MIA: Where are the Milky Way’s Missing Orbiting Dwarf Galaxies?



Astronomers have used Hubble Space Telescope to study some of the smallest and faintest galaxies in our cosmic neighborhood –fossils of the early Universe that have barely changed for 13 billion years. The discovery could help explain the so-called “missing satellite” problem, where only a handful of satellite galaxies have been found around the Milky Way, versus the thousands that are predicted by theories. Astronomers think the rest of the sky should contain dozens more of these ultra-faint dwarf galaxies with few stars, and the evidence for squelched star formation in the smallest of these dwarfs suggests that there may be still thousands more with essentially no stars at all.

The image above is a model of the Milky Way, showing stars streams torn from smaller dwarf galaxies. The structures seen in the new Sloan survey support this prediction of a complicated outer galaxy for the Milky Way. The region shown is about one million light years across.

Astronomers have puzzled over why some extremely faint dwarf galaxies spotted in our Milky Way galaxy's backyard contain so few stars. The galaxies are thought to be some of the tiniest, oldest, and most pristine galaxies in the Universe. They have been discovered over the past decade by astronomers using automated computer techniques to search through the images of the Sloan Digital Sky Survey. But an international team of astronomers needed the NASA/ESA Hubble Space Telescope to help solve the mystery of why these galaxies are starved of stars, and why so few of them have been found.

Hubble views of three of these small galaxies, the Hercules, Leo IV and Ursa Major dwarf galaxies, reveal that they all started forming stars more than 13 billion years ago — and then abruptly stopped — all in the first billion years after the Universe was born in the Big Bang. In fact, the extreme age of their stars is similar to Messier 92, the oldest known globular cluster in the Milky Way.

"These galaxies are all ancient and they're all the same age, so you know something came down like a guillotine and turned off the star formation at the same time in these galaxies," said Tom Brown of the Space Telescope Science Institute in Baltimore, USA, the study's leader. "The most likely explanation is a process called reionisation."

The relic galaxies are evidence for a transitional phase in the early Universe that shut down star-making factories in tiny galaxies. This phase seems to coincide with the time when the first stars burned off a fog of cold hydrogen, a process called reionisation. In this period, which began in the first billion years after the Big Bang, radiation from the first stars knocked electrons off primeval hydrogen atoms, ionising the Universe's cool hydrogen gas.

The same radiation that sparked universal reionisation also appears to have squelched star-making activities in dwarf galaxies, such as those in Brown's study. The small irregular galaxies were born about 100 million years before reionisation began and had just started to churn out stars at that time. Roughly 2000 light-years wide, these galaxies are the lightweight cousins of the more luminous and higher-mass star-making dwarf galaxies near our Milky Way. Unlike their higher-mass relatives, the puny galaxies were not massive enough to shield themselves from the harsh ultraviolet light. What little gas they had was stripped away as the flood of ultraviolet light rushed through them. Their gas supply depleted, the galaxies could not make new stars.

The discovery could help explain the so-called "missing satellite problem," where only a few dozen dwarf galaxies have been observed around the Milky Way while the computer simulations predict that thousands should exist. One possible explanation for the low number discovered to date is that there has been very little, or even no star formation in the smallest of these dwarf galaxies, leaving them virtually invisible.

The image below shows seven — and perhaps eight — dwarf galaxies orbiting Earth's home galaxy, the Milky Way discovered as part of the Sloan Digital Sky Survey (SDSS-II), the most ambitious survey of the sky ever undertaken.



"These dwarf galaxies have been captured by the gravity of the Milky Way and most eventually will merge with our own galaxy," explains Penn State Professor of Astronomy and Astrophysics Donald Schneider, a coauthor of the investigation. "The discovery of these dwarf galaxies demonstrates the unexpected power of large astronomical surveys like the Sloan Digital Sky Survey, which originally was designed to investigate objects millions and billions of light years from Earth, not to discover faint companions of our own galaxy." Schneider is the Chairman of the Sloan Digital Sky Survey (SDSS) Quasar Science Group and the SDSS Scientific Publications Coordinator.

According to Daniel Zucker of Cambridge University in the United Kingdom, theories predict that there should be tens to hundreds more dwarf galaxies yet to be discovered in the "Local Group" of galaxies, which includes the Milky Way. Dwarf galaxies contain, at most, a few million stars.

The new dwarfs have some unusual properties. "They're more like Hobbits than dwarfs," comments Zucker's co-investigator Vasily Belokurov, also of Cambridge, "since they are smaller and fainter than most previously known satellites. Several of the newly discovered systems appear to be on the verge of disruption — probably by the tidal gravity of the Milky Way — and the 'Ursa Major II' dwarf already seems to be in several pieces. "They look as though they're being ground up," notes Belokurov.

Current theories of galaxy assembly suggest that many — perhaps all — of the stars in the halo and thick disk of the Milky Way originated in smaller dwarf galaxies, which were dissolved when they merged into the Milky Way itself. "The new dwarfs are really just the crumbs from the galactic feast," says Zucker. "Most of the merging happened early on — billions of years ago — and what we're seeing here are the leftovers."

The Sloan survey recently uncovered more than a dozen of these galaxies in our cosmic neighborhood. These have very few stars — only a few hundred or thousand — but a great deal of dark matter, the underlying scaffolding upon which galaxies are built. Normal dwarf galaxies near the Milky Way contain 10 times more dark matter than the ordinary matter that makes up gas and stars, while in these so-called ultra-faint dwarf galaxies, dark matter outweighs ordinary matter by at least a factor of 100.

"By measuring the star formation histories of the observed dwarfs, Hubble has supported the theoretical explanation for the paucity of such objects, according to which star formation in the smaller clumps would be shut down by reionisation," said Jason Tumlinson of the Space Telescope Science Institute, a member of the research team.

The Image below unmasks the dim, star-starved dwarf galaxy Leo IV. This Hubble image demonstrates why astronomers had a tough time spotting this small-fry galaxy: It is practically invisible. The image shows how the handful of stars from the sparse galaxy are virtually indistinguishable from the background. Residing 500 000 light-years from Earth, Leo IV is one of more than a dozen ultra-faint dwarf galaxies found lurking around our Milky Way galaxy.



The results are published in the 1 July issue of The Astrophysical Journal Letters

The Daily Galaxy via ESA/Hubble Information Center and http://science.psu.edu/news-and-events/2007-news/Schneider1-2007.htm

Image credit: NASA, ESA, and T. Brown (STScI); image top of page: K. Johnston, J. Bullock


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