Earlier this year, Ars Technica celebrated the Hubble Space Telescope's 20th anniversary with a retrospective of what we considered to be some of its best images returned to date. The last image we highlighted was one from the Hubble Ultra Deep Field, which shows some of the earliest stars and oldest galaxies known to exist. Buried within a similar image was a speck known as UDFy-38135539, a galaxy with a redshift of approximately 8.6. Since the shifting of light towards the red end of the spectrum increases with distance (and hence time), a redshift of this magnitude means that the light seen by Hubble was generated over 13 billion years ago.

The apparently extreme redshift seen in the Hubble data was only a guess—albeit a scientifically supported one. The same data was consistent with the unlikely possibility that UDFy-38135539 existed at a more pedestrian redshift of 2.12, and was a much closer galaxy that appeared unusually young. To figure out which of these is the case, we needed to accurately measure the galaxy's distance from Earth by carrying out a spectroscopic analysis of the light that is just now reaching Earth.

The spectroscopic findings are published in this week's edition of Nature. Using the SINFONI spectrograph at the ESO Very Large Telescope, a British and French team measured the Lyman-α emission line from the light emanating from UDFy-38135539. The Lyman-α line is the light emitted when an electron falls from the n=2 quantum level to the n=1 (ground state) quantum level in a hydrogen atom. In its non-redshifted form, this light resides in the ultraviolet portion of the EM spectrum. The astronomers found the Ly-α spectral line at a wavelength of 11,615.6±2.4Å, resulting a redshift value of 8.5549±0.0002. The original UV light had been redshifted all the way to the near-infrared portion of the spectrum.

The other possibility, that UDFy-38135539 resides at a redshift of only 2.12, was also examined. If this were the case, then the spectral line the astronomers thought was a highly redshifted Ly-α line could actually be some other spectral line, like a doublet produced by oxygen that has wavelengths of 3,726 and 3,729Å. However, these lines would be very clearly resolved. Since they weren't, the authors conclude that the original assessment of the galaxy's immense age was indeed correct—UDFy-38135539 is officially the oldest object ever seen.

Not only is UDFy-38135539 the oldest object known, it comes from a critical time in the Universe's history. In between redshift values of 6 and 20—somewhere between 150 million and 1 billion years after the big bang—the neutral hydrogen atoms that permeated the Universe began to become reionized (a period known unsurprisingly as reionization) and formed the cold, low-density plasma that makes up the Universe today.

Observations of stars and galaxies from this time have the ability to provide key constraints on the physical parameters of the early Universe, including the locations of sources of reionization energy, which could clear up the muddy picture we have of it today.

Assuming that UDFy-38135539 shares some characteristics with galaxies of similar luminosity at lower redshifts, the authors can make some predictions about the nature of the galaxy. Using the Ly-α luminosity, the authors estimate a lower bound for the star formation rate to be between 0.3 and 2.1 solar mass per year.

With this number in hand, the authors attempt to answer a question: how much of the neighboring space could UDFy-38135539 have ionized? They estimate that it could impact a sphere of space no more than a couple of megaparsecs in diameter. To account for the relatively strong signal that is observed on Earth 13 billion years afterwards, they conclude that UDFy-38135539 did not act alone. There must be other unseen sources ionizing atomic hydrogen within a few megaparsecs of UDFy-38135539.

As is usual with papers of this sort, the authors conclude by suggesting that new data from the next generation of Earth-based and LEO telescopes will be able to shed much more light onto this period of the Universe's history. For now, we will have to be content with simply knowing we can see something that was formed 13+ billion years in the past.

Nature, 2010. DOI: 10.1038/nature09462

Listing image by NASA, ESA, G. Illingworth (UCO/Lick Observatory and the University of California, Santa Cruz), R. Bouwens (UCO/Lick Observatory and Leiden University), and the HUDF09 Team