MACS J0717.5+3745 is an extraordinarily dynamic galaxy
cluster with a total mass greater than 1015 (a million billion) times the mass
of the sun or more than 1,000 times the mass of our own galaxy. It appears to
contain three relatively stationary subclusters (A, C, and D) and one
subcluster (B) that is being drawn into the larger galaxy cluster, moving at a
speed of 3,000 kilometers per second.
The galaxy cluster was observed by a team led by Sunil
Golwala, professor of physics
at Caltech and director of the Caltech
Submillimeter Observatory (CSO) in Hawaii. Subcluster B was observed during
what appears to be its first fall into MACS J0717.5+3745. Its momentum will carry
it through the center of the galaxy cluster temporarily, but the strong
gravitational pull of MACS J0717.5+3745 will pull subcluster B back again.
Eventually, subcluster B should settle in with its stationary counterparts,
subclusters A, C, and D.
Though subcluster B's behavior is dramatic, it fits neatly
within the standard cosmological model. But the details of the observations of
MACS J0717.5+3745 at different wavelengths were puzzling until they were
analyzed in terms of a theory called the kinetic Sunyaev-Zel'dovich (SZ)
effect.
In 1972, two Russian physicists, Rashid Sunyaev and Yakov
Zel'dovich, predicted that we should be able to see distortions in the cosmic
microwave background (CMB) -- the afterglow of the Big Bang -- whenever it
interacts with a collection of free electrons. These free electrons are present
in the intracluster medium, which is made up primarily of gas. Gas within dense
clusters of galaxies is heated to such an extreme temperature, around 100
million degrees, that it no longer coheres into atoms. According to Sunyaev and
Zel'dovich, the photons of the CMB should be scattered by the high-energy
electrons in the intracluster medium and take on a measurable energy boost as
they pass through the galaxy cluster.
This phenomenon, known as the thermal SZ effect, has been
well supported by observational data since the early 1980s, so it was no
surprise when MACS J0717.5+3745 showed signs of the effect. But recent
observations of this galaxy cluster yielded some curious data. A team led by
Golwala and Jamie Bock -- also a Caltech professor of physics -- observed MACS
J0717.5+3745 with the CSO's Bolocam instrument, measuring microwave radiation
from the cluster at two frequencies: 140 GHz and 268 GHz. Through a simple
extrapolation, the 140 GHz measurement can be used to predict the 268 GHz
measurement assuming the thermal SZ effect.
Yet observations of subcluster B at 268 GHz did not match
those expectations. The trio of Caltech and JPL postdocs who had first proposed
observations of MACS J0717.5+3745 -- Jack Sayers, Phil Korngut, and Tony
Mroczkowski -- puzzled over these images for some time. Trying to sort out the
discrepancy, Korngut kept returning to subcluster B's rapid velocity relative
to the rest of the cluster. Prompted by Korngut's interest, Mroczkowski decided
one weekend to calculate whether the kinetic SZ effect might explain the
discrepancy between the 140 GHz and 268 GHz data. To everyone's surprise, it
could. In order to show this conclusively, the signals from dusty galaxies
behind MACS J0717.5+3745 also had to be accounted for, which was done using
data at higher frequencies from the Herschel Space Observatory analyzed by Mike
Zemcov, a senior postdoctoral scholar at Caltech. The model combining the two
SZ effects and the dusty galaxies was a good match to the observations.
The kinetic SZ effect, like the thermal SZ effect, is caused
by the interaction of the extremely hot and energetic electrons in the gas of
the intracluster medium with the CMB's photons. However, in the kinetic effect,
the photons are affected not by the heat of the electrons, which gives a
random, uncoordinated motion, but instead by their coherent motion as their
host subcluster moves through space. The size of the effect is proportional to
the electrons' speed -- in this case, the speed of subcluster B.
Prior to this study of MACS J0717.5+3745, the best
indication of the kinetic SZ effect came from a statistical study of a large
number of galaxies and galaxy clusters that had been detected by the Atacama
Cosmology Telescope and the Sloan Digital Sky Survey. This is the first time,
Golwala says, "that you can point to a single object and say, 'We think we
see it, right there.'"
"By using the kinetic SZ effect to measure the
velocities of whole clusters relative to the expanding universe, we may be able
to learn more about what causes the universe's accelerating expansion,"
Golwala explains. The next step in the process is the development of new, more
sensitive instrumentation, including the new Multiwavelength Sub/millimeter
Inductance Camera recently commissioned on the CSO.
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