SN: Dark matter eldorado

[bystander]

Dark matter eldorado
Science News | 30 July 2010

Nearby galaxy holds record for densest concentration of mysterious mass

When astronomers discovered the galaxy Segue 1 in 2007, they weren’t sure if it was anything more than a cluster of stars, perhaps stripped from the nearby Sagittarius dwarf galaxy. But observations with the Keck II telescope atop Hawaii’s Mauna Kea now confirm the status of Segue 1 as a galaxy by showing that its stars have a diverse chemical composition, Simon says.

Observations by the Anglo-Australian Telescope near Coonabarabran, Australia, have also found a diversity of stellar composition in Segue 1, a team including Rosemary Wyse of Johns Hopkins University in Baltimore, Md., report in an article scheduled to be posted at arXiv.org in early August.

After examining the stars’ compositions, Simon’s team calculated the total amount of mass in Segue 1 — both the unseen dark matter and the small number of faint, visible stars — by measuring how fast the stars move. The faster the stars orbit about the center of Segue 1, the heavier the galaxy.

The team found that although the stars in Segue 1 have a combined mass of no more than about 1,000 suns, the mass of the whole galaxy is about 500 times larger. “That tells us that Segue 1 is made almost entirely of dark matter,” Simon says.

Segue 1 is both dark matter–dominated and compact, yielding a dark matter density higher than any known galaxy. The galaxy’s high density and proximity to Earth — about 80,000 light-years distant — make it an ideal place to look for proposed signatures of dark matter.

A Complete Spectroscopic Survey of the Milky Way Satellite Segue 1: The Darkest Galaxy -- JD Simon et al

• arXiv.org > astro-ph > arXiv:1007.4198 > 23 Jul 2010 (v1), 27 Jul 2010 (v2)

Indirect Dark Matter Detection Limits from the Ultra-Faint Milky Way Satellite Segue 1 -- R Essig

• arXiv.org > astro-ph > arXiv:1007.4199 > 23 Jul 2010 (v1), 27 Jul 2010 (v2)
  Physical Review D 82 123503 (Dec 2010) DOI: 10.1103/PhysRevD.82.123503

Astronomers Discover Most Dark Matter-Dominated Galaxy In Universe
Science Daily | Yale University | 19 Sept 2008

A team led by a Yale University astronomer has discovered the least luminous, most dark matter-filled galaxy known to exist.

The galaxy, called Segue 1, is one of about two dozen small satellite galaxies orbiting our own Milky Way galaxy. The ultra-faint galaxy is a billion times less bright than the Milky Way, according to the team’s results, to be published in an upcoming issue of The Astrophysical Journal (ApJ). But despite its small number of visible stars, Segue 1 is nearly a thousand times more massive than it appears, meaning most of its mass must come from dark matter.

The Least Luminous Galaxy: Spectroscopy of the Milky Way Satellite Segue 1 -- M Geha et al

• arXiv.org > astro-ph > arXiv:0809.2781 > 16 Sep 2008
  2009 ApJ 692 1464 doi: 10.1088/0004-637X/692/2/1464

The Most Dark Matter Dominated Galaxies: Predicted Gamma-ray
Signals from the Faintest Milky Way Dwarfs -- LE Strigari et al

• arXiv.org > astro-ph > arXiv:0709.1510 > 11 Sep 2007 (v1), 29 Sep 2007 (v2)
  2008 ApJ 678 614 doi: 10.1086/529488

[bystander]

Alone In The Dark?
Universe Today | Tammy Plotner | 2011 July 31

Click to view full size image 1 or image 2

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

Two years ago, Marla Geha, a Yale University astronomer, Joshua Simon from the Carnegie Institution of Washington, and their colleagues discovered something unusual while studying with the Keck II telescope and information for the Sloan Digital Sky Survey. Their observations turned up a contrasting group of stars which all appeared to be moving in unison – not just a moving cluster of similar stars which could have been torn away from the nearby Sagittarius dwarf galaxy. The team knew they were on to something, but a competing group of astronomers at Cambridge University was skeptical. Too bad… there was a dark treasure right there before their eyes.

Not to be dissuaded, Simon, Geha and their group returned to Keck and turned the photographic eye of the telescope’s Deep Extragalactic Imaging Multi-Object Spectrograph (DEIMOS) towards their target area. Even though it was only about 1,000 small, dim stars, they wanted to know how they migrated both in respect to the Milky Way and to each other. Named Segue 1, the target the team was looking at could possibly have 3,400 times more mass than can be accounted for by its visible stars… a galaxy dominated by dark matter and salted with a handful of ancient suns. If the 1,000 or so stars were all there was to Segue 1, with just a touch 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.”

But the information from DEIMOS didn’t stop there… It also revealed an eclectic collection of nearly primordial metal-poor stars. 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 the Sun. “That suggests these are some of the oldest and least evolved stars that are known,” said Simon. This is fascinating data considering investigations for stars of this type out of the Milky Way’s billions have produced 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.”

By confirming Segue 1′s massive concentration of dark matter, other types of research into this dark galaxy’s lifestyle now become more dedicated. The space-based Fermi Gamma Ray Telescope has also been looking its way in hopes of catching a gamma-ray event created 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.”

Let’s hope Segue 1 isn’t alone in the dark.

Found: Heart of Darkness
W.M. Keck Observatory | 2011 July 29

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

---

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.

A Complete Spectroscopic Survey of the Milky Way Satellite Segue 1: The Darkest Galaxy -- JD Simon et al

• Astrophysical Journal 733(1) 46 (2011 May 20) DOI: 10.1088/0004-637X/733/1/46
  arXiv.org > astro-ph > arXiv:1007.4198 > 23 Jul 2010 (v1), 22 Feb 2011 (v3)

Probing a Galactic Heart of Darkness
Discovery News | Jennifer Ouellette | 2011 Aug 08