A survey of ultra-distant galaxies has zeroed in on one, poetically named z8_GND_5296, that is the oldest galaxy ever seen.
Astronomers measure distance by the strength of the red shift, caused by the same Doppler effect that alters the horn sound from a passing train, and “z8” came in at an unprecedented red shift of 7.5.
That means the galaxy is being seen as it was 13.1 billion years ago, just 700 million years after the Big Bang, says study first author Steven Finkelstein, an associate professor of astronomy at the University of Texas at Austin.
That’s the second-oldest astronomical object ever observed: One star explosion, from an unknown galaxy, occurred 70 million years before z8.
The discovery came during a study of 43 elderly galaxies identified by the Hubble Space Telescope. Using a spectroscope attached to the giant Keck 1 telescope in Hawaii, Finkelstein and his colleagues sought a distinctive breed of electromagnetic radiation called Lyman alpha.
Lyman alpha radiation is released when hydrogen’s sole electron relaxes into its lowest orbit. Hydrogen is the most abundant element in the universe, and Lyman alpha’s wavelength is relatively easy to measure with a spectroscope, even after a severe red-shifting due to the expansion of the universe.
So why was Lyman alpha only visible from one of the 43 geezerly galaxies? One answer could be a fundamental change in the universe. Today, hydrogen between the galaxies is ionized into bare protons, and Lyman alpha can move freely. But early in the universe, intergalactic hydrogen was apparently made of neutral atoms, which block Lyman alpha like a political campaign drowns difficult truths.
And thus finding Lyman alpha from only one the ancient galaxies may mean the change from neutral to ionized state had already begun 700 million years post-Bang.
Stars are star of the show
As best Finkelstein and his crew can figure it, z8 contains the mass of 1 billion suns; in round numbers, about 2 percent the mass of our Milky Way galaxy.
But even though the Big Bang created only hydrogen and helium, the concentration of heavier atoms in Z8 reaches 20 to 40 percent of the level in the sun. Because these heavier atoms are made only in star explosions, plenty of supernovas must already have occurred by 700 million year after the Bang.
The most surprising finding about z8, however, concerned the quick congealing of hydrogen and helium into stars. “Before, we thought a decent guess for a typical rate of star formation at this time was 10 solar masses per year per galaxy, and this is 30 times higher,” says Finkelstein.
“Maybe it’s just a weirdo galaxy, but another galaxy nearby has a rate of star formation of 100 solar masses per year; which is still pretty high compared to the 10 you would have expected,” Finkelstein says. “This tells us that extreme star formation is more common than we thought at this time. We really don’t know why; the galaxy could be merging with another galaxy. It’s a question for our theoretician friends.”
– David J. Tenenbaum