When we look back at the Universe through cosmic time, we see too few galaxies at the greatest distances to account for the Universe becoming transparent to light. The newest discovery, going beyond what Hubble can see, discovered the low-mass, low-brightness early galaxies that had been missing up until now. (Credit: NASA, ESA, P. Oesch (University of Geneva), and M. Montes (University of New South Wales))

Astronomers discover “missing” population of normal, young galaxies before reionization

Previously, only the brightest, most active galaxies could pierce the obscuring wall of cosmic dust. At last, normal galaxies break through.

Back in the Universe’s early days, starlight couldn’t travel very far at all.

The first stars in the Universe will be surrounded by neutral atoms of (mostly) hydrogen gas, which absorbs the starlight. As more generations of stars subsequently form, the universe becomes reionized, allowing us to fully see the starlight and investigate the underlying properties of the observed objects. (Credit: Nicole Rager Fuller / NSF)

After the Big Bang, the Universe formed neutral atoms, creating a problem.

An artist’s impression of the environment in the early Universe after the first few trillion stars have formed, lived and died. While there are sources of light in the early Universe, the light is very rapidly absorbed by the interstellar/intergalactic matter until reionization is complete. (Credit: NASA/ESA/ESO/W. Freudling et al. (STECF))

Although they self-gravitate, forming stars and galaxies, atoms also exist between these luminous entities.

Although the Milky Way is full of stars, this stellar density map of the sky, constructed with data from the ESA’s space-based Gaia mission, is only accurate to the extent that visible light gives us accurate information. The ultraviolet and visible light emitted by the Milky Way’s stars is obscured by the light-blocking dust in our galaxy, requiring longer-wavelength views to reveal them. Dust can block ultraviolet and visible light at all redshifts and locations in the Universe. (Credit: ESA/Gaia)

Most emitted starlight is energetic ultraviolet light: easily absorbed by these neutral atoms.