A juxtaposition of a distant galaxy and, within our Milky Way , a red dwarf star.
Sloan Sky Survey
Newberg and several colleagues have been searching for insights into this process hidden in the Milky Way’s spheroid. “When I started working on the spheroid,” she says, “everyone said it was just a featureless cloud.” But in the mid-1990s, astronomers spotted an intriguing cloud of stars in the spheroid. They realized it must be leftovers from a pulverized dwarf galaxy, which they called the Sagittarius dwarf. This galaxy was only partially digested, and a faint stream of stars was still hemorrhaging from it.
With the Sloan’s exquisite sensitivity, Newberg and her colleagues have been able to map the stars of the Sagittarius stream trailing all the way around the Milky Way. They have also found more than a half dozen other streams of stars crisscrossing one another, a formation that they call the Field of Streams. One of them, the Monoceros stream, is as big as the Sagittarius, although there is no definitive remnant of the original galaxy that was destroyed to make it. “That one is still controversial because it’s in the plane of the Milky Way’s disk,” Newberg says. “Some people argue that it’s part of the disk itself.” But other streams that make up the field are unmistakable remains of cannibalized dwarf galaxies. Taking advantage of the Sloan telescope’s ability to record the precise color and brightness of stars, Newberg can now determine the distances to individual stars in the streams. That information then allows her to create a 3-D map of our galaxy and its surroundings. “You can’t really see these structures when you’re looking in only two dimensions,” she says.
The results confirm that dark energy is no illusion; there really is an unseen force pushing the universe apart.
Surveying the star streams helps us piece together the life history of our galaxy. It also brings the dark universe closer to home. Since most of the gravitational force ripping dwarf galaxies apart comes from dark matter, astronomers hope to deduce the distribution of dark-matter particles lurking around the Milky Way by tracing the structure of the streams.
The Infant Universe Grew Up Fast
The Sloan survey functions as a time machine, looking not just far out into space but also far back into the early history of the universe. This type of research focuses particularly on quasars, the core of certain hyperactive galaxies. Quasars easily outshine the rest of their galaxies, and yet they are so compact that they look like mere points of light. The engine behind a quasar’s efficient brilliance is a monster black hole, as massive as a billion or more suns, which consumes gas so voraciously that the stuff heats to millions of degrees as it falls in. The Sloan telescope can study the resulting blaze of radiation even if it originates clear across the cosmos.
The distance to quasars, as well as to most galaxies, is established by measuring a change in their light known as a redshift. Because of the expansion of the universe, light from faraway objects is stretched and shifted toward the red end of the spectrum. The farther away the object, the longer its light has taken to reach us and the bigger its redshift. Until the Sloan survey came along, the most distant known quasars had a redshift of between 4 and 5 (the number is a measure of how substantially the light has been stretched). That means we are seeing these quasars as they were when the universe was just about 1.1 billion years old, some 12.7 billion years ago.
“We had found a few dozen of those quasars at most,” says Donald Schneider, a Pennsylvania State University quasar expert who helped plan the Sloan survey project. But he and his colleagues were puzzled that we could see these extremely distant quasars at all. Standard cosmological models implied that matter in the universe was not concentrated tightly enough to have formed black holes so early on. Clearly the models were wrong. Unfortunately, there were too few of these superdistant quasars known for astrophysicists to say much more than that—until the SDSS added 100,000 new quasars to the rolls.
Among this set are no fewer than 1,000 quasars with redshifts higher than 4. A handful of them have redshifts greater than 6, dating them to a time no more than 900 million years after the Big Bang. Just as astrophysicists have used the clustering of nearby galaxies to measure the modern structure of the universe, they can now—finally—start to do the same for the distant, young universe. The preliminary conclusion: Luminous matter—stars and their galaxies—was already gathering on a grand scale at a very early point in cosmic history, probably seeded by dense clouds of dark matter.
The most ancient of the quasars found by the Sloan survey also show signs of being shrouded by clouds of hydrogen gas, another clue about conditions in the early universe. Such clouds formed about 400,000 years after the Big Bang, when the cosmos cooled sufficiently to allow charged protons and electrons to bind together to form electrically neutral hydrogen atoms. These atoms absorb certain frequencies of light very efficiently, making the young universe much more opaque than it is today. For obvious reasons, astronomers know little about what was happening during that obscured era, known as the Dark Ages. When the first stars began to form, perhaps 100 million years later, their radiation drove the electrons and protons back apart, making interstellar space highly transparent—as it remains today. Being able to examine extremely distant quasars, still surrounded by long-vanished neutral hydrogen clouds, “means we’re finally probing into the Dark Ages,” Schneider says.
