Massive galaxy cluster verifies predictions of cosmological theory

Dec 13, 2013 by Cynthia Eller
Massive galaxy cluster verifies predictions of cosmological theory
Hubble space telescope optical image (green), mass map (limousin et al 2012; contours), and cso/bolocam 140 ghz (red) and 268 ghz (blue) maps of the galaxy cluster macs j0717+3745. The lack of 268 ghz signal at subcluster b (second large concentration from upper right) is due to the kinetic sunyaev-zeldovich effect. Credit: p. Korngut

( —By observing a high-speed component of a massive galaxy cluster, Caltech/JPL scientists and collaborators have detected for the first time in an individual object the kinetic Sunyaev-Zel'dovich effect, a change in the cosmic microwave background caused by its interaction with massive moving objects.

MACS J0717.5+3745 is an extraordinarily dynamic 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 (CMB)—the afterglow of the Big Bang—whenever it interacts with a collection of . These free electrons are present in the , 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.

Explore further: Hubble view: Wolf-Rayet stars, intense and short-lived

More information: The paper detailing these observations is titled "A Measurement of the Kinetic Sunyaev-Zel'dovich Signal Towards MACS J0717.5+3745," and appears in Astrophysical Journal.

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2 / 5 (4) Dec 13, 2013
I wonder how far away these clusters were? a good distance i would guess.
Dec 13, 2013
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4.2 / 5 (5) Dec 13, 2013
moving at a speed of 3,000 kilometers per second

wow, that's 1% of the speed of light, and that's a lot of mass. A fast moving asteroid is around 40,000 mph as it passes Earth, which is 'only' about 18 kilometers per second, for comparison.

Anyway, as for the rest of the article, it's always cool when someone figures out a new way to directly measure a large scale property of the Universe, such as distance or, in this case, velocity.

I wonder if this only works for radial velocity relative to us, or if it is absolute velocity relative to us, regardless of direction?
4.2 / 5 (5) Dec 13, 2013
I wonder how far away these clusters were? a good distance i would guess.

Skippy it's 5,400,000,000 light years away. If you want to know how far it is in miles, you'll have to cipher that out on your own.

@ Zephyr fan,

I'm impressed, that's not at all like what ya usually post. I hope I'm not making myself a target of your affections, but I really have to ask: How did ya know that? It's exactly correct.
Dec 13, 2013
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2 / 5 (4) Dec 14, 2013
That image reminds me of ones I've see of the Bullet Cluster showing dark matter distribution (similar red/blue colorization, but highlighting x-ray emission instead). Going by the distribution of optical emission in subcluster b (upper right with no red/blue), might the bulk of that dimly lit oval shape in the mass contour that's ahead of the inward plunge be a fair amount of the subcluster's DM? I'm guessing the mass contours came from micro-lensing data analysis, and that the DM has a more obstruction-free path on the way in. Looks like a great example of SZ effect, at any rate.
1 / 5 (4) Dec 14, 2013
thats the difference between the man and a technician. thank you skippy.
2 / 5 (4) Dec 14, 2013
I wonder how far away these clusters were? a good distance i would guess.

Skippy it's 5,400,000,000 light years away. If you want to know how far it is in miles, you'll have to cipher that out on your own.

About 1.656 Billion parsecs (1.7)...sorry, just thought I'd say something...just a joke ha ha.
I think the Hubble photo referred to goes back to the second half of the last decade, at least that's the one I have.
@GSwift7The 3000km/s was then given as 'average relative radial velocity'.
At that time the Bullet Cluster (1ES0657–56) was a comparison but the MACS dynamics remained speculation suggesting accretion connected to cluster+filament. In addition, although there is mention of x-rays & temerature figures the is no mention of the SZ effect.
So this is where this article goes a step further. Will have to add this to my picture details.
2 / 5 (4) Dec 15, 2013
At that time the Bullet Cluster (1ES0657–56) was a comparison but the MACS dynamics remained speculation suggesting accretion connected to cluster+filament. In addition, although there is mention of x-rays & temerature figures the is no mention of the SZ effect.

Thanks, Mimath224. I think I misidentified subcluster B with A. I found a nice picture of MACSJ0717 as a color image from the HST with contours showing the light distribution from cluster members, the Chandra X-ray Luminosity map, and DM distribution determined from weak lensing shear analysis (Fig. 12 bottom right, p.16 in this paper: ). It looks like my guess was completely opposite to where it appears—the dynamics of that region have a history, duh. :)
2.3 / 5 (3) Dec 15, 2013
Protoplasmix, thanks for the arxiv link..have updated my info.

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