Small Galaxies Helped Light Up Universe

Hubble observations confirm that much of the light that broke up the early universe’s hydrogen came from the smallest galaxies.

three galaxy clusters

These images from the Hubble Space Telescope show three galaxy clusters (top to bottom): MACS J0717.5+3745, Abell 2744, and MACS J0416.1–2403. Astronomers are studying these and three other clusters as part of the Hubble Frontier Fields program, which uses the phenomenon of gravitational lensing to study distant, early galaxies that would otherwise be too faint to be detected. Observations of more than 250 small galaxies lensed by these clusters reveal that such galaxies contributed substantially to the ultraviolet radiation that reionized the early universe’s neutral hydrogen.
NASA / ESA / HST Frontier Fields team (STScI)

In the universe’s early years, after the primordial plasma cooled down and became transparent, the cosmos was a sea of neutral hydrogen. The sea was a clumpy one. These clumps ultimately became the first galaxies, their stars beginning to shine only a few hundred million years after the Big Bang.

The first stars poured ultraviolet radiation into the universe. But ultraviolet rays are like kryptonite to neutral hydrogen: the photons ionize the atoms. In fact, within the first billion years of cosmic birth, there was so much UV around that most of the universe’s hydrogen was ionized, turning the space between galaxies into a cooled-down version of the primordial plasma.

It’s surprisingly difficult to say just when this epoch of reionization occurred. The timing is important because it tells us when stuff happened in the early universe. Pin down when UV flooded the cosmos, and you can say how fast the things producing that UV — galaxies, madly gobbling black holes — arose.

Astronomers think that almost all of the ultraviolet radiation responsible for reionization came from star-forming galaxies (not, say, their black holes). Observers also have evidence that later on — about 2 to 3 billion years later — the smallest galaxies were pumping out 70% of the universe’s stars. So they’ve started wondering just how much of a role galaxies a fraction of the Milky Way’s size played in reionization. Computer simulations last year suggested that they might contribute 30% of the UV to reionize the universe.

New observations with the Hubble Space Telescope confirm that small galaxies played a big role. Hakim Atek (Ecole Polytechnique Fédérale de Lausanne, Switzerland, and Yale) and colleagues went hunting for early, faint galaxies using the magnifying power of three massive galaxy clusters’ gravity, an effect called gravitational lensing. They detected 252 galaxies from about 650 to 950 million years after the Big Bang. That’s a notable uptick in astronomers’ early galaxy census: less than two years ago, we’d observed only about 1,400 galaxies from this era.

These new galaxies are at least two magnitudes fainter than those previously used to study early galaxies’ energy output. Atek’s team confirmed that indeed, these small galaxies put out lots of UV, enough to make up for what was lacking from big galaxies and totally reionize the universe by about 750 million years after the Big Bang. Depending on where the astronomers draw the line for what qualifies as a small, faint galaxy, these stellar cities contributed between about 20% (just the faintest, newly found) and 60% (including the previous, brighter ones) of the ultraviolet radiation. At first glance that's in keeping with the 30% from last year's simulations, but Atek cautions comparing them isn't straightforward because of the different estimates involved.

You can read more about the study in the press release, or the team’s paper, which will appear in Astrophysical Journal.


Reference: H. Atek et al. “Are Ultra-faint Galaxies at z=6-8 Responsible for Cosmic Reionization ? Combined Constraints from the Hubble Frontier Fields Clusters and Parallels.” To appear in Astrophysical Journal.


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