section_path: Formation and evolution/Formation
section_name: Formation
paragraph_text: Formation
  
  Current models of the formation of galaxies in the early universe are based on the ΛCDM model. About 300,000 years after the Big Bang, atoms of hydrogen and helium began to form, in an event called recombination. Nearly all the hydrogen was neutral (non-ionized) and readily absorbed light, and no stars had yet formed. As a result, this period has been called the " dark ages ". It was from density fluctuations (or anisotropic irregularities) in this primordial matter that larger structures began to appear. As a result, masses of baryonic matter started to condense within cold dark matter halos. These primordial structures allowed gasses to condense in to protogalaxies, large scale gas clouds that were precursors to the first galaxies.
  
  As gas falls in to the gravity of the dark matter halos, its pressure and temperature rise. To condense further, the gas must radiate energy. This process was slow in the early universe dominated by hydrogen atoms and molecules which are inefficient radiators compared to heavier elements. As clumps of gas aggregate forming rotating disks, temperatures and pressures continue to increase. Some places within the disk reach high enough density to form stars.
  
  Once protogalaxies began to form and contract, the first halo stars, called Population III stars, appeared within them. These were composed of primordial gas, almost entirely of hydrogen and helium.Emission from the first stars heats the remaining gas helping to trigger additional star formation; the ultraviolet light emission from the first generation of stars re-ionized the surrounding neutral hydrogen in expanding spheres eventually reaching the entire universe, an event called reionization. The most massive stars collapse in violent supernova explosions releasing heavy elements ("metals") into the interstellar medium. This metal content is incorporated into population II stars.
  
  Theoretical models for early galaxy formation have been verified and informed by a large number and variety of sophisticated astronomical observations. The photometric observations generally need spectroscopic confirmation due the large number mechanisms that can introduce systematic errors. For example, a high redshift (z ~ 16) photometric observation by James Webb Space Telescope (JWST) was later corrected to be closer to z ~ 5. Nevertheless, confirmed observations from the JWST and other observatories are accumulating, allowing systematic comparison of early galaxies to predictions of theory.
  
  Evidence for individual Population III stars in early galaxies is even more challenging. Even seemingly confirmed spectroscopic evidence may turn out to have other origins. For example, astronomers reported He emission evidence for Population III stars in the Cosmos Redshift 7 galaxy, with a redshift value of 6.60. Subsequent observations found metallic emission lines, O, inconsistent with an early-galaxy star.