Universe Today

Image credit: University of Virginia

A new survey of the stars surrounding the Milky Way has produced a detailed map of how streams of stars and debris are being added to our galaxy. Researchers from the University of Virginia used data from the 2MASS sky survey to map out the Sagittarius galaxy which wraps around the Milky Way in a long stream of stars. They were able to distinguish between galaxies because a certain class of stars, called M giants, are much more common in Sagittarius – when they tuned their search to just look for these stars, Sagittarius “popped into view”.

Thousands of stars stripped from the nearby Sagittarius dwarf galaxy are streaming through our vicinity of the Milky Way galaxy, according to a new view of the local universe constructed by a team of astronomers from the University of Virginia and the University of Massachusetts.

Using volumes of data from the Two-Micron All Sky Survey (2MASS), a major project to survey the sky in infrared light led by the University of Massachusetts, the astronomers are answering questions that have baffled scientists for decades and proving that our own Milky Way is consuming one of its neighbors in a dramatic display of ongoing galactic cannibalism. The study, to be published in the Dec. 20 issue of the Astrophysical Journal, is the first to map the full extent of the Sagittarius galaxy and show in visually vivid detail how its debris wraps around and passes through our Milky Way. Sagittarius is 10,000 times smaller in mass than the Milky Way, so it is getting stretched out, torn apart and gobbled up by the bigger Milky Way.

“It’s clear who’s the bully in the interaction,” said Steven Majewski, U.Va. professor of astronomy and lead author on the paper describing the results.

In model images made to show the interaction in 3-D, available at http://www.astro.virginia.edu/~mfs4n/sgr/, the Milky Way appears as a flattened disk with spiral arms, while Sagittarius is visible as a long flourish of stars swirling first under and then over and onto the Milky Way disk.

“If people had infrared-sensitive eyes, the entrails of Sagittarius would be a prominent fixture sweeping across our sky,” Majewski said. “But at human, visual wavelengths, they become buried among countless intervening stars and obscuring dust. The great expanse of the Sagittarius system has been hidden from view.”

Not any more. By using infrared maps, the astronomers filtered away millions of foreground stars to focus on a type of star called an M giant. These large, infrared-bright stars are populous in the Sagittarius galaxy but uncommon in the outer Milky Way. The 2MASS infrared map of M giant stars analyzed by Majewski and collaborators is the first to give a complete view of our galaxy’s meal of Sagittarius stars, now wrapping like a spaghetti noodle around the Milky Way. Prior to this work, astronomers had detected only a few scattered pieces of the disrupted Sagittarius dwarf. Even the existence of Sagittarius was unknown until the heart of this nearest satellite galaxy of the Milky Way was discovered by a British team of astronomers in 1994.

“We sifted several thousand interesting stars from a catalog of half a billion,” said co-author Michael Skrutskie, U.Va. professor of astronomy and principal investigator for the 2MASS project. “By tuning our maps of the sky to the ‘right’ kind of star, the Sagittarius system jumped into view.”

“This first full-sky map of Sagittarius shows its extensive interaction with the Milky Way,” Majewski said. “Both stars and star clusters now in the outer parts of the Milky Way have been ‘stolen’ from Sagittarius as the gravitational forces of the Milky Way nibbled away at its dwarf companion. This one vivid example shows that the Milky Way grows by eating its smaller neighbors.”

“Astronomers used to view galaxy formation as an event that happened in the distant past,” noted David Spergel, a professor of astrophysics at Princeton University after viewing the new finding. “These observations reinforce the idea that galaxy formation is not an event, but an ongoing process.”

The study’s map of M giants depicts 2 billion years of Sagittarius stripping by the Milky Way, and suggests that Sagittarius has reached a critical phase in what had been a slow dance of death.

“After slow, continuous gnawing by the Milky Way, Sagittarius has been whittled down to the point that it cannot hold itself together much longer,” said 2MASS Science Team member and study co-author Martin Weinberg of the University of Massachusetts. “We are seeing Sagittarius at the very end of its life as an intact system.”

Does this mean we are at a unique moment in the life of our galaxy? Yes and no.

“Whenever possible, astronomers appeal to the principle that we are not at a special time or place in the universe,” Majewski said. “Because over the 14 billion-year history of the Milky Way it is unlikely that we would just happen to catch a brief event like the death of Sagittarius, we infer that such events must be common in the life of big spiral galaxies like our own. The Milky Way probably dined on a number of dwarf galaxy snacks in the past.”

On the other hand, Majewski and his colleagues have been surprised by the Earth’s proximity to a portion of the Sagittarius debris.

“For only a few percent of its 240 million-year orbit around the Milky Way galaxy does our Solar System pass through the path of Sagittarius debris,” Majewski said. “Remarkably, stars from Sagittarius are now raining down onto our present position in the Milky Way. Stars from an alien galaxy are relatively near us. We have to re-think our assumptions about the Milky Way galaxy to account for this contamination.”

The new findings will help astronomers measure the total mass of the Milky Way and Sagittarius galaxies, and probe the quantity and distribution of the invisible dark matter in these systems.

