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Zooming in on the earliest galaxies


An international team of astronomers has used the Hubble Space Telescope to take a census of some of the universe’s earliest galaxies. The results, reported in an upcoming issue of the Astrophysical Journal Letters, confirm that galaxies formed gradually in the early universe and not in a dramatic spurt.

This new image of the Hubble Ultra Deep Field 2012 campaign reveals a previously unseen population of several faraway galaxies, which are observed as they appeared in a period 350 million to 600 million years after the Big Bang. NASA / ESA / R. Ellis (Caltech) / UDF 2012 Team

The astronomers 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 team focused for 100 hours on the Hubble Ultra Deep Field (HUDF), a region of sky about one-tenth the diameter of the full Moon. They then combined these observations with 2009 HUDF work to produce the new results. Using galaxies’ detectability in the different infrared wavelength bands, the astronomers calculated the galaxies’ photometric redshifts, providing estimates of how far back in cosmic time the galaxies are.

These and other observations reveal that early galaxies are smaller and intrinsically fainter than today’s galaxies, in keeping with expectations that smaller, feebler objects grew and merged to form larger structures, says Loeb. “These early galaxies represent the building blocks of the present-day galaxies that we have . . . this is very exciting.”

The new results might mark the limit for Hubble: by design Hubble cannot see earlier than a redshift of about 12, notes study coauthor James Dunlop (University of Edinburgh, Scotland). That’s because heat from the telescope will ruin images at infrared wavelengths longer than about 1.7 microns.

That’s where the James Webb Space Telescope will come in. Its reputation for being a black hole for NASA’s cash aside, JWST will have infrared capabilities that Hubble does not, possibly peering even back to the formation of the very first stars. Dunlop says there’s no technical redshift limit for the scope; its success will depend on how many things there are to see.


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