Hubble Takes Galaxy Census

New observations by the Hubble Space Telescope reveal some of the earliest galaxies in the universe.

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 seven 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.

Photometric redshifts are less precise than spectroscopic ones, which measure specific spectral lines and determine how much their wavelengths have shifted due to cosmic expansion. Photometric redshifts are comparatively a broad-brush approach and require less exposure time, making them faster and easier to measure than their spectroscopic counterparts, but also less dependable.

Using this filter approach, the astronomers found seven galaxies that fit in the time range they set out to study. The redshifts range from 8.6 (about 590 million years after the Big Bang) to 9.5 (510 million years), with one outlier potentially at 11.9. That source, UDFj-39546284, was first reported at a redshift of 10.3 in 2009 using photometric measurements from the HUDF. But the new 11.9 measurement, which would put the galaxy 380 million years after the Big Bang and make it the most distant object detected thus far, is not air-tight. The astronomers only detected the galaxy in one filter, and there is a chance — small, but extant — that it’s actually an exotic foreground source peeking in, says study leader Richard Ellis (Caltech).

“We set out in this campaign to add a new filter to the array of filters in which this field has been imaged, purposefully to avoid single-filter detections,” he says. “So we were a little surprised, to be frank, that we found an object which had only been seen in one filter.”

Ellis says that the ultimate test will be an infrared spectrum, which he hopes to obtain using one of the Keck telescopes next year. It won’t be easy: taking an infrared spectrum of this galaxy will probably require a two-night exposure, he says. “Until then, I think my best bet is that this object is at 11.9.”

But Ellis stresses that the take-away message is not the 11.9 galaxy, but rather the census, which provides a crucial look at the so-called reionization era. During this cosmic era, ultraviolet radiation from luminous sources — such as early galaxies' burgeoning star populations — broke the universe’s hydrogen atoms into protons and electrons. These sources started the synthesis of heavy elements, including those that we're made of. 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), a theorist who studies the formation of the universe’s first stars. “[It's] similar to having the first ultrasound of an infant.”

Comparing the number of objects observed to those seen at redshifts of 7 and 8 reveals a steady increase in the number of galaxies as the universe aged. That contrasts with previous work, which turned up fewer objects at the highest redshifts and therefore suggested some sort of spike in formation and an abrupt reionization era, the authors say. If the transition is smooth, that implies reionization was a gradual process, extending over several hundred million years.

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.

Reference: R.S. Ellis et al. The Abundance of Star-forming Galaxies in the Redshift Range 8.5 to 12: New Results from the 2012 Hubble Ultra Deep Field Campaign.” Posted to November 29, 2012.

You can read more in the Space Telescope Science Institute's press release.