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Saturday, July 21, 2012

BX442, a spiral galaxy from the depths of time

A Grand-Design Spiral Galaxy before its Time – Dunlap Institute for Astronomy & Astrophysics.

Artist's rendering of galaxy BX442 and companion. Credit: Dunlap Institute for Astronomy & Astrophysics; Joe Bergeron
A team led by an astronomer at the Dunlap Institute for Astronomy & Astrophysics, University of Toronto, 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. According to Dunlap Institute postdoctoral fellow and Principal Investigator David Law, “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.”

Law says, “The fact that this galaxy exists is astounding. 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.  

The rest of Law’s team comprises 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 were published in the 19 July 2012 issue of the science journal Nature.  

HST rest-frame optical continuum image of galaxy BX442. Credit: David Law; Dunlap Institute for Astronomy & Astrophysics
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 galactic 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 incoming 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. According to Shapley, “Immediately, we started wondering how such a spiral galaxy might form in the early universe.” 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.  

HST/Keck false colour composite image of galaxy BX442. Credit: David Law; Dunlap Institute for Astronomy & Astrophysics
Kinematic velocity and velocity dispersion maps of BX442. Credit: David Law; Dunlap Institute for Astronomy & Astrophysics

Astronomers using the Hubble Space Telescope report the earliest spiral galaxy ever seen - UCLA

Astronomers have witnessed for the first time a spiral galaxy in the early universe, billions of years before many other spiral galaxies formed. In findings reported July 19 in the journal Nature, the astronomers said they discovered it while using the Hubble Space Telescope to take pictures of about 300 very distant galaxies in the early universe and to study their properties. This distant spiral galaxy is being observed as it existed roughly three billion years after the Big Bang, and light from this part of the universe has been traveling to Earth for about 10.7 billion years.  

"As you go back in time to the early universe, galaxies look really strange, clumpy and irregular, not symmetric," said Alice Shapley, a UCLA associate professor of physics and astronomy, and co-author of the study. "The vast majority of old galaxies look like train wrecks. Our first thought was, why is this one so different, and so beautiful?"

Galaxies in today’s universe divide into various types, including spiral galaxies like our own Milky Way, which are rotating disks of stars and gas in which new stars form, and elliptical galaxies, which include older, redder stars moving in random directions. The mix of galaxy structures in the early universe is quite different, with a much greater diversity and larger fraction of irregular galaxies, Shapley said.

"The fact that this galaxy exists is astounding," said David Law, lead author of the study and Dunlap Institute postdoctoral fellow at the University of Toronto’s Dunlap Institute for Astronomy & Astrophysics. "Current wisdom holds that such ‘grand-design’ spiral galaxies simply didn’t exist at such an early time in the history of the universe." A ‘grand design’ galaxy has prominent, well-formed spiral arms.

The galaxy, which goes by the not very glamorous name of BX442, is quite large compared with other galaxies from this early time in the universe; only about 30 of the galaxies that Law and Shapley analyzed are as massive as this galaxy.

To gain deeper insight into their unique image of BX442, Law and Shapley went to the W.M. Keck Observatory atop Hawaii’s dormant Mauna Kea volcano and used a unique state-of-the-science instrument called the OSIRIS spectrograph, which was built by James Larkin, a UCLA professor of physics and astronomy. They studied spectra from some 3,600 locations in and around BX442, which provided valuable information that enabled them to determine that it actually is a rotating spiral galaxy — and not, for example, two galaxies that happened to line up in the image.

"We first thought this could just be an illusion, and that perhaps we were being led astray by the picture," Shapley said. "What we found when we took the spectral image of this galaxy is that the spiral arms do belong to this galaxy. It wasn’t an illusion. We were blown away." Law and Shapley also see some evidence of an enormous black hole at the center of the galaxy, which may play a role in the evolution of BX442.

Why does BX442 look like galaxies that are so common today but were so rare back then?

Law and Shapley think the answer may have to do with a companion dwarf galaxy, which the OSIRIS spectrograph reveals as a blob in the upper left portion of the image, and the gravitational interaction between them. Support for this idea is provided by a numerical simulation conducted by Charlotte Christensen, a postdoctoral scholar at the University of Arizona and a co-author of the research in Nature. Eventually the small galaxy is likely to merge into BX442, Shapley said.

"BX442 looks like a nearby galaxy, but in the early universe, galaxies were colliding together much more frequently," she said. "Gas was raining in from the intergalactic medium and feeding stars that were being formed at a much more rapid rate than they are today; black holes grew at a much more rapid rate as well. The universe today is boring compared to this early time."

Law, a former Hubble postdoctoral fellow at UCLA, and Shapley will continue to study BX442.

"We want to take pictures of this galaxy at other wavelengths," Shapley said. "That will tell us what type of stars are in every location in the galaxy. We want to map the mixture of stars and gas in BX442."

Shapley said that BX442 represents a link between early galaxies that are much more turbulent and the rotating spiral galaxies that we see around us. "Indeed, this galaxy may highlight the importance of merger interactions at any cosmic epoch in creating grand design spiral structure," she said.

Studying BX442 is likely to help astronomers understand how spiral galaxies like the Milky Way form, Shapley said.

Co-authors are Charles Steidel, the Lee A. DuBridge Professor of Astronomy at the California Institute of Technology; Naveen Reddy, assistant professor of physics and astronomy at UC Riverside; and Dawn Erb, assistant professor of physics at the University of Wisconsin, Milwaukee.  

David Law. Credit: Dunlap Institute of Astronomy & Astrophysics

High velocity dispersion in a rare grand-design spiral galaxy at redshift z = 2.18 – Nature (abstract)

Although grand-design spiral galaxies are relatively common in the local Universe, only one has been spectroscopically confirmed to lie at redshift z > 2 (HDFX 28; z = 2.011); and it may prove to be a major merger that simply resembles a spiral in projection. The rarity of spirals has been explained as a result of disks being dynamically ‘hot’ at z > 2, which may instead favour the formation of commonly observed clumpy structures. Alternatively, current instrumentation may simply not be sensitive enough to detect spiral structures comparable to those in the modern Universe. At z < 2, the velocity dispersion of disks decreases, and spiral galaxies are more numerous by z ≈ 1. Here we report observations of the grand-design spiral galaxy Q2343-BX442 at z = 2.18. Spectroscopy of ionized gas shows that the disk is dynamically hot, implying an uncertain origin for the spiral structure. The kinematics of the galaxy are consistent with a thick disk undergoing a minor merger, which can drive the formation of short-lived spiral structure. A duty cycle of <100 Myr for such tidally induced spiral structure in a hot massive disk is consistent with its rarity.

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