Searching for Dwarf Galaxies

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By Unknown November 22, 2008 21:37

Nearly three-quarters of the mass in the Universe was missing. In the second half of the twentieth century, astronomers began to realize that, if their observations were correct, the universe contained more mass than the brightness of the universe suggested.

Scientists concluded that nearly 75% of the mass of the universe was composed of ‘dark matter:’ matter that cannot be detected by electromagnetic radiation.

Dark matter does not interact directly with ordinary “bright matter” and is not composed of the same elementary particle as bright matter. It is observed primarily through gravitational lensing, when massive objects bend light around themselves like a lens. Adding dark matter into theories of the universe clarified many old observations, but raised new questions.

The new theory suggested that the Milky Way Galaxy would be surrounded by ‘dwarf galaxies.’ These dwarf galaxies were expected to include several billion stars, a tiny fraction compared to the 200-400 billion stars in the Milky Way.

By 2004, astronomers had found approximately twelve dwarf galaxies matching the predicted criteria. The trouble was that computer simulations of this model predicted anywhere between 100 and 1000 galaxies; the observations differed from the model by at least an order of magnitude.

In November 2007, Yale Professor of Astronomy Marla Geha and coauthor Josh Simon, a post-doctoral student at the California Institute of Technology, published a paper in The Astrophysical Journal demonstrating the existence of new dwarf galaxies.

Working at the Keck Observatory in Hawaii, Geha and Simon identified galaxies that contained less than one-ten-thousandth of the mass of the Milky Way.

Their discoveries were made possible by imaging from the Sloan Digital Sky Survey, an attempt to create a 3-D optical map of over a quarter of the night sky. These broad, uniform images were critical for Geha and Simon’s attempt to identify new dwarf galaxies.

These galaxies were difficult to find because they are composed primarily of dark matter: the ratio of dark to bright matter in these galaxies is over 100 to 1. In comparison, the ratio for the Milky Way is less than 10 to 1. The total luminosity of all of the stars in all of eleven of Geha’s dwarf galaxies is less than that of a single red giant star.

To identify these dark dwarfs, Geha had to filter the images for a narrow band of light. Using this approach, she and Simon have identified at least eleven dark dwarfs visible in the Northern Hemisphere alone over the past two years.

Further extrapolating from their observations, they estimate that there must be at least 50 to 60 dark dwarf galaxies surrounding the Milky Way, approaching the magnitude of dwarfs predicted.

Speed Guns for Galaxies

Because the galaxies are so small, and, proportionally, so little of their mass is visible, simply proving that these star clusters were galaxies was an important step. If the clusters really were galaxies, the movement of any star in the cluster would be small relative to its neighboring stars.

If Geha tried to observe this motion in the plane of the sky, it would take several years to measure the velocities accurately. Instead, she measured velocities along the ‘line of the sight’ (its movement directly toward or away from the observer).

Instead of making several observations over the course of years and comparing the relative positions of the stars, line-of-sight velocity can be determined from only one observation.

To determine the velocities of the galaxies, Geha measured the redshift of the light emitted by stars in the cluster. Because light has a finite speed, when objects move at high speeds away from a particular point, the light appears to be shifted into the redder end of the light spectrum.

By measuring the size of the shift, scientists can estimate how fast the object is moving away. When Geha measured the line-of-sight velocities, she found that the relative velocity of the stars in the cluster was only about 5 km/s. The cluster was a galaxy.

A map of Geha’s newly discovered dark dwarf galaxies and their locations relative to our own Milky Way.

“We Are Starstuff”

Geha is optimistic that these new galaxies will help inform the theory of star formation. Astronomer Carl Sagan declared, “We are starstuff ” since all elements heavier than helium are forged in the hearts of stars. Every atom in your body was once in the core of a star.

Stars in Geha’s dark matter universes contain more hydrogen and helium and less of all the other metals than comparable stars in bright matter galaxies. If more of the universe is composed of dark matter galaxies, this property would affect the amount of heavier metals in the universe. It could affect the formation of planetary bodies in these galaxies and the distribution of planets in the universe.

Geha’s work also has implications on the macroscopic level. Currently, there are two dueling hypotheses: the Hot and Cold Theories of dark matter. Although scientists aren’t sure what dark matter consists of, they can change their models of the universe based on the speed at which any dark matter particles move.

The faster the particles move, the hotter the particles are considered to be. If the Hot Theory were true, and dark matter particles were small and fast, their motion would be likely to smooth out irregularities in the structure of the universe. The Hot Dark Matter theory predicted fewer irregularities, like dwarf galaxies.

Geha’s discoveries lend credence to the Cold Dark Matter theory, which predicts that, because the first stars to form in a Cold Dark Matter universe would be relatively short-lived, we will observe few very old stars today. Her discovery will help astronomers model the life of the Universe from the Big Bang to the present and beyond.

On Beyond Andromeda

Geha plans to expand her search beyond our own Milky Way Galaxy. She plans to use the WIYN (Wisconsin, Indiana, Yale, and NOAO (National Optical Astronomy Observatory)) observatory in Arizona to peer approximately 2.5 million light-years away at our closest neighbor, the Andromeda Galaxy.

The structure of the Andromeda Galaxy is similar enough to that of the Milky Way that scientists expect that Andromeda should have similar dwarf galaxies.

These observations will be made possible by the addition of a new One-Degree- Imager (ODI) camera to the WIYN observatory.

The new camera (due to come online in 2010) will be able to take snapshots of a one-degree square of the sky (most cameras can handle only images about one-halfdegree square).

Additionally, the camera will minimize the ‘twinkling star’ fuzziness caused by eddies in the atmosphere. The pixels in the camera will ‘wiggle’ in approximate synchronization with the eddies to produce sharper images.

Finding dwarf galaxies near Andromeda would provide a lot of support to Geha’s dark matter dwarf galaxy solution. “If we didn’t find similar dwarfs near Andromeda, it would be very strange,” she said. “We’d have to start to asking new questions.”

Head in the Stars, Feet on the Ground

Geha received a B.S. in Applied and Engineering Physics from Cornell, and continued her studies in astronomy, receiving a Ph. D. in Astronomy and Astrophysics from the University of Florida at Santa Cruz in 2003. Her studies have taken her all over the world, making observations in Arizona, Hawaii, and Peru.

Geha says she is lucky that her research doesn’t require observations made from outside the Earth’s atmosphere. When asked about the future of astronomy, Geha was optimistic about scientific ideas but was worried about support for observatories like the Hubble Telescope.

She still remembers the 2005 fight in Congress over a servicing mission to keep Hubble in the air. President Bush and some members of Congress opposed a mission to keep the telescope running.

The mission was eventually approved after contentious public hearings and thousands of supportive letters from students around the county. “The public went nuts,” Geha said, “and it worked.”

With astronomers (on planet and off) turning up results like Geha’s, it’s no wonder the public has turned out in support of the space program. Hopefully, that support will continue for years and help Geha keep asking questions and uncovering more about the structure of our universe.

ABOUT THE AUTHOR
LEAH LIBRESCO is a sophomore in Jonathan Edwards College. She is very grateful to the stars that forged her atoms.

ACKNOWLEDGEMENTS
The author would like to thank Assistant Professor Marla Geha.

FURTHER READING
The Kinematics of the Ultra-faint Milky Way Satellites: Solving the Missing Satellite Problem, The Astrophysical Journal, Volume 670, Issue 1, pp. 313-331.
Money to Fix Space Telescope May Be Cut by White House (NYT, 1/23/05).

Unknown
By Unknown November 22, 2008 21:37