No galaxies have been seen before at such early epochs as that seen in this deepest image of the universe ever taken in near-infrared light by NASA's Hubble Space Telescope. The faintest and reddest objects (left inset) in the image are galaxies that correspond to "look-back times" of approximately 12.9 billion years to 13.1 billion years ago.
A longstanding enigma is that it still appears that these early galaxies did not emit enough radiation to "reionise" the early Universe by stripping electrons from the neutral hydrogen that cooled after the Big Bang. This "reionisation" event occurred between about 400 million and 900 million years after the Big Bang, but astronomers still don't know which light sources caused it to happen. These newly discovered galaxies date from this important epoch in the evolution of the Universe.
It took about the first billion years to completely ionize the Universe; before that, the Universe was opaque to light, with neutral atoms acting like dust. As the Universe reionizes, it becomes easier to see the light from whatever objects are behind it. The youngest object ever discovered in the universe, Gamma Ray Burst GRB 090423, born when the Universe was under 0.7 billion years old. This thing is so far away that no visible light actually got out; we can only see the X-rays from it
These early Hubble galaxies are much smaller than the Milky Way and other spiral galaxies and have populations of stars that are intrinsically very blue. This may indicate the galaxies are so primordial that they are deficient in heavier elements, and as a result, are quite free of the dust that reddens light through scattering.
Ross McLure of the Institute for Astronomy at Edinburgh University and his team detected 29 galaxy candidates, of which twelve lie beyond redshift 6.3 and four lie beyond redshift 7 (where the redshifts correspond to 890 million years and 780 million years after the Big Bang respectively). He notes that "the unique infrared sensitivity of Wide Field Camera 3 means that these are the best images yet for providing detailed information about the first galaxies as they formed in the early Universe".
"These galaxies could have roots stretching into an earlier population of stars. There must be a substantial component of galaxies beyond Hubble's detection limit," according to James Dunlop of the University of Edinburgh.
"These ancient galaxies are only one twentieth of the Milky Way's diameter," reports HUDF09 team member Pascal Oesch of the Swiss Federal Institute of Technology in Zurich. "Yet they must be the seeds from which the great galaxies of today were formed," adds HUDF09 team member Marcella Carollo of the Swiss Federal Institute of Technology.
"The masses are just 1 percent of those of the Milky Way. To our surprise, the results show that these galaxies at 700 million years after the Big Bang must have started forming stars hundreds of millions of years earlier, pushing back the time of the earliest star formation in the Universe," explains team member Ivo Labbe of the Carnegie Institute of Washington,
"This is about as far as we can go to do detailed science with the new HUDF09 image. It shows just how much the James Webb Space Telescope is needed to unearth the secrets of the first galaxies," says Illingworth.
The challenge is that spectroscopy is needed to provide definitive redshift values, but the objects are too faint for spectroscopic observations (until JWST is launched), and the redshifts have to be inferred from the apparent colours of the galaxies.
Perhaps the density of very faint galaxies below the current detection limit is so high that there may be enough of them to support reionisation. Or there was an earlier wave of galaxy formation that decayed and then was "rebooted" by a second wave of galaxy formation. Or, possibly the early galaxies were extraordinarily efficient at reionising the Universe.
Due to these uncertainties it is not clear which type of object or evolutionary process did the "heavy lifting" by ionising the young Universe. The calculations are inconclusive, and so galaxies may do more than currently expected, or astronomers may need to invoke other phenomena such as mini-quasars (active supermassive black holes in the cores of galaxies) — current estimates suggest that quasars are even less likely than galaxies to be the cause of reionisation. This is an enigma that still challenges astronomers and the very best telescopes.
"We know the gas between galaxies in the Universe was ionised early in history, but the total light from these new galaxies may not be sufficient to achieve this." said Andrew Bunker of the University of Oxford, a researcher on one of the European teams.
Did dark matter destroy this early universe? You might be looking around at the way things "exist" and thinking "No", but we're talking about ancient history. Three hundred million years after the start of the universe, things had finally cooled down enough to form hydrogen atoms out of all the protons and electrons that were zipping around - only to have them all ripped up again around the one billion year mark. Why?
Most believe that the first quasars, active galaxies whose central black holes are the cosmic-ray equivalent of a firehose, provided the breakup energy, but some Fermilab scientists have another idea. Dan Hooper and Alexander Belikov posit that invisible, self-destructing dark matter may have blown up every atom in the universe. At least it's plausible in that if we wanted to ionize an entire universe, we'd want something that sounded that awesome.
Dark matter is a candidate for providing ionizing radiation because, if it exists at all, it's its own antiparticle: if two dark matter particles hit each other they can blow up. Insane as it sounds, the theory predicts that despite making up most of everything the particles themselves are so tiny, and so terribly fussy about colliding, that they can form huge structures without destroying themselves. Positron emissions which may be an indication of exactly this kind of self-destruction have been observed by the European PAMELA satellite currently orbiting the Earth.
As theories go, this one is more awesome than accepted. The quasar hypothesis has wide support, and crediting something we've never even seen with reshaping the universe may be going a little far. Then again, that's what modern cosmology is doing with dark matter anyway, so maybe this idea will fit right in. With the launch of the James Webb Space Observatory, perhaps we'll find out for certain.
Posted by Casey Kazan with Luke McKinney
Source material provided by ESA/Hubble Information Center.
Image Credit: Credit: NASA, ESA, G. Illingworth (UCO/Lick Observatory and University of California, Santa Cruz), and the HUDF09 Team