“Fossil Galaxy” Discovered From the Early Universe

A small galaxy circling the Milky Way may be a fossil left over from the early Universe.

The stars in the galaxy, known as Segue 1, are virtually pure with fewer heavy elements than those of any other galaxy known. Such few stars (roughly 1,000 compared to the Milky Way’s 100 billion) with such small amounts of heavy elements imply the dwarf galaxy may have stopped evolving almost 13 billion years ago.

If true, Segue 1 could offer a window into the early universe, revealing new evolutionary pathways among galaxies in the early Universe.

Only hydrogen, helium, and a small trace of lithium emerged from the Big Bang nearly 13.8 billion years ago, leaving a young universe that was virtually pure.  Over time the cycle of star birth and death produced and dispersed more heavy elements (often referred to as “metals” in astronomical circles), planting the seeds necessary for rocky planets and intelligent life.

The older a star is, the less contaminated it was at birth, and the fewer metals lacing the star’s surface today. Thus the elements detectible in a star’s spectrum provide a key to understanding the generations of stars, which preceded the star’s birth.

The Sun, for example, is metal-rich, with roughly 1.4% of its mass composed of elements heavier than hydrogen and helium. It formed only 4.6 billion years ago — two thirds of the way from the Big Bang to now — and sprung from multiple generations of earlier stars.

But three stars visible in Segue 1 have an iron abundance that is roughly 3,000 times less than the Sun’s iron. Or to use the proper jargon, these three stars have metallicities below [Fe/H] = -3.5.

Researchers led by Anna Frebel of the Massachusetts Institute of Technology report that Segue 1 “may be a surviving first galaxy that experienced only one burst of star formation” in the Astrophysical Journal.

Not only do the low chemical abundances suggest this galaxy is composed of extremely old stars, but they provide tantalizing hints about the types of supernovae explosions that helped create these stars. When high-mass stars explode they disperse a mix of elements; But when low-mass stars explode they almost exclusively disperse iron.

The lack of iron suggests the stars in Segue 1 are the products of high mass stars, which explode much more quickly than low mass stars. It appears that Segue 1 underwent a rapid burst of star formation shortly after the formation of the galaxy in the early universe.

Additionally, six stars observed show some of the lowest levels of neutron-capture elements ever found, with roughly 16,000 fewer elements than those seen in the Sun. These elements are created within stars when an atomic nucleus grabs an extra neutron. So a low level indicates a lack of repeated star formation.

Segue 1 burned through its first generation of stars quickly. But after the young galaxy produced a second generation of stars it completely shut off star formation, remaining a relic of the early universe.

The findings here suggest there may be a greater diversity of evolutionary pathways among galaxies in the early universe than had previously been thought.

But before we can make any sweeping claims “we really need to find more of these systems,” said Frebel in a press release. Alternatively, “if we never find another one, it would tell us how rare it is that galaxies fail in their evolution. We just don’t know at this stage because this is the first of its kind.”

The paper will be published in the Astrophysical Journal and is available for download here.

Alone In The Dark?


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.

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

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

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.

Original News Source: Keck Observatory Science News.