October 8, 2008
Astronomers at the California Institute of Technology (Caltech) and their colleagues gained unique insight into the nature of a young star-forming galaxy as it appeared only 2 billion years after the Big Bang. The team also determined how the galaxy may eventually evolve to become a system like our own Milky Way.
To make the observations, the team coupled two techniques, gravitational lensing — in which the gravitational field of massive objects, such as foreground galaxies, bends light rays from objects located a distance behind, thus magnifying the appearance of distant sources — and laser-assisted guide star (LGS) adaptive optics (AO) on the 10-meter Keck Telescope in Hawaii.
Gravitational lensing enlarged the distant galaxy in angular size by a factor of about 8 in each direction. Adaptive optics corrects the blurring effects of Earth's atmosphere by real-time monitoring of the signal from a natural guide star or an artificial guide star. The enhanced resolution provided by adaptive optics allowed the team to determine the internal velocity structure of the remote galaxy, located 11 billion light-years from Earth, and hence its likely future evolution.
The researchers found that the distant galaxy, which is typical in many respects to others at that epoch, shows clear signs of orderly rotation. The finding was associated with observations conducted at millimeter wavelengths, which are sensitive to cold molecular gas (an indicator of galactic rotation). The finding also suggests that the source is in the early stages of assembling a spiral disk with a central nucleus similar to those seen in spiral galaxies at the present day.
Using the Hubble Space Telescope, the team located a distinctive galaxy dubbed the "Cosmic Eye" because a foreground galaxy and gravitational field distorts its from into a ring-shaped structure.
"Gravity has effectively provided us with an additional zoom lens, enabling us to study this distant galaxy on scales approaching only a few hundred light-years. This is 10 times finer sampling than hitherto possible," explains Dan Stark of Caltech, the leader of the study. "As a result, we can see, for the first time, that a typical-sized young galaxy is spinning and slowly evolving into a spiral galaxy much like our own Milky Way."
The research, described in the October 9 issue of the journal Nature, provides a demonstration of the likely power of the future Thirty Meter Telescope (TMT), the first of a new generation of large telescopes designed to exploit AO.
When completed in the latter half of the next decade, TMT's large aperture and improved optics will produce images with an angular resolution three times better than the 10-meter Keck and 12 times better than the Hubble Space Telescope, at similar wavelengths. Because of the significant improvement in angular resolution provided by AO, the TMT will be able to study the internal properties of small distant galaxies, seen as they were when the universe was young.
Likewise, the Atacama Large Millimeter Array (ALMA), a large interferometer being completed in Chile, will provide a major step forward in mapping the extremely faint emission from cold hydrogen gas #8212 the principal component of young, distant galaxies and a clear marker of cold molecular gas #8212 compared to the coarser capabilities of present facilities. In their recent research, the Caltech-led team has provided a glimpse of what can be done with the superior performance expected of TMT and ALMA.
The key spectroscopic observations were made with the OSIRIS
instrument. Astrophysicist James Larkin and collaborators at the University of California, Los Angeles developed this instrument specifically for the Keck AO system. Stark and his coworkers used the OSIRIS instrument to map the velocity across the source in fine detail, allowing them to demonstrate that it has a primitive rotating disk.
To aid in their analysis, the researchers combined data from the Keck Observatory with data taken at millimeter wavelengths by the Plateau de Bure Interferometer (PdBI), located in the French Alps. This PdBI instrument is sensitive to the distribution of cold gas that has yet to collapse to form stars. These observations give a hint of what will soon be routine with the ALMA interferometer.
"Remarkably, the cold gas traced by our millimeter observations shares the rotation shown by the young stars seen in the Keck observations. The distribution of gas seen with our amazing resolution indicates we are witnessing the gradual buildup of a spiral disk with a central nuclear component," explains co-investigator Mark Swinbank of Durham University, who was involved in both the Keck and PdBI observations.
This work demonstrates how important angular resolution has become in ensuring progress in extragalactic astronomy. This will be the key gain of both the TMT and ALMA facilities.