“The shape of the Sagittarius debris trail shows us that the Milky Way’s unseen dark matter is in a spherical distribution, a result that is quite unexpected,” Weinberg said.

“The observations provide new insights into the nature of the mysterious dark matter,” said Princeton’s Spergel. “Either our galaxy is unusual or the dark matter has richer properties than postulated by conventional models.”

2MASS was a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology. The National Aeronautics and Space Administration, and the National Science Foundation funded the project. Additional funding for the Sagittarius study with 2MASS came from the David and Lucile Packard Foundation and the Research Corporation.

Original Source: University of Virginia News Release

Image credit: NASA

Sarah Horst, a planetary sciences major at Caltech, helped astronomers track cloud formations on Saturn’s moon Titan using only a fourteen inch telescope – in Los Angeles. Researchers needed a way to track Titan night after night for several months, but no large observatory could provide this much time to carry out detailed observations. Horst set up an old teaching telescope to track the intensity of light coming from Titan. Whenever something unusual happened, her associates would contact Keck for detailed photographs.

Meet Sarah Horst, throwback. The planetary science major, a senior at the California Institute of Technology, spent six months engaged in a bit of old-time telescope observing. The work led to some breakthrough research about Saturn’s moon Titan, and indirectly led to funding for a new telescope at Caltech’s Palomar Observatory.

Horst, 21, was looking for a part-time job in the summer of her sophomore year, and was hired by Mike Brown, an associate professor of planetary astronomy. Brown and graduate student Antonin Bouchez knew there had been previous evidence of “weather” on Titan in the form of clouds. But that evidence was elusive. “Someone would look one year and think they saw a cloud, then look the next year and not see a cloud,” explains Brown. “What we were after was a way to look at Titan, night after night after night.”

The problem, of course, is that all of the large telescopes like Keck are incredibly busy, booked by astronomers from around the world who use the precious time for their own line of research. So Brown and Bouchez knew that obtaining large amounts of time for a single project like this was not going to happen.

The solution: Use an old teaching telescope–the hoary 14-inch Celestron telescope located on top of Caltech’s Robinson Lab–to do cutting edge science that couldn’t be done at the largest telescopes in the world, in Hawaii.

Though the power of the Robinson telescope is weak, and light pollution from Pasadena strong, which prevents imaging the actual clouds, the light reflecting from clouds could be imaged (the more clouds, the more light that’s reflected). All that was needed was someone who could come night after night and take multiple images.

Enter Horst, the self-described “lowly undergraduate.” For months, Horst spent her evenings in Robinson. “I did the setup, which involved a wheel that contained four light filters,” she explains. Each filter would capture a different wavelength of light. Software switched the filters; all she had to do, says Horst, was to orientate and focus the telescope.

Now, modern-day astronomers have it relatively easy when using their telescope time. Sure they’re up all night, but they sit on a comfortable chair in a warm room, hot coffee close at hand, and do their observing through a computer monitor that’s connected to a telescope.

Not Horst. She did it the old way, in discomfort. “A lot of times in December or January I’d go in late at night, and it would be freezing,” says Horst, who runs the 800-meter for the Caltech track team. “I’d wrap myself up in blankets.” Horst spent hours in the dark, since the old dome itself had to be dark. “I couldn’t even study,” she says, “although sometimes I tried to read by the light of the moon.”

A software program written by Bouchez plotted the light intensity from each image on a graph. When a particular image looked promising, Bouchez contacted Brown. As a frequent user of the Keck Observatory, which is powerful enough to take an image of the actual clouds, Brown was able to call colleagues who were using the Keck that night and quickly convince them that something exciting was going on. “It only took about ten minutes to get a quick image of Titan,” says Brown. “The funny part was having to explain to them that we knew there were clouds because we had seen the evidence in our 14-inch telescope in the middle of the L.A. basin.”

The result was “Direct Detection of Variable Tropospheric Clouds Near Titan’s South Pole,” which appeared in the December 19 journal Nature. It included this acknowledgement: “We thank . . . S. Horst for many nights of monitoring Titan in the cold.”

The paper has helped Brown obtain the funding to build a new 24-inch custom-built telescope. It will be placed in its own building atop Palomar Mountain, on the grounds of Caltech’s existing observatory. It’s also roboticized; Brown will control the scope from Pasadena via a computer program he has written.

He’ll use it for further observation of Titan and for other imaging, as well, such as fast-moving comets. “Most astronomy is big,” notes Brown; “big scopes looking at big, unchanging things, like galaxies. I like to look at changing things, which led to this telescope.”

What really made this project unique, though, according to Brown, is the Robinson scope. “Sarah was able to do something with this little telescope in Pasadena that no one in the world, on any of their larger professional telescopes on high, dark mountaintops, had been able to do,” he says. “Sometimes a good idea and stubbornness are better than the largest telescope in town.”

For Horst, while the work wasn’t intellectually challenging–“a trained monkey could have done it,” she says with a laugh–it was, nonetheless, “a cool project. Everything here is so theoretical and tedious, and so classroom orientated. So in that way it was a nice experience and reminded me what real science was about.”

Original Source: Caltech News Release