Celestial Divide: Finding two families of galaxies

By analyzing data from a huge sky survey, researchers have found that galaxies divide into two distinct families. Those galaxies whose stars collectively weigh more than 30 billion times the mass of the sun are old and staid and appear to have finished formation of most of their stars billions of years ago. In contrast, galaxies with stellar populations weighing less than 30 billion suns are young and still forming stars.

The finding may spark new insights into galaxy evolution, says Timothy M. Heckman of Johns Hopkins University in Baltimore. He described the work last week at the annual October Astrophysics Conference at the University of Maryland in College Park.

The analysis, Heckman notes, provides further evidence that dark matter–the invisible material that outweighs all the visible matter by a factor of 10–can’t by itself account for the diversity of galaxies seen today.

Although it was dark matter’s gravity that first pulled gas together, forcing it to condense into galaxies, other factors may be equally important in determining how galaxies change over time, says study collaborator Guinevere Kauffmann of the Max Planck Institute for Astrophysics in Garching, Germany.

Astronomers have known for years that the most massive galaxies are elliptical and that most low-mass galaxies are spiral or irregularly shaped. “What is new with our analysis is that every single galaxy property we have looked at transitions abruptly at this fundamental scale of 30 billion solar masses of stars,” says Kauffmann.

A significant fraction of total light from higher-mass galaxies in the study emanates from matter falling onto a central black hole, says Heckman. In contrast, starlight accounts for much more of the radiation from the lower-mass galaxies.

In their study of 80,000 nearby galaxies recorded by the Sloan Digital Sky Survey, the astronomers examined two distinct wavelengths of light absorbed by galaxies. These data revealed the average age of each galaxy’s stars and the fraction of stars that formed in bursts over the past few billion years. Combining this information with the amount of light emitted over a broad range of wavelengths, the researchers deduced the total mass of the stars in each galaxy.

Kauffmann and her colleagues suggest that fierce winds produced by a succession of supernova explosions in galaxies in the early universe may partly account for the differences between high-mass and low-mass galaxies. These so-called superwinds would have had no effect on dark matter, which is impervious to everything but gravity. But the superwinds were strong enough to blow a substantial amount of gas out of galaxies, preventing it from condensing into stars for hundreds of millions of years (SN: 4/20/02, p. 244: Cosmic Remodeling: Superwinds star in early universe). Because lower-mass galaxies can’t exert a tight gravitational grip on their gas, the winds may have slowed or halted star formation for longer there than in higher-mass galaxies, says Heckman. As a consequence, the smaller galaxies still have an abundance of gas for making stars.

So far, however, the winds can’t explain why galactic personality changes so abruptly and at the particular mass observed. The findings do provide new clues about why galaxies look the way they do, says James S. Bullock of Harvard University.


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