天文学：宇宙再電離の初期兆候

One of the earliest known galaxies caught in the act of reionization has been detected by the JWST. The observations, reported in Nature this week, place the onset of cosmic reionization (an important transition that occurred in the early Universe) to at least 330 million years after the Big Bang.

After the ultra-hot Big Bang, the Universe gradually cooled down until the free protons and electrons combined into neutral (chargeless) gas, mostly hydrogen and helium, a period sometimes referred to as the cosmic dark ages. The first galaxies illuminated the Universe. In particular, photons of specific ultraviolet (UV) wavelengths, known as the Lyman continuum, would be absorbed by neutral hydrogen, whereas shorter wavelengths would reionize the gas, making the Universe transparent to Lyman photons and letting them pass through to Earth. The timing of this event, known as cosmic reionization, has been uncertain. Recent JWST observations have identified bright UV-producing galaxies when the Universe was less than 300 million years old, but direct evidence of reionization has been lacking.

Joris Witstok and colleagues report the detection of a signal of reionization occurring in a galaxy (named JADES-GS-z13-1-LA) at just 330 million years after the Big Bang, observed by the JWST. The bright emission is identified as Lyman-α, a signal of neutral hydrogen transitioning from an excited stated to the ground state. This means that the galaxy produced enough UV photons to excite its neutral hydrogen, and there was little neutral hydrogen between it and Earth to re-absorb the Lyman-α photons the hydrogen released when it returned to its stable, lowest-energy state. The authors note that the likely source of ionization is either massive hot stars or a supermassive black hole. The findings offer new insights into the nature of the earliest galaxies, helping to constrain the onset and timeline of cosmic reionization, they conclude.