Ancient Dwarf Galaxies Orbiting the Milky Way -Clues to Dark Matter?

Ancient Dwarf Galaxies Orbiting the Milky Way -Clues to Dark Matter?

 

Dwarf galaxies are faint, inconspicuous systems with only a few million stars, but they may ultimately play a key role in understanding dark matter. Measurements of the random motions of stars in nearby dwarf galaxies show that these galaxies may require a much larger fraction of dark matter than normal galaxies. They may be the best places to search for X-rays or gamma-rays which could result from the decay or annihilation of dark matter particles.

Astronomers recently discovered a relic from the early universe -- a star that may have been among the second generation of stars to form after the Big Bang. Located in the dwarf galaxy Sculptor some 290,000 light-years away, the star has a remarkably similar chemical make-up to the Milky Way's oldest stars. Its presence supports the theory that our galaxy underwent a "cannibal" phase, growing to its current size by swallowing dwarf galaxies and other galactic building blocks.

Dwarf galaxies are small galaxies with just a few billion stars, compared to hundreds of billions in the Milky Way. In the "bottom-up model" of galaxy formation, large galaxies attained their size over billions of years by absorbing their smaller neighbors.

"If you watched a time-lapse movie of our galaxy, you would see a swarm of dwarf galaxies buzzing around it like bees around a beehive," explained Anna Frebel of the Harvard-Smithsonian Astrophysics Center. "Over time, those galaxies smashed together and mingled their stars to make one large galaxy -- the Milky Way."

If dwarf galaxies are indeed the building blocks of larger galaxies, then the same kinds of stars should be found in both kinds of galaxies, especially in the case of old, "metal-poor" stars. To astronomers, "metals" are chemical elements heavier than hydrogen or helium. Because they are products of stellar evolution, metals were rare in the early Universe, and so old stars tend to be metal-poor.

Old stars in the Milky Way's halo can be extremely metal-poor, with metal abundances 100,000 times poorer than in the Sun, which is a typical younger, metal-rich star. Surveys over the past decade have failed to turn up any such extremely metal-poor stars in dwarf galaxies, however.

"The Milky Way seemed to have stars that were much more primitive than any of the stars in any of the dwarf galaxies," says co-author Josh Simon of the Observatories of the Carnegie Institution. "If dwarf galaxies were the original components of the Milky Way, then it's hard to understand why they wouldn't have similar stars."

The team suspected that the methods used to find metal-poor stars in dwarf galaxies were biased in a way that caused the surveys to miss the most metal-poor stars. Team member Evan Kirby, a Caltech astronomer, developed a method to estimate the metal abundances of large numbers of stars at a time, making it possible to efficiently search for the most metal-poor stars in dwarf galaxies.

"This was harder than finding a needle in a haystack. We needed to find a needle in a stack of needles," said Kirby. "We sorted through hundreds of candidates to find our target."

Among stars he found in the Sculptor dwarf galaxy was one faint, 18th-magnitude speck designated S1020549. Spectroscopic measurements of the star's light with Carnegie's Magellan-Clay telescope in Las Campanas, Chile, determined it to have a metal abundance 6,000 times lower than that of the Sun; this is five times lower than any other star found so far in a dwarf galaxy.

The researchers measured S1020549's total metal abundance from elements such as magnesium, calcium, titanium, and iron. The overall abundance pattern resembles those of old Milky Way stars, lending the first observational support to the idea that these galactic stars originally formed in dwarf galaxies.

The researchers expect that further searches will discover additional metal-poor stars in dwarf galaxies, although the distance and faintness of the stars pose a challenge for current optical telescopes. The next generation of extremely large optical telescopes, such as the proposed 24.5-meter Giant Magellan Telescope, equipped with high-resolution spectrographs, will open up a new window for studying the growth of galaxies through the chemistries of their stars.

