Some big, old galaxies gave up starmaking early
When it comes to gauging a galaxy's age, nothing says "old" like big, red, and spheroid. These are the hallmarks of bloated "island universes" whose star-forming days are virtually over.
Now, two teams of astronomers working independently have uncovered what some say is the first unambiguous evidence that mature, even over-the-hill galaxies were much more common in the early universe than previously believed.
The two teams have discovered what they say is an unexpectedly large number of enormous, quiescent, and in some cases elliptical galaxies that reached their advanced stages only three billion to six billion years after the big bang. According to current theories, this is a period when stars should be forming at a furious pace, driven in large part by galaxies merging to form ever larger galaxies.
"We should be seeing young stars in these galaxies," says Gregory Wirth, an astronomer with the Keck Observatory in Hawaii. Instead, the two teams saw few if any stars forming. "The fact these galaxies were both big and old poses an interesting problem for the theorists to try to resolve."
One team, led by Karl Glazebrook, an astronomer at the Johns Hopkins University in Baltimore, Md., used the eight-meter Gemini telescope atop Hawaii's Mauna Kea to gather spectra of galaxies over a range of distances - and, hence, age periods - for the universe.
At least two-thirds of the 150 massive galaxies in their sample reached their advanced stages after the period in question. But they also found that a significant fraction of these galaxies appeared as early as three billion years after the big bang.
The second team, led by Italian astronomer Andrea Cimatti, used the European Southern Observatory's eight-meter telescopes in Chile to discover four elliptical galaxies as huge as the largest ellipticals seen in today's universe. These four galaxies appear at distances that correspond to a time when the universe was perhaps one-fourth its current age - roughly the same period Dr. Glazebrook's team covered. Both teams reported their results in the journal Nature.
Just getting useful data from this period was a tremendous feat, astronomers say. At these distances, galaxies are extremely faint. And as viewed from Earth, their light appears as near-infrared radiation, rather than as visible light - thanks to the universe's expansion and the stretching effect it has on light from distant objects. The feeble glow from these galaxies gets swamped by the bright near-infrared background radiation Earth's atmosphere presents. This region of the cosmic timeline has been called the "redshift desert" for its paucity of observations, compared with the larger number of observations on either side of the desert.
By taking 30-hour exposures of each object, and frequently nudging their telescopes slightly off the objects in a technique dubbed "nod and shuffle," the teams were able to cancel out the atmosphere's effect and get usable spectrographic information from the galaxies - including their mass and composition.
Both teams suggest that leading models of galaxy evolution have problems accounting for the galaxies they observed.
Yet newer simulations suggest that such large galaxies could have formed as early as the observations suggest, according to Rachel Somerville, a theorist at the Space Telescope Science Institute in Baltimore. She notes that the results from a space probe called the Wilkinson Microwave Anisotropy Probe show that the early universe had "plenty of mass at early times" to yield the large galaxies the two teams measured within the time span available.
To her, the lack of active star formation in the observed galaxies presents the true puzzle.
"The real question is: Why are these galaxies not making stars?" she asks.
One possibility, she says, is that black holes in the center of these galaxies may act as shutoff valves for star formation.
Closer to home, she notes, galaxies of similar mass and shape are known to host supermassive black holes at their centers. During these galaxies' more active phases, the black holes suck matter toward them at prodigious rates, generating enormous amounts of radiation.
Astronomers have observed that these active galactic nuclei can blast vast amounts of gas out of the galaxy. Thus, it's possible for a galaxy to grow only so large before the processes surrounding its supermassive black hole grow powerful enough to sweep the galaxy free of the gas required to form new stars.
Indeed, she adds, even the process of star formation alone, if it occurs in sufficiently intense bursts, also could clean a galaxy's gaseous clock and quench further star formation.
The trick now, she concludes, is to find and observe enough of these objects to build a statistically meaningful sample for detailed study.