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

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.

Galaxies comparable to the present-day Milky Way are numerous, but younger galaxies that are Milky Way-like are inherently smaller, bluer, and richer in gas in general than the galaxies we see today. For the first galaxies of all, this is taken to the extreme, and with their presence behind a “wall” of cosmic dust, most of them remain obscured even with 2021-level technology. (Credit: NASA, ESA, P. van Dokkum (Yale U.), S. Patel (Leiden U.), and the 3-D-HST Team)

Only enough ultraviolet photons, cumulatively, can fully reionize these intergalactic atoms.

Until they do, the Universe lives in “the dark ages,” where emitted starlight gets absorbed before it’s observable.

This diagrammatic view of the Universe’s history highlights the dark ages, which begins once neutral atoms form, and continues through the end of reionization, which happens everywhere, on average, 550 million years after the Big Bang. In the intermediate times, early stars and galaxies exist, but are difficult to see owing to the light-blocking presence of neutral atoms. (Credit: S. G. Djorgovski et al., Caltech. Produced with the help of the Caltech Digital Media Center)

Only the brightest galaxies, along the most serendipitously reionized line-of-sights, have previously been seen.

Only because this distant galaxy, GN-z11, is located in a region where the intergalactic medium is mostly reionized, can Hubble reveal it to us at the present time. To see further, we require a better observatory, optimized for these kinds of detection, than Hubble. (Credit: NASA, ESA, P. Oesch and B. Robertson (University of California, Santa Cruz), and A. Feild (STScI))

This includes the current cosmic record-holder: GN-z11.

This deep-field region of the GOODS-South field contains 18 galaxies forming stars so quickly that the number of stars inside will double in just 10 million years: just 0.1% the lifetime of the Universe. The deepest views of the Universe, as revealed by Hubble, also contain many of the most distant and extreme galaxies ever seen, particularly if they’re near another large mass that can enhance their light due to gravitational lensing. (Credit: NASA, ESA, A. van der Wel (Max Planck Institute for Astronomy), H. Ferguson and A. Koekemoer (Space Telescope Science Institute), and the CANDELS team)

But the brightest early galaxies, alone, can’t account for all of the photons that we need.

At the earliest times, starlight from the first luminous objects would be blocked by the neutral matter permeating space at that time. But by measuring longer-wavelength signatures, such as those emitted by carbon monoxide molecules in the gas, distant galaxies can be seen by other observatories, like ALMA, that ultraviolet, optical, and near-infrared observatories would otherwise miss. (Credit: R. Decarli (MPIA); ALMA (ESO/NAOJ/NRAO))

There must be additional early galaxies, yet unseen, contributing to the reionization process.

In this comparison view, the Hubble data is shown in violet, while ALMA data, revealing dust and cold gas (which themselves indicate star-formation potential), is overlaid in orange. Clearly, ALMA is revealing not only features and details that Hubble cannot, but sometimes, it shows the presence of objects that Hubble cannot see at all. (Credit: B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO); NASA/ESA Hubble)

ALMA, the Atacama Large Millimetre/submillimetre Array, can detect longer-wavelength photons beyond Hubble’s limits.

Different instruments can reveal different details about any astronomical object, dependent on wavelength and resolution. ALMA, owing to its uniquely high-resolution capabilities, can see details of new star formation and very cool gas better than any other observatory. (Credit: ESO, NASA, ALMA, CXC, VLA et al.)

Combining ALMA with infrared Spitzer data has revealed the first normal, pre-reionization galaxies.

These two newly discovered galaxies, REBELS-29-2 and REBELS-12-2, are beyond the wall of cosmic dust that renders all but the very brightest ones invisible to Hubble-like telescopes. However, mid/far-infrared observatories or those operating at longer wavelengths, like ALMA, can still reveal them even if they’re not very luminous or massive. These are the two faintest, smallest galaxies ever seen at such distances. (Credit: Y. Fudamoto et al., Nature, 2021)

Known as REBELS-29-2 and REBELS-12-2, they’re the first “less extreme” galaxies found before reionization completes.

The pre-reionizations galaxies REBELS-29-2 and REBELS-12-2 represent the lowest-mass, lowest-luminosity galaxies ever seen at a redshift of ~7 or higher. This is only possible owing to the combination of observatories like Spitzer and ALMA that were unavailable a few years ago. NASA’s James Webb should find many more galaxies such as these. (Credit: Y. Fudamoto et al., Nature, 2021)

Altogether, these previously unseen galaxies should contribute 10-25% of the needed early starlight.

Although star-formation should reach its peak significantly later in the Universe, between a redshift of 2 and 3, the early stars and galaxies are vital for their role in reionizing the Universe. These lower-mass galaxies, seen now for the first time, contribute between 10-25% of the needed ultraviolet, ionizing radiation. (Credit: Y. Fudamoto et al., Nature, 2021)

James Webb’s novel capabilities, at last, will abundantly reveal and characterize these earliest galaxies.

James Webb will have seven times the light-gathering power of Hubble, but will be able to see much farther into the infrared portion of the spectrum, revealing those galaxies existing even earlier than what Hubble could ever see. Galaxy populations seen prior to the epoch of reionization should abundantly be discovered, including at low masses and low luminosities, by James Webb beginning in 2022. (Credit: NASA/JWST Science Team; composite by E. Siegel)

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