Cosmic Neon Sign: Gas Glows Brighter Than Stars express
A cosmic neon sign lit up in an incredibly distant galaxy: almost all its light comes from hot gas, and the stars are lost in this glow. The James Webb telescope recorded that the gas, a mix of hydrogen and helium, is so heated that it glows on its own.
The secret of such brightness is the first stars of the Universe. A faint satellite galaxy was noticed nearby with no traces of heavy elements: it is likely a clump of remnants of stars born right after the Big Bang. These giants, hundreds of times more massive than the Sun, faded long ago, but their energy, like an invisible current, heats the gas and makes it shine. Johann Balmer back in the 19th century described a special jump in hydrogen's glow — today it serves as a thermometer for distant galaxies.
Without Webb, such a discovery would have been impossible. It peers into the era when the first galaxies were forming, and helps understand how the expansion of the Universe changed the light, discovered by Edwin Hubble.
Original abstract
as written by the authors arXiv:2603.13471
The discovery of galaxies with extremely strong nebular continuum emission at high redshifts provide novel, unique insights into the conditions under which the first super-massive stars formed. Here we identify a galaxy at redshift $$z=5.124$$ observed by the JWST CAPERS survey that exhibits a prominent turnover in the rest-frame UV continuum and a pronounced Balmer `jump'. We model the entire JWST/NIRSpec Prism spectrum from rest-frame UV to optical wavelength, finding that a dominant ($$>95\%$$) nebular continuum emission can accurately reproduce the spectral shape across all wavelengths. We tested an alternative model with strong damped Ly$$\alpha$$ absorption (DLA), but found that it is not able to match the shape of the turnover without invoking a large freedom in the redshift of the absorber. The nebular continuum emission model reveals a hot ($$T = (5.3\pm 0.2)\times 10^{4}$$ K) and dense ($$n_e = (5.4\pm 0.8)\times 10^{3} {\rm cm^{-3}}$$) nebular region powering the origin of the spectral shape. We also note the presence of a `blue' candidate companion source potentially at the same redshift, offset by 3 kpc to the main galaxy. Intriguingly, the spectrum of this source show several hints of hydrogen and helium lines, but no metal lines are detected. We theorize that this companion galaxy might be comprised mainly of Population III (PopIII) stellar remnants and potentially powers the nebular continuum emission seen in the main galaxy. These results have important implications for the presence of a potential dominant population of super-massive and PopIII stars and their consequent excess UV brightness for a significant fraction of galaxies at cosmic dawn.
The Balmer jump works like a cosmic thermometer: by its magnitude astronomers determine gas temperature with an accuracy of a few thousand degrees.