In the news this month: record-breaking galaxy
The Hubble Ultra Deep Field, the deepest image of the Universe ever taken, in near-infrared light. CREDIT: NASA/ESA
The most distant object in the known universe is a highly luminous gamma ray burst, a single explosion discovered near maximum light, at a redshift of 8.2, a time when the universe was only 630 million years old, less than 5 percent of its current age. The most distant known galaxy lies at a redshift of 6.96, the light we see now left the galaxy just 750 million years after the Big Bang. However, both these records have now been broken by a galaxy discovered by the Hubble Space Telescope which has a redshift of 8.56 and an estimated distance of 87 Gpc, making it the most distant object currently known.
First seen in the Hubble Ultra Deep Field, the deepest single image ever taken in near-infrared light, the galaxy (known as UDFy-38135539) was initially classified as a candidate high redshift object based on its colours. Now, a team led by Matthew Lehnert at the Observatoire de Paris in France, has used spectroscopic observations to confirm that the object is the most distant galaxy so far detected.
Since the universe is expanding, the further away an object is, the faster it appears to be moving away from us. This results in a shift in wavelength of the light emitted from an object (known as redshift) with the size of the shift relating to the distance between us and the object. (This is similar to the shift in pitch you hear when a police car travels past at high speed.) This effect allows distances to be calculated by measuring the shift in spectral lines from known chemicals. Lehnert's team used a sensitive spectrograph on the Very Large Telescope located in Chile to observe the spectrum of this galaxy and found an emission line which is likely to be caused by hydrogen shifted to redder wavelengths by the relative motion between the galaxy and us.
This is an exciting discovery because it is the first galaxy discovered in the so-called epoch of reionisation, the period in the history of the universe where the neutral material between the newly formed galaxies was being ionised - the light from young, hot stars stripped electrons from hydrogen atoms. The authors used the measured light from the galaxy to calculate the size of the region of surrounding gas which the galaxy should have been able to ionise on its own and found that, in order to explain the size of the ionised bubble which is consistent with the observations, there must be other sources of radiation. One suggestion is that dwarf galaxies clustering around larger, more easily observed galaxies, may be responsible for this additional radiation, but there are other explanations.
While observations such as these are difficult with current ground-based telescopes due to the faint nature of these distant sources, the planned next generation of larger and more sensitive ground- and space-based instruments should make observations of such sources much easier.
This blog post is a news story from the Jodcast, aired in the November 2010 edition.
Lehnert, M., Nesvadba, N., Cuby, J., Swinbank, A., Morris, S., Clément, B., Evans, C., Bremer, M., & Basa, S. (2010). Spectroscopic confirmation of a galaxy at redshift z = 8.6 Nature, 467 (7318), 940-942 DOI: 10.1038/nature09462