Fish and bears
Even farther out, a 59-million-light-year jaunt, lies the Dorado Group. This large collection comprises 70 galaxies in the direction of the southern constellation Dorado the Dolphinfish (a species that appears on menus as “mahi-mahi”). One of its most prominent members, NGC 1483, shows a luminous central bulge and jumbled spiral arms that host bright star-forming regions and young star clusters.
At approximately the same distance but in the direction of Ursa Major, two groups form a defined band. Most of its major galaxies are spirals, and these combine with the minor ones to produce 30 percent as much light as the Virgo Cluster. Thirty percent may not sound impressive, but consider that the Ursa Major groups contain just 5 percent of Virgo’s mass.
Thirty-two galaxies call the Ursa Major North Group home, including the bright spirals NGC 3631, NGC 4088, NGC 3953, and M109. The last of these looks most similar to the Milky Way. Of all 109 objects in the Messier catalog, it lies farthest away, meaning that 18th-century telescopes couldn’t decipher things beyond this point. Now, however, large scopes on Earth and in space give us glimpses of galaxies billions of light-years more distant.
Moving targets
But classifying those galaxies into categories has always been a problem. They orient themselves differently relative to us, and a face-on spiral looks quite different from one seen edge-on. We only have a 2-D sense of galaxies, as if they’re projected on a screen, rather than a true 3-D picture. The problem is particularly acute for disk-shaped galaxies, such as spirals, and spheroidals. You can tell the difference, though, by watching how their stars move. If they rotate slowly and disorderly, they’re spheroidal. If they rotate fast and in an orderly manner, they are disk galaxies.
A team of astronomers led by Nicholas Scott of the University of Sydney in Australia set about to study the stellar motions in the Fornax I Cluster, 62 million light-years away. They found that 93 percent of the cluster’s galaxies were fast rotators (spirals) and just 7 percent were slow rotators (spheroidals). Combining this study with those of other clusters, including the larger one in Virgo, Scott’s team found that the spheroidals tended to hover near a cluster’s center — either because they began there or migrated there.
“One of the long-debated ideas is the question of nature versus nurture in galaxy evolution,” says Scott. Is how a galaxy looks today totally determined by its properties in the early universe, or do its surroundings play a role too? “This study shows that, for at least some galaxies, the environment does play a major role.” He adds that many questions still exist, which further cluster studies can answer.
More questions always exist about the cosmos — its stars, galaxies, groups, clusters, superclusters, and beyond. And because the universe is organized into superclusters, their constituents stay “close” together. Astronomers can swing their telescopes and find a stadium alive with variety. They learn not just what the more distant universe is like, but also what it used to be like, what it will be like, and what has happened and will happen even closer to home.