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.