Ancient Galaxy Reveals Chemistry of First Stars, Early Universe.

Understanding the universe’s infancy requires identifying the earliest galaxies and analysing their chemical composition, a task complicated by both distance and faintness. Recent observations, however, have yielded a particularly revealing specimen, offering insights into a period known as the Epoch of Reionization, when the first stars and galaxies began to illuminate the cosmos. This research, detailed in a forthcoming publication, presents spectroscopic analysis of LAP1-B, an exceptionally faint galaxy observed at a redshift of 6.625, corresponding to a cosmic age of just 800 million years post-Big Bang. The collaborative effort involves Kimihiko Nakajima, Masami Ouchi, Yuichi Harikane, Eros Vanzella, Yoshiaki Ono, Yuki Isobe, Moka Nishigaki, Takuji Tsujimoto, Fumitaka Nakamura, Yi Xu, Hiroya Umeda, and Yechi Zhang, and is formally titled “An Ultra-Faint, Chemically Primitive Galaxy Forming at the Epoch of Reionization”. Their findings indicate that LAP1-B possesses an extraordinarily low oxygen abundance, making it the most chemically primitive galaxy identified to date, and providing crucial data on stellar formation in the early universe.

The galaxy LAP1-B represents a significant advancement in understanding the universe’s formative years, offering a detailed glimpse into conditions shortly after the Big Bang. Observations reveal LAP1-B to be the most chemically primitive, actively star-forming galaxy discovered to date, existing approximately 800 million years post-Big Bang, corresponding to a redshift of 6.625. Redshift is a measure of how much the light from an object has been stretched due to the expansion of the universe; higher redshift values indicate greater distances and earlier times.

The galaxy’s exceptional chemical primitiveness is evidenced by its extremely low gas-phase oxygen abundance, measured at (4.2 ± 1.8) x 10^-3 times the solar value. This indicates a very early stage of stellar evolution, before substantial quantities of heavier elements, forged within stars, were created and dispersed through galactic winds and supernovae. These heavier elements, collectively termed ‘metals’ by astronomers, are crucial for subsequent star formation and planetary development.

Notably, LAP1-B emits a surprisingly hard ionizing radiation field. Ionizing radiation possesses sufficient energy to strip electrons from atoms, and a ‘hard’ field indicates a high proportion of high-energy photons. This characteristic is unusual for galaxies and aligns with theoretical models predicting the emission spectra of galaxies forming from pristine gas, devoid of the shielding effects provided by heavier elements. Metals absorb ionizing radiation, so their absence allows it to escape more readily.

LAP1-B is a compact galaxy, possessing a low stellar mass of less than 2700 times that of our Sun. Despite its modest visible mass, the galaxy’s dynamical mass – the mass inferred from the motion of its constituent stars and gas – is over 100 times greater. This substantial discrepancy suggests a dominant dark matter halo, an invisible structure providing the gravitational scaffolding for the galaxy’s formation and evolution. Dark matter, which constitutes approximately 85% of the matter in the universe, interacts gravitationally but does not emit, absorb, or reflect light, making it difficult to detect directly.

The galaxy also exhibits an elevated carbon-to-oxygen ratio, a characteristic consistent with predictions for the earliest generations of stars, known as Population III stars. These stars, theorised to be massive and short-lived, are believed to have produced carbon before oxygen through nuclear fusion processes. The observed ratio provides further evidence supporting the prevailing models of early stellar nucleosynthesis, the creation of elements within stars.

These findings are based on detailed spectroscopic observations, which analyse the light emitted by the galaxy to determine its chemical composition, temperature, and velocity. Crucially, the research leveraged the capabilities of the James Webb Space Telescope (JWST), which allows astronomers to observe the early universe with unprecedented sensitivity and resolution. The faint light from LAP1-B was further magnified through gravitational lensing, a phenomenon where the gravity of a massive foreground object bends and amplifies the light from a distant background source.