Earliest Spiral Galaxy Surprises Astronomers

HST/Keck false colour composite image of galaxy BX442.

Credit: David Law; Dunlap Insitute for Astronomy & Astrophysics

Artist's rendering of galaxy BX442 and companion

Credit: Dunlap Institute for Astronomy & Astrophysics; Joe Bergeron

Kinematic velocity and velocity dispersion maps of BX442
Credit: David Law; Dunlap Institute for Astronomy & Astrophysics

Kamuela, HI – In the beginning, galaxies were hot and clumpy – too hot to settle down and form grand spirals like the Milky Way and other galaxies seen in the nearby universe today. But astronomers have now been surprised by the discovery of a solitary grand design spiral galaxy in the early universe which could hold clues to how spirals start to take shape. The find was announced in a report in the July 19 edition of the journal Nature.

The ancient spiral, called BX442, was found by astronomers who first surveyed 300 distant galaxies using the Hubble Space Telescope, then followed up and confirmed using detailed observations and analyses from the W. M. Keck Observatory in Hawaii.

“As you go back in time to the early universe — about three billion years after the Big Bang; the light from this galaxy has been travelling to us for about 10.7 billion years —galaxies look really strange, clumpy and irregular, not symmetric” said astronomer Alice Shapley of UCLA. “The vast majority of old galaxies look like train wrecks. Our first thought was, why is this one so different, and so beautiful?”

Not only was the spiral shape clearly visible, but by using Keck’s OSIRIS instrument (OH-Suppressing Infrared Imaging Spectrograph), astronomers were able to study different parts of BX442 and determine that it is, in fact, rotating and not just two unrelated disk galaxies along the same line of sight that give the appearance of being a single spiral galaxy.

“We first thought this could just be an illusion and that perhaps we were being led astray by the picture,” said Shapley, a coauthor on the Nature paper. “What we found when we took spectra of this galaxy is that the spiral arms do belong to this galaxy; it wasn’t an illusion. Not only does it look like a rotating spiral disk galaxy; it really is. We were blown away.”

Using laser adaptive optics (AO) to cancel out much of the Earth’s atmospheric distortions, the Keck II Telescope is able to get equal or better resolution than the Hubble Space Telescope. This was critical in this case, said astronomer David Law of the Dunlap Institute for Astronomy & Astrophysics at the University of Toronto and the lead author on the paper.

“Galaxies at this distance appear super, super faint and super, super tiny,” said Law. “We needed every inch of Keck’s light collecting area, exquisite image quality from the AO system, and a sensitive instrument to not only detect the galaxy but chop up its light into 3,600 pieces to analyze. OSIRIS is really one of the only instruments in the world that could do what we needed, and everything came together beautifully.”

In the end, it took thirteen hours over three nights with the Keck II Telescope to gather the spectra from BX422 needed to confirm the nature of the distant and early spiral.

“We got a beautiful map that told us this thing is a rotating disk,” said Shapley.

What also sets BX442 apart from other galaxies of its epoch is that it appears to be in the process of merging with another galaxy. That, in fact, could be the reason it is beginning to form a spiral.

“Indeed, many of the most well-known grand design spiral galaxies in the nearby universe (e.g., M51, M81, M101) are observed to have nearby companions, and small satellites such as the Sagittarius dwarf galaxy may even be partly responsible for producing spiral patterns in our own Milky Way galaxy,” the researchers wrote in their paper.

They tested the idea with a simulation and found that the spiral pattern could be formed by such a merger. The simulations indicate that its glory may be fleeting though; the spiral may dissipate again in just 100 million years.

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

***

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; Charlotte Christensen, postdoctoral scholar at the University of Arizona, and Dawn Erb, assistant professor of physics at the University of Wisconsin, Milwaukee.

Shapley’s research is funded by the David and Lucile Packard Foundation.

The W. M. Keck Observatory operates two 10-meter optical/infrared telescopes on the summit of Mauna Kea on the Big Island of Hawaii. The twin telescopes feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectroscopy and a world-leading laser guide star adaptive optics system which cancels out much of the interference caused by Earth’s turbulent atmosphere. The Observatory is a private 501(c) 3 non-profit organization and a scientific partnership of the California Institute of Technology, the University of California and NASA.

