Kinetic Sunyaev-Zel'dovich Effect Observed In Individual Object For The First Time
January 17, 2014

Kinetic Sunyaev-Zel’dovich Effect Observed In Individual Object For The First Time

redOrbit Staff & Wire Reports - Your Universe Online

Scientists from the California Institute of Technology (Caltech) and NASA’s Jet Propulsion Laboratory (JPL) report in a recent study that they have observed a change in an individual object’s cosmic microwave background radiation resulting from its interaction with massive moving objects for the first time ever.

According to the recently-published paper in The Astrophysical Journal, this phenomenon, which is known as the kinetic Sunyaev-Zel'dovich effect, was detected by observing a high-speed component of the MACS J0717.5+3745 galaxy cluster. MACS J0717.5+3745 has a total mass of more than 1,000 times that of our own galaxy, and includes four subclusters – three that are stationary, and one traveling about 1,800 miles per second.

Caltech physics professor Sunil Golwala and his colleagues observed the galaxy cluster, witnessing the moving subcluster (subcluster B) during what is believed to be its first fall into MACS J0717.5+3745. Its momentum is expected to carry it through the galaxy cluster’s center temporarily, but the cluster’s strong gravitational pull will eventually pull subcluster B back in again, until it eventually becomes stationary.

Despite the subcluster’s dramatic movement, the researchers said that it fits “neatly within the standard cosmological model.” However, the details of MACS J0717.5+3745 observations conducted at different wavelengths puzzled the researchers until they looked at them under the lens of the kinetic 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,” Caltech explained in a statement. “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,” the institute added. “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.”

However, recent observations of the galaxy cluster conducted by Golwala and JPL scientist/Caltech professor Jamie Bock using the Caltech Submillimeter Observatory (CSO)’s Bolocam instrument led to some unusual findings. They measured microwave radiation from MACS J0717.5+3745 in two frequencies (140 GHz and 268 GHz) and, assuming the thermal SZ effect is accurate, they should have been able to use the former measurement in order to correctly predict the latter one.

As it turned out, though, the observations of subcluster B conducted at 268 GHz did not turn out as expected, leaving the researchers to try and figure out why. Ultimately, they decided to calculate whether or not the kinetic SZ effect could help explain the discrepancy between the 140 GHz and 268 GHz data.

“To everyone's surprise, it could,” the institute said. “In order to show this conclusively, the signals from dusty galaxies behind MACS J0717.5+3745 also had to be accounted for,” which the researchers did using data collected at higher frequencies from the NASA/ESA Herschel Space Observatory. “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,” it added. “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.”

According to the researchers, the effect’s size is proportional to the speed of the electrons (in this case, subcluster B’s speed). Previously, the best indication of the kinetic SZ effect came from multiple galaxies and galaxy clusters originally detected by the Atacama Cosmology Telescope and the Sloan Digital Sky Survey, they said.

However, this is the first time scientists have observed the phenomenon on a single object, said Golwala.

“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,” he added.

The next step in the research, the institute said, is the development of more sensitive instruments, including one recently commissioned on the CSO, known as the Multiwavelength Sub/millimeter Inductance Camera.