July 18, 2012 — Astronomers discover "astounding" spiral galaxy
U of T's David Law pictured in front of an artist's rendering of the spiral galaxy BX442. (Photo courtesy of Dunlap Institute for Astronomy & Astrophysics)
Formed a billion years before other spiral galaxies
By Chris Sasaki
A team led by an astronomer at the University of Toronto's Dunlap Institute for Astronomy & Astrophysics, has discovered a spiral galaxy that appears to have formed a billion years before other spirals.
The galaxy is 10.5 billion light-years from Earth, putting it at a time when the Universe was only three billion years old and spirals were extremely rare.
“Seeing this galaxy amongst the irregular, young galaxies of that epoch is like seeing a fully-formed adult in a room of grade-school children,” said David Law, Dunlap Institute postdoctoral fellow and Principal Investigator.
“The fact that this galaxy exists is astounding," Law said. "Current wisdom holds that such grand-design spiral galaxies simply didn’t exist at such an early time in the history of the Universe.”
Most galaxies in the three billion year old Universe are clumpy and irregularly-shaped; they haven’t formed the well-defined spiral arms we see in galaxies like the iconic M51 Whirlpool Galaxy.
Law’s team includes researchers from UCLA, Caltech, UC Riverside, Steward Observatory, and UW Milwaukee. The Space Telescope Science Institute provided principal funding for the work, the results of which will be published in the 19 July 2012 issue of the science journal Nature.
The researchers noticed the galaxy, identified as BX442, in images they obtained using the Hubble Space Telescope (HST). Law’s co-investigator Alice Shapley, from UCLA, remembers coming across the oddity.
"Among the irregular and clumpy galaxies of the early Universe, this well-ordered spiral stuck out like a sore thumb — a beautiful and amazing sore thumb."
But, while the Hubble image revealed the galaxy’s spiral structure, it didn’t prove conclusively that the galaxy rotated.
In order to settle this question, Law and Shapley used the Keck II telescope in Hawaii to study the object’s internal motions. The twin Keck telescopes, each with 10-metre diameter primary mirrors, are the largest optical/infrared telescopes in the world. The Keck II is equipped with a laser-guide-star adaptive-optics system which corrects for the distortion of in-coming light caused by the Earth’s turbulent atmosphere, resulting in images as sharp as those taken with the HST.
Law and Shapley used an integral-field spectrograph called OSIRIS (OH-Suppressing Infrared Imaging Spectrograph) on the Keck II telescope to sample light from different parts of the galaxy. These samples showed that those parts were moving at different speeds relative to us — revealing that it is indeed a spiral disk, rotating roughly as fast as our own Milky Way Galaxy, but much thicker and forming stars more rapidly.
While the spiral structure and rotation have been confirmed, the reason for the spiral structure remains a mystery; it’s unclear why this galaxy has been able to form such sweeping spiral structures so much earlier than other galaxies.
"Immediately, we started wondering how such a spiral galaxy might form in the early universe,” said Shapley.
One possibility, Law suggests, is the presence of a dwarf companion galaxy that they observe in the process of merging with the main galaxy. Just as Messier 51 is subject to tidal forces from a dwarf companion of its own, gravitational interaction with the newly-discovered galaxy’s dwarf companion might help excite transient spiral structure within the main galaxy.
Understanding this mechanism in greater detail could help explain the formation and evolution of modern spirals like our own Milky Way Galaxy.