be enhancing the emissions it produces. If correct, this theory could mean that the broadening of the emission line by a significant degree could be influenced by such effects.
Ultimately, the discovery of JADES-GS-z13-1-LA and its Lyα emission represents a significant new development in our understanding of the mechanics of the early universe, and hits at future surprises that are likely to arise from observations by Webb in the years ahead.
The new discovery was detailed in a recent paper, “Witnessing the onset of Reionisation via Lyman-α emission at redshift 13,” which can be read in its entirety on the preprint server arXiv.org.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. He can be reached by email at micah@thedebrief.org. Follow his work at micahhanks.com and on X: @MicahHanks.