Hubble Spies Faint Galaxies, Bountiful in Early Universe

Hubble has spotted more than 100 small, faint galaxies in the young universe, common as dust bunnies but previously out of reach of even the best telescopes.

Astronomers using the Hubble Space Telescope have spotted the faintest galaxies yet in the distant universe. The result isn’t just a new entry in the Guinness Book of World Records; it also sheds light on a poorly understood epoch in the early history of our cosmos.

MACS 0416 galaxy cluster and faint galaxies behind it

Hubble views the galaxy cluster MACS 0416, which acts as a gravitational lens to magnify more distant background galaxies. Cyan marks the distribution of mass in the cluster, mostly in the form of dark matter. Magenta highlights the degree to which the background galaxies are magnified, which is related to the mass distribution.
STScI / NASA / CATS Team / R. Livermore (UT Austin)

According to Rachael Livermore (University of Texas, Austin), these galaxies emit less than a hundredth of the Milky Way’s luminosity; the dimmest of them is 2,000 times fainter than our galaxy. “They are the precursors of dwarf systems like the nearby Fornax galaxy,” she says.

Livermore and her colleagues found 167 faint galaxies between redshifts of 5.3 and 8.8, so their light has traveled between 12.6 and 13.1 billion years to Earth. Normally, even Hubble would never be able to spot them: Hubble can detect objects down to 31st magnitude, but the combination of these objects’ intrinsic faintness plus their incredible distance puts them out of Hubble’s reach. They were only detected because gravitational lensing boosted their brightness.

Over the past several years, Hubble has taken long looks at six massive galaxy clusters in what is called the Frontier Fields program (see Sky & Telescope’s January 2015 issue). Astronomers are searching for faint galaxies in these clusters’ backgrounds, making use of the fact that the foreground cluster’s gravity magnifies the images of remote galaxies. The stronger the magnification, the fainter the galaxies Hubble can detect.

There’s a problem, though: The magnification produced by gravitational lensing is strongest in the central regions of the foreground cluster. But that’s also where the cluster’s galaxies are brightest and crowded together. Any magnified background galaxies are swamped in this foreground light.

Together with her colleague Steven Finkelstein (also at University of Texas, Austin) and Frontier Fields principal investigator Jennifer Lotz (Space Telescope Science Institute), Livermore has now succeeded in filtering out the foreground galaxies’ light in two Frontier Fields clusters: Abell 2744, in the constellation Sculptor, also known as Pandora’s Cluster, and MACS 0416.1-2403 in Eridanus.

Abell 2744 galaxy cluster

A similar view of the galaxy cluster Abell 2744, where cyan and magenta again highlight the mass distribution and magnification, respectively.
STScI / NASA / CATS Team / R. Livermore (UT Austin)

“We used a technique known as wavelet decomposition,” explains Livermore. “You basically analyze an image on many possible physical scales to isolate the largest structures. It’s a bit comparable to the way image compression works, or to the technique behind noise-canceling earphones.” Wavelet decomposition has been used in astronomy before, but not for this particular purpose, she adds.

Some of the faint background galaxies that this analysis has revealed are magnified by a factor of 50 or 60. “They are up to a hundred times less luminous than the faintest galaxies observed in the [unlensed] Hubble Ultra Deep Field,” says Livermore.

And there are many of them — Hubble is finally seeing the most common galaxies from this time period. Thanks to the combination of gravitational lensing, Hubble’s unprecedented sensitivity, and the team’s wavelet technique, astronomers now have a better picture of how much light these galaxies emitted as a population.

In particular, as the authors observe in the February 1st Astrophysical Journal, there’s now strong evidence that these small, faint galaxies acted as an important source of light in the young universe — even though the less massive galaxies emitted less light, they made up for it by their sheer numbers.

That’s good news for cosmologists trying to understand the so-called Epoch of Reionization: A few hundred million years after the Big Bang, neutral hydrogen atoms in intergalactic space became ionized, losing their single electron due to energetic ultraviolet radiation. (It’s called re-ionization because hydrogen was also in an ionized state right after the Big Bang, before the universe cooled down and grew dark.) However, the details of this reionization process are sketchy.

Here's how the infant universe might have looked when ultraviolet light from young galaxies started to created ionized "clearings" in the otherwise-dark cosmic fog of hydrogen gas:

Credit: ESO / L. Calçada

In the past, scientists haven’t been able to agree on the main source of the energetic radiation. The light might have come from the first generation of stars in a huge population of small, just-formed galaxies, or rare but ultra-luminous quasars, powered by supermassive black holes, could have supplied the radiation. The former scenario now looks to be the most promising: according to Livermore and colleagues, their faint-galaxy trove is “consistent with the number of faint galaxies needed to reionize the universe.”


To learn more about the Hubble Space Telescope's Frontier Fields project, read Sky & Telescope's January 2015 issue.

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