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Hubble finds faraway galaxies.

Astronomers have used the Hubble Space Telescope to take a census of the universe's first galaxies. The results, reported in the Astrophysical Journal Letters, confirm that galaxies started forming gradually in the early universe and not in a dramatic spurt.

The team used four near-infrared filters on Hubble's Wide Field Camera 3 to search for star-forming galaxies about 400 to 600 million years after the Big Bang. The scope stared for 100 hours at a square of sky in Fornax about one-tenth the diameter of the Moon, known as the Hubble Ultra Deep Field (HUDF). The team then combined these observations with 2009 HUDF work to produce the new results.

Cosmic expansion shifts distant galaxies' light to longer wavelengths, and highly redshifted galaxies are visible only at infrared wavelengths. Judging by the galaxies' visibility with different Hubble filters, the astronomers calculated the galaxies' photometric redshifts--estimates of how much space has expanded since the galaxies shone as we see them.

Photometric redshifts are less accurate than spectroscopic ones, which measure narrow spectral lines. But the broad-brush approach requires less exposure time.

Using this method, the team found seven galaxies that fit the target time range. The redshifts range from 8.6 (590 million years after the Big Bang) to 9.5 (520 million years). One outlier is potentially at redshift 11.9. That source, UDF j-39546284, was first reported at redshift 10.3 in 2009 using HUDF photometry. But the new 11.9 measurement, which would put the galaxy 380 million years after the Big Bang, is not airtight. The astronomers only detected the galaxy in one filter, and there's a chance that it's some exotic foreground source, says study leader Richard Ellis (Caltech). He says that the ultimate test will be a true infrared spectrum, which he hopes to obtain using one of the Keck telescopes.

The census provides a crucial look deep into the reionization era. During this epoch, ultraviolet radiation from the first post-Big-Bang light sources knocked electrons from the neutral hydrogen atoms filling space. These sources started the synthesis of heavy elements. The new study is basically "the deepest archaeological dig that we have" into this part of cosmic history, says Abraham Loeb (Harvard-Smithsonian Center for Astrophysics).

When compared with lower redshift studies, the new results show that the number of galaxies grew steadily as the universe aged. If the transition was smooth, reionization was probably gradual, extending over several hundred million years.

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Title Annotation:GALAXIES
Author:Carlisle, Camille M.
Publication:Sky & Telescope
Geographic Code:1USA
Date:Mar 1, 2013
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