KU professor creates 'topographic' maps to determine shape of galaxies

LAWRENCE -- Sergei Shandarin, professor of physics and astronomy at the University of Kansas, has found a way to help solve a problem that has plagued astronomers since they started photographing the stars.

A galaxy can stretch for trillions of miles in every direction. A picture of a galaxy, however, is two-dimensional and flat as a pancake.

So flattening a galaxy means losing a lot of information about it, including information about the complexity of its shape.

Looking at a two-dimensional photo of it is like looking at an aerial snapshot taken directly over a mountain range. It's hard to tell which peaks are high and which are low.

Shandarin said that a galaxy's shape can reveal important information. The shape gives clues about how the galaxy rotates and about whether stars formed relatively early or late in the galaxy's life.

So Shandarin and KU graduate student Nurur Rahman have come up with a way to retrieve some of that information by looking at the brightness of different parts of a galaxy.

The technique involves drawing lines, called isophots, to connect areas of equal brightness on a galaxy's image.

Shandarin does not draw the isophots with a pencil after squinting through a magnifying glass. Instead, he uses computers to compare the brightness of the picture's pixels.

Pixels are the little dots that make up digital images and that are visible on a digital TV or computer screen when it's examined closely.

Shandarin said he starts drawing the isophots somewhere in the interior of the galaxy, wherever light is most brilliant.

Shandarin and Rahman described their method in the August issue of Monthly Notices of the Royal Astronomical Society.

He first draws an irregular ring connecting all those bright spots.

Then, he said, he moves on to the next brightest set of pixels and draws a second isophot. He keeps this up until he works his way to the galaxy's outer edge.

The result is a series of concentric rings of varying shapes that form what looks like a topographical map of a hill.

Just as the topographical map shows you both the hill's height and its shape, Shandarin's map illustrates the three-dimensional complexity of a galaxy, he said.

"A hill can be circular at the very top, then get more elliptical as you move toward the bottom," he said. "As you map the shape at each altitude, the picture becomes more complex and more realistic. We do the same thing using brightness instead of altitude."

So far, Shandarin has tested his technique on elliptical galaxies with very simple shapes.

However, galaxies have a wide variety of forms. For instance, elliptical galaxies run the gamut from being flat like a lens -- astrophysicists call these lenticular galaxies -- to being rounded -- a spherical galaxy.

There are whirlpool-shaped galaxies, such as our own Milky Way, known as spiral galaxies. There also are peculiar galaxies, which do not have any easily classified shape.

Shandarin's results make him optimistic, he said, about the ability of the technique to deal with the more complex shapes.

One problem with Shandarin's method is that it looks only at relatively shiny stars. It can miss parts of a galaxy populated by dim objects.

"It's like trying to figure the population of Earth from a satellite picture taken at night," Shandarin said. "In the U.S., there are lots of lights because people consume lots of energy. In India, the cities are much darker, even though more people live there."