June 7, 2012 | 14
The universe is a big place, and by peering across it astronomers get to look back in time. A galaxy or supernova so far away that it takes two billion years for its light to reach us will be seen here as it appeared two billion years ago. Remarkably, today’s best telescopes can look across the majority of cosmic time, spying on galaxies as they looked just hundreds of millions of years after the big bang.
That’s just what a team of Japanese researchers has now done with the 8.2-meter Subaru Telescope in Hawaii. The group, led by Takatoshi Shibuya of the Graduate University for Advanced Studies in Tokyo and the National Astronomical Observatory of Japan, found what appears to be a galaxy 750 million years or so after the big bang. The study is now available online and has been accepted for publication in the Astrophysical Journal.
Distant galaxies are hard to find, but they’re exceedingly numerous, so they do not get names as snappy as those assigned to nearby galaxies such as Andromeda or Fornax. The newfound galaxy is known as SXDF-NB1006-2, after the area of sky in which it was found (the Subaru/XMM-Newton Deep Field, or SXDF) and the infrared filter at Subaru with which it was identified (NB1006).
Light from objects so distant has been stretched toward longer wavelengths as it travels across the expanding universe, and astronomers and cosmologists use the degree of stretching, or redshift, as a measure of distance traveled. In the case of SXDF-NB1006-2, the redshift implies that the object existed nearly 13 billion years ago.
Such objects are incredibly useful markers of the universe’s overall state during an early, transitional phase. Around the time that SXDF-NB1006-2 emitted the light now reaching telescopes on Earth, the neutral hydrogen atoms of intergalactic space were being ionized by newly formed stars and galaxies. Shibuya and his colleagues looked for objects in the sky emitting a specific wavelength of light from hydrogen atoms known as the Lyman alpha line. Lyman alpha photons can pass through ionized hydrogen but are blocked by neutral hydrogen. Measuring the number of galaxies visible as Lyman alpha emitters at various redshifts, then, can help pinpoint when the universe switched from neutral to ionized.
To calculate the redshift of SXDF-NB1006-2, the researchers took spectra of the object with the 10-meter Keck II telescope in Hawaii, breaking down the galaxy’s light into its component wavelengths. They identified a spectral line that seems to be Lyman alpha emission from a redshift of 7.215. A few other candidate galaxies at similar redshifts were discarded due to inconclusive spectra or variations in brightness that indicated that the object in question was a flaring black hole rather than an ordinary, distant galaxy.
It’s a very impressive piece of work, but it’s too bad that it was accompanied by a misleading press release proclaiming SXDF-NB1006-2 “the most distant galaxy ever found.” The researchers make no such claim in their study, and in recent years astronomers have located dozens and dozens of galaxies at redshifts of approximately 8 and one probable galaxy at a redshift of about 10, corresponding to a time 500 million years after the big bang. Those exceedingly faint objects have not generally been followed up with spectral observations, in the way that Shibuya and his team have done, limiting the precision of the cosmic distance estimates. But there is at least one galaxy more distant than SXDF-NB1006-2 that received spectral follow-up. In a 2010 study, astronomers found a spectral line, albeit a faint one, that placed a galaxy at a redshift of 8.55, using the Hubble Space Telescope and the European Southern Observatory’s Very Large Telescope in Chile. That object, UDFy-38135539, existed just 600 million years after the big bang.