Hubble Discovers Oldest Spiral Galaxy

An international team of astronomers using the Hubble Space Telescope has discovered the Universe’s oldest spiral galaxy yet seen, an object that dates back more than 10.7 billion years.

An artist’s rendering of galaxy BX442 and its companion dwarf galaxy (Joe Bergeron / Dunlap Institute for Astronomy & Astrophysics)

The astronomers said they discovered it while using the Hubble to capture pictures of about 300 very distant galaxies in the early Universe.

This distant spiral galaxy, called BX442, 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 Dr Alice Shapley, an associate professor of physics and astronomy at the University of California in Los Angeles and co-author of a paper published July 19 in the journal Nature. “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,” Dr Shapley said.

“The fact that this galaxy exists is astounding,” said Dr David Law, a 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.”

The galaxy is quite large compared with other galaxies from this early time in the Universe; only about 30 of the galaxies that Dr Law and Dr Shapley analyzed are as massive as this galaxy.

To gain deeper insight into their unique image of BX442, the team went to the 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 Prof James Larkin of the University of California in Los Angeles.

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.

False color composite image of BX442 (David Law / Dunlap Institute for Astronomy & Astrophysics)

“We first thought this could just be an illusion, and that perhaps we were being led astray by the picture,” Dr 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.”

The astronomers 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.

“We want to take pictures of this galaxy at other wavelengths,” Dr 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.”

“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,” Dr Shapley said.

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Bibliographic information: Law DR et al. 2012. High velocity dispersion in a rare grand-design spiral galaxy at redshift z = 2.18. Nature 487, 338–340 (19 July 2012); doi: 10.1038/nature11256