In the meantime, says Simon, the extremely low metal abundance in S1020549 study marks a significant step towards understanding how our galaxy was assembled. "The original idea that the halo of the Milky Way was formed by destroying a lot of dwarf galaxies does indeed appear to be correct."

New observations using ESO’s Very Large Telescope have been used to solve an important astrophysical puzzle concerning the oldest stars in our galactic neighborhood hidden until recently in dwarf galaxies orbiting the Milky Way. In comparison to the Milky Way, most dwarf galaxies are blob-like, 85% smaller (around 6700 vs 100,000 light-years across), containing around 30 billion stars.

When the Universe was just a fraction of its current age, and galaxies such as the one we inhabit were nowhere to be seen, stars formed inside odd structures, that have long since disappeared.  These structures are called dwarf irregulars, and the scientist believes that the peculiar type of stellar formation processes they display may resemble the original one and may provide clues as to how stars appeared shortly after the Big Bang. 

Unlike massive galaxies, such as the Milky Way, with highly-defined central regions, spiral arms and so on, dwarf irregulars are very small and diffuse groups of stars, which are the last thing to spring to mind when thinking of the word “galaxy”. Star formation in dwarfs today is similar to star formation right after the Big Bang.

“We have, in effect, found a flaw in the forensic methods used until now,” says Else Starkenburg, lead author of the paper reporting the study. “Our improved approach allows us to uncover the primitive stars hidden among all the other, more common stars.”

Primitive stars are thought to have formed from material forged shortly after the Big Bang, 13.7 billion years ago. They typically have less than one thousandth the amount of chemical elements heavier than hydrogen and helium found in the Sun and are called “extremely metal-poor stars." They belong to one of the first generations of stars in the nearby Universe. Such stars are extremely rare and mainly observed in the Milky Way.

Cosmologists think that larger galaxies like the Milky Way formed from the merger of smaller galaxies. Our Milky Way’s population of extremely metal-poor or “primitive” stars should already have been present in the dwarf galaxies from which it formed, and similar populations should be present in other dwarf galaxies. Metals” are all the elements other than hydrogen and helium. Such metals, except for a very few minor light chemical elements, have all been created by the various generations of stars.

“So far, evidence for them has been scarce,” says co-author Giuseppina Battaglia. “Large surveys conducted in the last few years kept showing that the most ancient populations of stars in the Milky Way and dwarf galaxies did not match, which was not at all expected from cosmological models.”

Element abundances are measured from spectra, which provide the chemical fingerprints of stars . Since the dwarf galaxies are typically 300 000 light years away — which is about three times the size of our Milky Way — only strong features in the spectrum could be measured, like a vague, smeared fingerprint. The team found that none of their large collection of spectral fingerprints actually seemed to belong to the class of stars they were after, the rare, extremely metal-poor stars found in the Milky Way.

The team of astronomers around Starkenburg has now shed new light on the problem through careful comparison of spectra to computer-based models. They found that only subtle differences distinguish the chemical fingerprint of a normal metal-poor star from that of an extremely metal-poor star, explaining why previous methods did not succeed in making the identification.

The astronomers also confirmed the almost pristine status of several extremely metal-poor stars thanks to much more detailed spectra obtained with the UVES instrument on ESO’s Very Large Telescope. “Compared to the vague fingerprints we had before, this would be as if we looked at the fingerprint through a microscope,” explains team member Vanessa Hill. “Unfortunately, just a small number of stars can be observed this way because it is very time consuming.”

“Among the new extremely metal-poor stars discovered in these dwarf galaxies, three have a relative amount of heavy chemical elements between only 1/3000 and 1/10 000 of what is observed in our Sun, including the current record holder of the most primitive star found outside the Milky Way,” says team member Martin Tafelmeyer.

“Not only has our work revealed some of the very interesting, first stars in these galaxies, but it also provides a new, powerful technique to uncover more such stars,” concludes Starkenburg. “From now on there is no place left to hide!”

Casey Kazan via materials provided by Harvard-Smithsonian Center for Astrophysics.