The collection of red dots seen near the center of this image show
one of several very distant galaxy clusters discovered by combining
ground-based optical data from the National Optical Astronomy
Observatory's Kitt Peak National Observatory with infrared data from
NASA's Spitzer Space Telescope. This galaxy cluster, named ISCS
J1434.7+3519, is located about 9 billion light-years from Earth.
The
large white and yellow dots in this picture are stars in our galaxy,
while the rest of the smaller dots are distant galaxies. The cluster,
comprised of red dots near the center, includes more than 100 massive
galaxies.
Spitzer was able to capture prodigious levels of star
formation occurring in the galaxies that live in this cluster. Some of
them are forming stars hundreds of times faster than our own Milky Way
galaxy.
Infrared light in this image has been colored red; and visible light, blue and green. Credit: NASA/JPL-Caltech/M. Brodwin (UMKC)
In the fable of the town and country mice, the country mouse visits
his city-dwelling cousin to discover a world of opulence. In the early
cosmos, billions of years ago, galaxies resided in the equivalent of
urban or country environments. Those that dwelled in crowded areas
called clusters also experienced a kind of opulence, with lots of cold
gas, or fuel, for making stars.
Today, however, these galactic
metropolises are ghost towns, populated by galaxies that can no longer
form stars. How did they get this way and when did the fall of galactic
cities occur?
A new study from NASA's Spitzer Space Telescope
finds evidence that these urban galaxies, or those that grew up in
clusters, dramatically ceased their star-making ways about 9 billion
years ago (our universe is 13.8 billion years old). These galactic
metropolises either consumed or lost their fuel. Galaxies in the
countryside, by contrast, are still actively forming stars.
"We
know the cluster galaxies we see around us today are basically dead, but
how did they get that way?" wondered Mark Brodwin of the University of
Missouri-Kansas City, lead author of this paper, published in the
Astrophysical Journal. "In this study, we addressed this question by
observing the last major growth spurt of galaxy clusters, which happened
billions of years ago."
Researchers studying distant galaxies get
a peek into the past since the galaxies' light takes time, sometimes
billions of years, to reach us. Brodwin and his colleagues used Spitzer
to study 16 galaxy clusters that existed between the time our universe
was 4.3 and 6 billion years old. Spitzer's infrared vision allows it see
the dust warmed by new stars, revealing star-formation rates. NASA's
Hubble Space Telescope and the W.M. Keck Observatory were used to
measure the galaxies' distances from Earth.
This is one of the
most comprehensive looks at distant galaxy clusters yet, revealing new
surprises about their environments. Previous observations of relatively
nearby clusters suggested that the urban, cluster galaxies produced all
their stars early in the history of our universe in one big burst. This
theory, called monolithic collapse, predicted that these tight-knit
galaxies would have used all their fuel at once in an early, youthful
frenzy. But the new study shows this not to be the case: The urban
galaxies continued to make stars longer than expected, until suddenly
production came to a halt around 9 billion years ago, or about 3 billion
years later than previously thought.
A second study using
observations from the Herschel Space Observatory, led by Stacey Alberts
at the University of Massachusetts-Amherst and published in the Monthly
Notices of the Royal Astronomical Society journal, finds a similar
transition epoch. Alberts and colleagues observed 300 clusters over a
longer period of time, dating back to when the universe was 4 to 10
billion years old. Herschel, which ran out of coolant in April of 2013
as expected, detected longer wavelengths of infrared light than Spitzer,
which is still up and running. The two telescopes complement each
other, allowing scientists to confirm results and probe different
aspects of cosmic conundrums.
"We find that around 9 billion
years ago, cluster galaxies were as active as their counterparts outside
of clusters; however, their rate of star formation decreases
dramatically between then and now, much more quickly than we see in
isolated galaxies," said Alberts.
Why do the urban galaxies shut
down their star formation sooner and more rapidly than the country
bumpkins? Brodwin says this may have to do with galaxy mergers. The more
crowded a galactic environment, as is the case in young, growing galaxy
clusters, the more often two galaxies will collide and merge. Galaxy
mergers induce bursts of fuel-consuming star formation, and also feed
growing supermassive black holes, which then blast out radiation that
heats up the gas and quickly shuts off the star formation.
"It's
as if boom times for galaxies in clusters ended with a sudden widespread
collapse," said Peter Eisenhardt of NASA's Jet Propulsion Laboratory,
Pasadena, Calif., who led a previous study that identified the distant
galaxy cluster sample used by Brodwin and Alberts. "They go from
vibrantly forming new stars to the quiescent state they've been in for
the last half of the history of the universe, and the switch happens
surprisingly fast."
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. Spacecraft operations are based at
Lockheed Martin Space Systems Company, Littleton, Colorado. Data are
archived at the Infrared Science Archive housed at the Infrared
Processing and Analysis Center at Caltech. Caltech manages JPL for NASA.
Source: Spitzer Space Telescope