(Partially adapted from a press release by UCLA)

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Found: Heart of Darkness

This is the portion of sky in which astronomers found the Segue 1 dwarf galaxy. Can you see it? Credit: Marla Geha

Using the DEIMOS instrument on the Keck II telescope, astronomers could identify which stars were moving together as a group. They are circled here in green. Credit: Marla Geha

By subtracting out all the other objects in the image and leaving the Segue I member stars, the “darkest galaxy” emerges. Credit: Marla Geha

All three images above are combined in this captioned mosaic

Credit: Marla Geha, Keck Observatory

Kamuela, HI – Astronomers using the 10-meter Keck II telescope in Hawaii have confirmed in a new paper that a troupe of about 1,000 small, dim stars just outside the Milky Way comprise the darkest known galaxy, as well as something else: a treasure trove of ancient stars.

By “dark” astronomers are not referring to how much light the galaxy, called Segue 1, puts out, but the fact that the dwarf galaxy appears to have 3,400 times more mass than can be accounted for by its visible stars. In other words, Segue 1 is mostly an enormous cloud of dark matter decorated with a sprinkling of stars.

The initial announcement of the “Darkest Galaxy” was made two years ago by Marla Geha, a Yale University astronomer, Joshua Simon from the Carnegie Institution of Washington, and their colleagues. This original claim was based on data from the Sloan Digital Sky Survey and the Keck II telescope. Those observations indicated the stars were all moving together and were a diverse group, rather than simply a cluster of similar stars that had been ripped out of the nearby and more star-rich Sagittarius dwarf galaxy. A competing group of astronomers at Cambridge University were, however, not convinced.

So Simon, Geha and their group returned to Keck and went to work with the telescope’s Deep Extragalactic Imaging Multi-Object Spectrograph (DEIMOS) to measure how the stars move not just in relation to the Milky Way, but also in relation to each other.

If the 1,000 or so stars were all there was to Segue 1, with just a smidgeon of dark matter, the stars would all move at about the same speed, said Simon. But the Keck data show they do not. Instead of moving at a steady 209 km/sec relative to the Milky Way, some of the Segue 1 stars are moving at rates as slow as 194 kilometers per second while others are going as fast as 224 kilometers per second.

“That tells you Segue 1 must have much more mass to accelerate the stars to those velocities,” Geha explained. The paper confirming Segue 1’s dark nature appeared in the May 2011 issue of The Astrophysical Journal.

The mass required to cause the different star velocities seen in Segue 1 has been calculated at 600,000 solar masses. But there are only about 1,000 stars in Segue 1, and they are all close to the mass of our Sun, Simon said. Virtually all of the remainder of the mass must be dark matter.

Stellar Old Folks Home

Equally exciting news from Segue 1 is its unusual collection of nearly primordial stars. One way to tell how long ago a star formed is by its heavy element content, which can be gleaned from the characteristic absorption features in the star’s spectrum. Very old or primitive stars come from a time when the universe was young and few large stars had yet grown old enough to fuse lightweight atoms like hydrogen and helium into heavier elements like iron and oxygen. Early, and therefore ancient, stars that formed from early gas clouds are therefore very low in heavy elements.

The researchers managed to gather iron data on six stars in Segue 1 with the Keck II telescope, and a seventh Segue 1 star was measured by an Australian team using the Very Large Telescope. Of those seven, three proved to have less than one 2,500th as much iron as our own Sun.

“That suggests these are some of the oldest and least evolved stars that are known,” said Simon.

Searches for such primitive stars among the Milky Way’s billions have yielded less than 30.

“In Segue 1 we already have 10 percent of the total in the Milky Way,” Geha said. “For studying these most primitive stars, dwarf galaxies are going to be very important.”

Dark Matter Demolition Derby

The confirmation of the large concentration of dark matter in Segue 1 underscores the importance of other research that has focused on Segue 1. In particular, some researchers have been looking with the space-based Fermi Gamma Ray Telescope in hopes of catching sight of a faint glimmer of gamma rays which could be created, theoretically, by the collision and annihilation of pairs of dark matter particles.

So far the Fermi telescope has not detected anything of the sort, which isn’t entirely surprising and doesn’t mean the dark matter isn’t there, said Simon.

“The current predictions are that the Fermi telescope is just barely strong enough or perhaps not quite strong enough to see these gamma rays from Segue 1,” Simon explained. So there are hopes that Fermi will detect at least the hint of a collision.
“A detection would be spectacular,” said Simon. “People have been trying to learn about dark matter for 35 years and not made much progress. Even a faint glow of the predicted gamma rays would be a powerful confirmation of theoretical predictions about the nature of dark matter.”

In the meantime, astronomers suspect there are other, perhaps even darker dwarf galaxies hovering around the Milky Way, waiting to be discovered. “We’d like to find more objects like Segue 1,” Simon said.

Source: W.M.Keck Observatory

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