This image shows two of the galaxy clusters
observed by NASA's Wide-field Infrared Survey Explorer (WISE) and
Spitzer Space Telescope missions. Galaxy clusters are among the most
massive structures in the universe. The central and largest galaxy in
each grouping, called the brightest cluster galaxy or BCG, is seen at
the center of each image. Image credit: NASA/JPL-Caltech/SDSS/NOA
. › Full image and caption
Figure 1 (left) shows the cluster known as Abell 2199, which is relatively
nearby at a distance of 400 million light-years from Earth (redshift of
0.0302). This image combines infrared data from WISE (in red) with
shorter wavelengths of light extending into the visible spectrum from
the Sloan Digital Sky Survey (in blue and green).
Figure 2 (right) is the
cluster ISCS 1433.9+3330, which is significantly farther away at a
distance of 4.4 billion light-years (redshift of 0.42). Infrared data
from Spitzer (red) is combined with similar shorter wavelength data
taken by the Mayall Telescope on Kitt Peak, Ariz.
Our universe is filled with gobs of galaxies, bound together by gravity
into larger families called clusters. Lying at the heart of most
clusters is a monster galaxy thought to grow in size by merging with
neighboring galaxies, a process astronomers call galactic cannibalism.
New research from NASA's Spitzer Space Telescope and Wide-field Infrared
Survey Explorer (WISE) is showing that, contrary to previous theories,
these gargantuan galaxies appear to slow their growth over time, feeding
less and less off neighboring galaxies.
"We've found that these massive galaxies may have started a diet in the
last 5 billion years, and therefore have not gained much weight lately,"
said Yen-Ting Lin of the Academia Sinica in Taipei, Taiwan, lead author
of a study published in the Astrophysical Journal.
Peter Eisenhardt, a co-author from NASA's Jet Propulsion Laboratory in
Pasadena, Calif., said, "WISE and Spitzer are letting us see that there
is a lot we do understand -- but also a lot we don't understand -- about
the mass of the most massive galaxies." Eisenhardt identified the
sample of galaxy clusters studied by Spitzer, and is the project
scientist for WISE.
The new findings will help researchers understand how galaxy clusters --
among the most massive structures in our universe -- form and evolve.
Galaxy clusters are made up of thousands of galaxies, gathered around
their biggest member, what astronomers call the brightest cluster
galaxy, or BCG. BCGs can be up to dozens of times the mass of galaxies
like our own Milky Way. They plump up in size by cannibalizing other
galaxies, as well as assimilating stars that are funneled into the
middle of a growing cluster.
To monitor how this process works, the astronomers surveyed nearly 300
galaxy clusters spanning 9 billion years of cosmic time. The farthest
cluster dates back to a time when the universe was 4.3 billion years
old, and the closest, when the universe was much older, 13 billion years
old (our universe is presently 13.8 billion years old).
"You can't watch a galaxy grow, so we took a population census," said
Lin. "Our new approach allows us to connect the average properties of
clusters we observe in the relatively recent past with ones we observe
further back in the history of the universe."
Spitzer and WISE are both infrared telescopes, but they have unique
characteristics that complement each other in studies like these. For
instance, Spitzer can see more detail than WISE, which enables it to
capture the farthest clusters best. On the other hand, WISE, an infrared
all-sky survey, is better at capturing images of nearby clusters,
thanks to its larger field of view. Spitzer is still up and observing;
WISE went into hibernation in 2011 after successfully scanning the sky
twice.
The findings showed that BCG growth proceeded along rates predicted by
theories until 5 billion years ago, or a time when the universe was
about 8 billion years old. After that time, it appears the galaxies, for
the most part, stopped munching on other galaxies around them.
The scientists are uncertain about the cause of BCGs' diminished
appetites, but the results suggest current models need tinkering.
"BCGs are a bit like blue whales -- both are gigantic and very rare in
number. Our census of the population of BCGs is in a way similar to
measuring how the whales gain their weight as they age. In our case, the
whales aren't gaining as much weight as we thought. Our theories aren't
matching what we observed, leading us to new questions," said Lin.
Another possible explanation is that the surveys are missing large
numbers of stars in the more mature clusters. Clusters can be violent
environments, where stars are stripped from colliding galaxies and flung
into space. If the recent observations are not detecting those stars,
it's possible that the enormous galaxies are, in fact, continuing to
bulk up.
Future studies from Lin and others should reveal more about the feeding habits of one of nature's largest galactic species.
JPL manages the Spitzer Space Telescope mission for NASA's Science
Mission Directorate, Washington. Science operations are conducted at the
Spitzer Science Center at the California Institute of Technology in
Pasadena. Data are archived at the Infrared Science Archive housed at
the Infrared Processing and Analysis Center at Caltech. Caltech manages
JPL for NASA. For more information about Spitzer, visit http://spitzer.caltech.edu and http://www.nasa.gov/spitzer .
JPL managed and operated WISE for NASA's Science Mission Directorate.
Edward Wright is the principal investigator and is at UCLA. The mission
was selected competitively under NASA's Explorers Program managed by the
agency's Goddard Space Flight Center in Greenbelt, Md. The science
instrument was built by the Space Dynamics Laboratory in Logan, Utah.
The spacecraft was built by Ball Aerospace & Technologies Corp. in
Boulder, Colo. Science operations and data processing take place at the
Infrared Processing and Analysis Center at the California Institute of
Technology in Pasadena. Caltech manages JPL for NASA. More information
is online at http://www.nasa.gov/wise and http://wise.astro.ucla.edu and http://jpl.nasa.gov/wise .