Radio mapping helps unravel first steps in how stars form
Posted: January 11, 2002

The first-ever map of all sites of star formation in a spiral galaxy reveals the important role played in the earliest steps of star formation by magnetic fields in the gas between stars.

The star formation areas, known as giant molecular clouds, should be rotating rapidly, spinning up as they collapse like spinning ice skaters drawing in their arms. Surprisingly, most are spinning between one and 10 percent of their expected rate, and many are rotating backward, said Leo Blitz, professor of astronomy and director of the University of California, Berkeley's Radio Astronomy Laboratory.

"We expected to see slower rotation than classical theories of star formation predict, but I was surprised that the effect was so very, very strong," said Erik Rosolowsky, a graduate student at UC Berkeley. "Magnetic field interactions within the forming clouds are the only thing that could explain this."

Observations of the Triangulum galaxy at millimeter wavelengths reveal the sites of star formation in the galaxy (orange). The background image is the galaxy as it appears in visible light. The star formation sites were discovered using the Berkeley-Illlinois-Maryland Association Millimeter Array. Credit: Erik Rosolowsky, UC Berkeley and Space Telescope Science Institute.
This report was presented this week at the American Astronomical Society Meeting in Washington, D.C., by Rosolowsky, Blitz, Greg Engargiola and Richard Plambeck -- all from UC Berkeley.

All of the stars in the Milky Way, including the Sun and the solar system, plus the stars in neighboring galaxies, were made in giant clouds of molecular hydrogen (H2) and other molecules. Like raindrops forming in the clouds of Earth's atmosphere, the stars form by accreting gas from the cloud.

When the stars ignite, the released energy rips the molecular cloud apart, revealing the newborn stars. Although rapid progress is being made on how individual stars form inside molecular clouds, Blitz said, little is known about how the giant molecular clouds themselves form. An understanding of how the birthplaces of stars come about is an important missing link in the star formation process.

The UC Berkeley astronomers reasoned that, if they could obtain a complete census of the star-forming molecular clouds in a galaxy, the cloud properties would illuminate the earliest phases of cloud formation. The survey presented this week is the first such census of any spiral galaxy. A similar census in the Milky Way is impossible because of our vantage point within the galaxy -- it's like being unable to see an entire city while surrounded by tall buildings.

The observations targeted M33 (the Triangulum galaxy), a spiral galaxy similar to, but smaller than the Milky Way. Like the Milky Way, M33 is one of three large spiral galaxies in the local group of galaxies, lying more than 2.7 million light years from Earth.

Using the Berkeley-Illinois-Maryland Association (BIMA) Array, a state-of-the-art array of 10 radio telescopes operating at millimeter wavelengths, the astronomers were able to produce a radio image of most of the galaxy. The image shows almost all of the sites of star formation in M33. The individual 6 meter (20 ft.) telescopes are linked together electronically to function as a single large telescope, which makes it possible to discern much finer detail than can be seen with each telescope alone.

The astronomers tuned the telescope to observe emission from carbon monoxide. Carbon monoxide is a pollutant on Earth, but is an essential tracer for locating molecular hydrogen (H2) in space. Emission from molecular hydrogen itself is undetectable at the cold temperatures of the giant molecular clouds, typically about 10 degrees Celsius above absolute zero. Astronomers turn to observations of molecules like carbon monoxide as well as water, ammonia, alcohol, vinegar and sugars to trace the molecular hydrogen.

The results presented come from using the BIMA Array to map a region of M33 about the size of the full moon. Nearly every giant molecular cloud in M33 within the survey region was detected, and the clouds have sizes and shapes similar to those found in our own galaxy.

Moreover, astronomers can use the array to measure how rapidly the clouds are spinning. The spins provide important clues for unraveling how they form. Most theories predict that giant molecular clouds form from gas spread over a large region in the galaxy. As this slowly spinning gas collapses into a dense cloud, the rotation speeds up. This is the same process by which skaters increase their rate of spin by bringing arms and legs closer to the body.

Surprisingly, the astronomers found that the clouds typically are spinning between 10 and 100 times slower than expected from this simple picture. Almost half of the clouds are rotating in a direction opposite to that of the very diffuse gas from which the clouds are produced.

"The situation we've observed is impossible unless something is slowing the clouds down as they form," Rosolowsky said.

"The most likely explanation is that the clouds are magnetized and tied to the rest of the gas in the galaxy," said Blitz. Large scale magnetic fields would slow the rotation, just as a giant rubber band tied between a desk and an office chair would slow a person spinning in the chair. Ultimately, enough energy can be stored in the magnetic fields to get the clouds to spin backwards. Direct evidence will require measurements of the magnetic fields with other specialized instruments.

Blitz said that only a tiny proportion of gas in the molecular cloud -- only one in 10 million atoms in the densest part of the cloud -- is charged and thus tied to the magnetic field. Nevertheless, that is sufficient for the magnetic field to slow down rotation of the molecular cloud.

"It's the tail wagging the dog," he said.

The team plans in the future to look at the relationship between areas of molecular clouds in M33 with areas of diffuse gas from which the clouds form.

This work was supported by the National Science Foundation and by research funds from the State of California.