How small can a galaxy be?
Astronomers are now finding small-fry galaxies that contain fewer than a million, possibly as few as a thousand, stars.
Until recently, these very faint, dwarf galaxies in the halo of the Milky Way have eluded discovery.
Now astronomers are using advanced techniques and instruments at The University of Arizona/Smithsonian 6.5-meter MMT Observatory at Mount Hopkins, Ariz., to find them.
They reported their latest discovery of such a galaxy, in the constellation Aries, in an online preprint last March. Their research article will be published in Monthly Notices, a publication of the Royal Astronomical Society, this summer.
"These are galaxies that might contain as few as a thousand stars, and those stars are being pulled out into the halo of our Milky Way," said UA astronomer Ed Olszewski. UA astronomy professor Jill Bechtold and MMTO astronomer Tim Pickering are also on the project.
"We're trying to understand whether these unbelievably faint objects are intact or have been mostly pulled apart by the Milky Way," Olszewski said. "We're trying to understand what the halo of the Milky Way really looks like, how many of these objects are in the halo, and whether our census of the population in the halo agrees or conflicts with the cosmological models.
"Knowing how many of these incredibly puny satellite galaxies populate our galactic neighborhood is important if we are to know whether cosmological models used to describe the evolution of the structure of galaxies are correct or way off base," he added.
"The sorts of objects we're finding have so few stars that one might think they're not galaxies at all, except that their internal motions imply that, unlike star clusters, they contain dark matter just like big galaxies do," Olszewski said.
A more accurate census of very faint, local dwarf galaxies is important because it will help scientists determine how much dark matter they might contain, he said. Scientists believe that "dark matter," or matter that is observed only by the effects of gravity but cannot be seen otherwise because it emits no radiation, makes up about 25 percent of the universe. "Normal" matter is thought to make up between 2 percent and 4 percent of the universe, with the remaining bulk being dark energy.
A decade ago, theoretical simulations showed that there must be 10 to 100 times as many objects in the Milky Way halo than observers had seen, Olszewski said. "Now we're coming closer to solving the ‘missing satellites' problem and, simultaneously, understanding how the Milky Way was put together."
Olszewski is a member of the observing team who has been using a wide-field imager called Megacam and a wide-field multi-fiber spectrograph called Hectochelle at the MMTO to confirm the existence of what British collaborators analyzing Sloan Digital Sky Survey data have identified as possible dwarf satellite galaxies.
"Until recently, astronomers could find small satellite galaxies just by looking at a photograph," Olszewski said. "We can image small galaxies that typically have one one-millionth as many stars as the Milky Way has. But the ones we're searching for now are 100 times fainter and won't show up in photographs. They are ridiculously hard to find, because they're so faint and because they're hidden in the foreground stars of the Milky Way itself."
The UA/MMT team collaborates with astronomers at the Institute for Astronomy in Cambridge, England. The British astronomers use a mathematical model of stars with the color and brightness of stars in galaxies they're searching for, moving their model as a kind of template against fields of stars recorded in Sloan Digital Sky Survey maps until they find a pattern match.
The Cambridge astronomers have found about 10 satellite galaxies by this "data mining" technique, Olszewski said, but now they're searching for fainter galaxies, which are harder to find. The Cambridge group works with the UA/MMT to confirm the fainter galaxies are real.
"It's a hard observational follow-up project that couldn't be done without the wonderful instruments built for the MMTO by people at the (Harvard-Smithsonian) Center for Astrophysics," Olszewski said.
Megacam has 36 CCDs that give it power to take deep sky images. Hectochelle has 300 fibers for gathering the spectra, or colors, of stars, which shows how far away stars are.
Observers use star velocity and star chemistry to determine if they have actually found an object in the Milky Way halo rather than in the Milky Way itself.
"Given what we know of the Milky Way halo in the context of all the discoveries so far, the model we made of the Milky Way 10 years ago is still a good one," Olszewski said.
The model can be visualized as a big meatball in a bowl of spaghetti. The meatball is the Milky Way, and spaghetti strands winding away in all different directions represent ripped apart small galaxies.
"We are finding not only little galaxies, we are finding some that are embedded in, or near the bigger, million-star sized galaxies," Olszewski said.
"It's getting to look more and more that that's what the halo of the Milky Way is like, and how the halo is assembled. The halo of a big galaxy largely arises from destruction of littler ones. Over time, the Milky Way will eat not only the little satellite galaxies falling in, but also the Magellanic clouds, " he added. "We're far from done forming the Milky Way."