The distant galaxy GN-z11 may owe much of its astonishing light to massive stars rather than to its central black hole.
GN-z11 is observed as it existed when the Universe was still very young. Its light traveled for more than 13 billion years before reaching us. Despite this distance, the galaxy appears remarkably bright and compact, prompting astronomers to search for the source of this energy.
Deep field observed by James Webb containing GN-z11.
Source: NASA / ESA / CSA / STScI
The James Webb Space Telescope makes it possible to break down its light according to its different wavelengths. This spectrum acts like a fingerprint: certain patterns reveal the presence of hot stars, gas, or matter heated around a black hole. The researchers compared several models capable of reproducing these signatures.
Two main explanations were competing. The first attributes a large part of the ultraviolet light to an active galactic nucleus, fed by matter falling toward a black hole. The second involves a population of very massive and extremely hot young stars.
The new fits clearly favor the star-dominated scenario. A particular nitrogen signature, known as a P Cygni profile, weighs heavily in this result. This type of spectral pattern appears when fast winds from massive stars absorb and emit light.
Nitrogen presents another puzzle. GN-z11 shows emissions that had been interpreted as an unusually high abundance of this element throughout the galaxy. The new measurements allow for another interpretation: the nitrogen may instead be localized in regions of very dense gas around certain stars.
The researchers specifically propose the involvement of nitrogen-rich massive Wolf–Rayet stars. Their winds can locally enrich the surrounding gas. Nearby, very hot stars would then provide enough radiation to make this gas glow and produce the lines observed by James Webb.
This explanation does not mean that GN-z11 lacks a particularly active black hole. Previous work has found evidence supporting a growing black hole. Rather, the new study seeks to determine which source dominates certain ultraviolet properties of the galaxy, and stellar models reproduce the analyzed data more effectively.
The authors have submitted their results to the Astrophysical Journal. Further spectroscopic observations will now need to distinguish more precisely between the different regions of GN-z11. They will help test whether its unusual nitrogen indeed comes from pockets of gas locally enriched by massive stars.