In a new study, University of Utah astronomer Adam Bolton and colleagues measured these Einstein rings to determine the mass of 79 lens galaxies that are massive elliptical galaxies, the largest kind of galaxy with 100 billion stars.
This is evidence that big galaxies are crashing into other big galaxies to make even bigger galaxies," says astronomer Adam Bolton, principal author of the new study.
"Most recent studies have indicated that these massive galaxies primarily grow by eating lots of smaller galaxies," he
Bolton conducted the study with three other University of Utah astronomers -- postdoctoral researcher Joel Brownstein, graduate student Yiping Shu and undergraduate Ryan Arneson -- and with these members of the Sloan Digital Sky Survey: Christopher Kochanek, Ohio State University; David Schlegel, Lawrence Berkeley National Laboratory; Daniel Eisenstein, Harvard-Smithsonian Center for Astrophysics; David Wake, Yale University; Natalia Connolly, Hamilton College, Clinton, N.Y.; Claudia Maraston, University of Portsmouth, U.K.; and Benjamin Weaver, New York University.
"They are the end products of all the collisions and mergers of previous generations of galaxies," perhaps hundreds of collisions," Bolton
"But if you have two roughly comparable galaxies and they are on a collision course, each one penetrates more toward the center of the other, so more mass ends up in the center," Bolton
Other recent studies indicate stars are spread more widely within galaxies over time, supporting the idea that massive galaxies snack on much smaller ones.
"We're finding galaxies are getting more concentrated in their mass over time even though they are getting less concentrated in the light they emit," Bolton
believes large galaxy collisions explain the growing mass concentration, while galaxies gobbling smaller galaxies explain more starlight away from galactic centers.
"Both processes are important to explain the overall picture," Bolton
new study was "almost gravy" that accompanied an SDSS-III project named BOSS, for Baryon Oscillation Spectrographic Survey.
BOSS is measuring the history of the universe's expansion with unprecedented precision.
"The more distant galaxy sends out diverging light rays, but those that pass near the closer galaxy get bent into converging light rays that appear to us as of a ring of light around the closer galaxy," says Bolton
The greater the amount of matter in a lens galaxy, the bigger the ring.
That seems counterintuitive, but the larger mass pulls with enough gravity to make the distant star's light bend so much that lines of light cross as seen by the observer, creating a bigger ring.
If there is more matter concentrated near the center of a galaxy, the faster stars will be seen moving toward or being slung away from the galactic center, Bolton
and colleagues acknowledge their observations might be explained by theories other than the idea that galaxies are getting denser in their centers over time:
- Gas that is collapsing to form stars can increase the concentration of mass in a galaxy.
argues the stars in these galaxies are too old for that explanation to work.
- Gravity from the largest massive galaxies strips neighboring "satellite" galaxies of their outskirts, leaving more mass concentrated in the centers of the satellite galaxies.
contends that process is not likely to produce the concentration of mass observed in the new study and explain how the extent of that central mass increases over time.
- The researchers merely detected the boundary in each galaxy between the star-dominated inner regions and the outer regions, which are dominated by unseen dark matter.
Under this hypothesis, the appearance of growing galaxy mass concentration over time is due to a coincidence in researchers' measurement method, namely that they are measuring younger galaxies farther from their centers and measuring older galaxies closer to their centers, giving an illusion of growing mass concentration in galactic centers over time.
says this measurement difference is too minor to explain the observed pattern of matter density within the lens galaxies.
Adam S. Bolton
, Joel R. Brownstein, Christopher S. Kochanek, Yiping Shu, David J. Schlegel, Daniel J. Eisenstein, David A. Wake, Natalia Connolly, Claudia Maraston, Ryan A. Arneson, Benjamin A. Weaver.