Astronomers thought they had a nice, clear picture of how galaxies formed billions of years ago — but now the picture is suddenly turning muddy. A team studying the faintest galaxies ever to have their spectra taken is finding far too many big, mature galaxies similar to our Milky Way much too early in cosmic history. "Theorists are not yet at the point of panic, but they're getting there," team member Roberto Abraham (University of Toronto) told a press conference at the American Astronomical Society meeting being held this week in Atlanta.
In the accepted theory, lots of little protogalaxies — clumps of dark matter, gas, and newborn stars — condensed out of the smooth material that emerged from the Big Bang. And indeed, deep images show that the universe several billion years ago was full of these little galaxylets and irregular conglomerations of them. Big, well-formed galaxies like the Milky Way were believed to take shape later, as more of these small objects fell together. Such mergers continue even today as the Milky Way collects a few remnant dwarf galaxies — the tail end of the process.
But that can't be the whole story. Abraham, Patrick McCarthy (Carnegie Institution), Karl Glazebrook (Johns Hopkins University), and several others have found that big, grown-up galaxies inhabited the early universe right along with the small fry — and may have held roughly two-thirds of all the stars that existed at that time.
The group used the 8-meter Frederick C. Gillett Gemini North telescope in Hawaii to study spectra, luminosities, and masses of more than 300 extremely dim red galaxies in the ongoing "Gemini Deep Deep Survey." They designed the project to capture the faintest galaxy spectra ever obtained. In particular, they zoomed in on objects seen 8 to 11 billion years ago (redshift 0.8 to 2), when the universe was only about 2.5 to 5 billion years old. During this era, the little protogalaxies should have been assembling into big ones at a furious rate. But this epoch has been hard to study because key spectroscopic features of galaxies are redshifted into a part of the infrared spectrum that's blanketed by airglow emission in Earth's nighttime atmosphere. Frustrated astronomers call the resulting gap in galaxy studies "the redshift desert."
But the Gemini team turned the redshift desert into "a redshift dessert," quipped Abraham. Glazebrook developed new tricks to separate faint galaxy light from the airglow even though the airglow has a surface brightness 300 times greater. These techniques allowed the group to take exposures as long 30 hours with the huge telescope.
The astronomers found that not only were many galaxies in this era big, "they were already old," said McCarthy. Spectra show that some of them contained about as many heavy elements as the Milky Way does now. To be so well matured, these anomalous giants must have been born in large numbers as early as 1 or 2 billion years after the Big Bang — which is right on the edge, or perhaps past the edge, of what galaxy-formation theorists can handle. Previous studies had overlooked this class of galaxies because they are no longer producing massive young stars in great numbers and therefore are not particularly luminous.
"Some of the largest galaxies must have formed from a different mechanism" than the standard assemblage of small parts, summed up McCarthy. Many more of them, he predicts, await discovery by the recently launched Spitzer Space Telescope (formerly named SIRTF), which is far out in space where airglow is not a problem. And theorists may have to start their job all over.