[Oct. 20, 2010]
Scientists pinpoint the farthest galaxy
Alan Boyle writes: Astronomers have confirmed that an incredibly faint galaxy in the constellation Fornax is the most distant known object in the universe, shining more than 13 billion light-years away and reflecting an era when stars were just beginning to emerge from a cosmic fog.
The galaxy, known as UDFy-38135539, is one of several super-distant objects picked out from the Hubble Ultra Deep Field, the most sensitive snapshot ever taken of deep space. In time, astronomers may well spot objects that are even farther away, but this particular galaxy was the first of its type to go through the arduous process of having its measurements checked.
In fact, the astronomers behind the observations say they couldn't have seen UDFy-38135539 unless there were other, fainter galaxies nearby to help clear out the space around it. "Without this additional help, the light from the galaxy, no matter how brilliant, would have been trapped in the surrounding hydrogen fog, and we would not have been able to detect it," Durham University's Mark Swinbank said in a news release from the European Southern Observatory.
The ESO researchers, led by Matt Lehnert of the Observatoire de Paris, published their findings in this week's issue of the journal Nature. Those findings shed unprecedented light (so to speak) on a mysterious period in the development of the universe, about 600,000 years after its big-bang origin, when the radiation of the first stars began clearing out the neutral hydrogen that filled the infant universe. That process, known as reionization, transformed the cosmos from an opaque haze to the mostly empty space we know today.
"Measuring the redshift of the most distant galaxy so far is very exciting in itself, but the astrophysical implications of this detection are even more important," Nicole Nesvadba of France's Institute d'Astrophysique Spatiale said. "This is the first time we know for sure that we are looking at one of the galaxies that cleared out the fog which had filled the very early universe."
Further observations are likely to flesh out the scientific story of how the universe emerged from its dark ages.
The Hubble Ultra Deep Field shows several candidates for breaking observational distance records, but confirming those distances is difficult. The inset picture highlights the galaxy UDFy-38135539, which is the farthest observed object to have its distance confirmed.
How the measurement was done
The story of UDFy-38135539 begins with last year's release of the latest Hubble Ultra Deep Field imagery, captured using the Hubble Space Telescope's brand-new Wide Field Camera 3. Astronomers checked the spectral signatures of thousands of faint objects in the picture, looking for the telltale signs of extreme redshift -- that is, a shift in the spectrum that is linked to how far away an object is in our expanding universe.
The ESO astronomers found several galaxies that had their light shifted so far to the red side of the spectrum that they knew those galaxies had to be incredibly distant. Numerically speaking, their redshift had to be greater than 8. But how much greater?
To figure out the precise redshift number, the astronomers booked 16 hours of time on the ESO's Very Large Telescope in Chile, which is equipped with an ultra-sensitive infrared spectroscopic instrument called SINFONI. After weeks of data analysis, the team ran the numbers and came up with a redshift of 8.55. That meant the galaxy was farther away than the most distant previously known galaxy (redshift 6.96) as well as the most distant previously known object (a gamma-ray burst at redshift 8.2).
That redshift means the light left the galaxy when the 600,000-year-old universe was in its era of reionization. But based on the models for the development of galaxies, UDFy-38135539 would not have had enough power at that time to clear out enough empty space for the light to shine through as it did. That's why scientists suspect that other, undetected galaxies were helping to clear out the bubble of space.
In a Nature commentary, Michele Trenti, an astronomer at the University of Colorado's Center for Astrophysics and Space Astronomy, hailed the results as "a fundamental leap forward in observational cosmology." He noted that there was "robust statistical confidence" that the team's interpretation was correct, with only a 0.1 percent chance that the interpretation of the galaxy's spectrum was incorrect.
Trenti said the study "opens up exciting proects for spectroscopy of high-redshift objects" -- not only using the data currently at hand, but also drawing upon future studies to be conducted by Hubble and its successor, the James Webb Space Telescope, as well as the European Extremely Large Telescope.
Q&A with the research team's leader
The leader of the research team, Matt Lehnert of the Observatoire de Paris, answered a couple of my follow-up questions in an e-mail exchange:
Cosmic Log: Could you explain why this observation is so difficult? Of course the faintness of the galaxy is one of the big issues, but I understand that the high redshift is another big issue.
Matt Lehnert: You are correct, it is not only the faintness. It becomes increasingly difficult because the night sky becomes brighter (which causes more background noise), contains a plethora of emission lines caused mainly by OH molecules in the upper atmosphere of the earth, and light is increasingly absorbed due to many molecules and other complex interactions. We cannot overcome all of these problems. Light lost is light lost. Having a very efficient spectrograph helps.
SINFONI is certainly that. Perhaps the best currently available. You also have to have good data reduction software. It's not very romantic, but removing those night sky lines is tricky -- they are strong, much, much stronger than the signal, and they vary with time. Because they are bright, they add lots of noise, but much of that "additional" noise is due to improper removal. My colleague, Nicole Nesvadba, has literally developed an excellent set of tools for extracting the most out of these data.
Q: Could you please also talk about the significance of the conclusions you reached on the galaxy's place in the epoch of reionization. I understand that the luminosity from the galaxy alone wouldn't have been enough to allow the redshifted photons to escape, and that the assumption is that there were surrounding smaller galaxies that aided in "carving" out a suitable bubble of ionized hydrogen gas. Does this fit with the existing models for galaxy formation during that epoch, or does it rule out any models that theorists have come up with? What do scientists hope to gain by learning more about the reionization epoch?
A: Well ... I always believe that models should be tested with results! Astronomy is still an empirical science and so much of what we model is based on observational results.
The underlying physics is very complicated. For example, we really do not have a robust picture of how individual stars form. As you might imagine, since galaxies are made up of stars, and are to some extent defined by these stars, it is difficult to understand how galaxies form without this essential understanding of how stars form. Having said all of that, our current models do in fact predict that reionization was mostly due to numerous faint objects and that the first places to be reionized were the ones that had higher densities of objects. Was it a surprise for me? Yes. Was it a surprise for all astronomers? No way!
What we hope to learn is, what types of galaxies were really responsible and in fact, were only galaxies responsible? There are other ideas, mini-quasars -- small black holes that accrete matter and contribute, to decaying particles, to several other [ideas that have been] at least proposed if not all that plausible.
We would like to know how reionization proceeded. Was it in fits and starts? Did it start in regions of the highest densities and then proceed to the lowest? How long did it take? How did this gas cool to form the first galaxies, and how did galaxy formation change because the universe was reionized?
These first galaxies literally changed the state of the universe. It was most neutral -- composed mainly of hydrogen and helium atoms -- to mostly ionized between galaxies -- composed mostly of protons, electrons, and helium nuclei (although helium re-ionization came later at lower redshifts).
It is a great challenge to understand how did these humble galaxies, humble because they are small, low-mass galaxies, change the state of the universe? It's an exciting puzzle and a challenge to our understanding of physics.