June 9, 2009
PASADENA, Calif.—Astronomers using the W. M. Keck Observatory have discovered distant galaxies as massive as the Milky Way yet ten to 1000 times more compact. The new results, announced June 9 at the 214th American Astronomical Society meeting in Pasadena, provide astronomers with surprising clues about early star and galaxy formation at a time when the Universe was just a few billion years old.
“The shapes of these galaxies tell us that it is not reasonable to expect they could occur from mergers. Instead, the kind of disks we’re seeing and the constituent stars seemed to have formed all at once, directly from the gas. In the old lingo, this is monolithic galaxy formation,” said astronomer Alan Stockton of the University of Hawai’i.
He and his colleagues Dr. Gabriela Canalizo of the University of California, Riverside and Dr. Elizabeth McGrath of the University of California, Santa Cruz used the Keck II telescope and its Laser Guide Star Adaptive Optics, or LGSAO, to image radio galaxies and quasars that are roughly 11 billion light years from Earth.
The Keck LGSAO system uses a powerful laser to excite sodium atoms in the upper atmosphere so that they emit light and appear as an artificial star. Astronomers use this artificial starlight to analyze how the atmosphere is distorting incoming light from their target astronomical sources. The distortion can then be corrected using a compensating distortion in a deformable mirror in the adaptive optics system.
From these AO-corrected observations of distant galaxies, Stockton and his colleagues could model the detailed structures of their target galaxies, which are quite unlike those of massive galaxies in the present-day Universe. The team found the objects had masses that were a hundred billion times the mass of the Sun, yet were compact and have diameters of roughly 3,000 to 15,000 light years. By comparison, the diameter of the Milky Way is 100,000 light years, yet it has a mass of about 500 billion solar masses.
Teams using the Hubble Space Telescope have also found that high redshift galaxies tend to be more compact than astronomers expected. Stockton said his team was able to obtain near-infrared images from the ground that were almost two times sharper than those they could obtain with the Hubble Space Telescope at similar wavelengths. These Keck LGSAO images allow not only the measurement of characteristic sizes of the distant galaxies, but also more detailed properties of the light distribution that may give clues to formation processes.
For example, Stockton’s team imaged a field of five galaxies two of which show a tidal tail (Fig. 2) that would be indistinguishable with Hubble. “The tail, however, can only form if the galaxies we observe were disk galaxies,” Stockton said. “This Keck data gives us further evidence of that conclusion.”
Astronomers expected that distant galaxies might be disk galaxies and would be more compact than today’s galaxies. They did not expect the galaxies to be as dense as Stockton’s observations indicate, and researchers have not yet identified objects in the local Universe that resemble these compact disk galaxies. This is surprising because dense, disk-like objects are like cannon balls and are therefore not easily destroyed by collisions, meaning some should survive today.
“It might therefore be possible that these disk galaxies have instead become the cores of today’s galaxies,” Stockton said.
The data cannot yet answer this or other questions about the morphology and evolution of these two billion-year-old galaxies. Stockton said that he is currently trying to obtain clearer spectral data of the distant galaxies to determine how fast their constituent stars are moving about their centers—this will enable astronomers to independently determine the galaxies’ masses. His team is also currently looking for examples of very compact galaxies that have survived to a time when the Universe was half its present age, about seven billion years old. It will be possible to obtain much more detailed observations of such galaxies, which may lead to a better physical understanding of these objects. Observations to find disk galaxies at more distant redshifts will also be done to determine if disk galaxies exist in the very early Universe, Stockton said.
The Keck II telescope and its LGSAO are operated by the W. M. Keck Observatory, which manages twin ten-meter optical/infrared telescopes on the summit of Mauna Kea on the island of Hawai’i. The two telescopes feature a suite of advanced instrumentation including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrographs and a world-leading laser-guide-star adaptive optics system. The Observatory is operated as a scientific partnership among the California Institute of Technology, the University of California and NASA. For information please call 808.885.7887 or visit http://www.keckobservatory.org.
This research was supported in part by the National Science Foundation, under grant AST 03-07335.