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Starts With A Bang

Universe’s Largest Structure Caught In The Act Of Forming

X-rays from Chandra reveal the cluster MACS J0717’s hot gas, while optical data show the individual galaxies in the system. Image credit: X-ray (NASA/CXC/IfA/C. Ma et al.); Optical (NASA/STScI/IfA/C. Ma et al.

When galaxy clusters merge together, they form the largest objects the Universe will ever create.

“On a cosmic scale, our life is insignificant, yet this brief period when we appear in the world is the time in which all meaningful questions arise.” 
Paul Ricoeur

Of all the astronomical, bound structures — planets, stars, galaxies, etc. — there’s none larger than a galaxy cluster.

The full-field image of MACSJ0717.5+3745 shows many thousands of galaxies in four separate sub-clusters within the large cluster, along with Chandra’s X-ray observations in purple. Image credit: X-ray (NASA/CXC/IfA/C. Ma et al.); Optical (NASA/STScI/IfA/C. Ma et al.

The largest ones contain many thousands of times the mass of the Milky Way, both in terms of galaxies and dark matter.

The individual galaxies within the cluster MACS J0717, revealed by Hubble in this optical/infrared composite. Image credit: NASA/STScI/IfA/C. Ma et al.

The Universe forms a vast cosmic web where filaments interconnect, with matter flowing along them into a nexus.

This Chandra image shows a larger scale view of the galaxy cluster MACSJ0717, where the white box shows the field-of-view of the Chandra/HST composite image. The green line shows the approximate position of the large-scale filament leading into the cluster. Image credit: NASA/CXC/IfA/C. Ma et al.

At the centers of these intersections, the most massive galaxy clusters form.

The X-ray data of the collision region of the cluster MACS J0717 reveals the varying temperatures found in the hot gas, where the coolest gas is reddish purple, the hottest gas is blue, and the temperatures in between are purple. Image credit: NASA/CXC/IfA/C. Ma et al.

Over time, more clusters fall in, creating the largest structures of all.

The Hubble Space Telescope recently observed one of them, MACS J0717, revealing four separate clusters in the collision process.

This composite image shows the massive galaxy cluster MACSJ0717.5+3745 (MACSJ0717, for short) where four separate galaxy clusters have been involved in a collision, the first time such a phenomenon has been documented. Image credit: X-ray (NASA/CXC/IfA/C. Ma et al.); Optical (NASA/STScI/IfA/C. Ma et al.).

Additional X-ray data from Chandra shows that a huge filament feeds into this cluster, creating this unprecedentedly messy cosmic mashup.

The combination of radio and X-ray data reveal background, lensed galaxies and signatures of active black holes within the colliding cluster. Image credit: X-ray: NASA/CXC/SAO/G.Ogrean et al.; Optical: NASA/STScI; Radio: NRAO/AUI/NSF.

Detailed radio observations show ultrafast shocks and, at the highest energies, supermassive black hole jets turned on in multiple separate galaxies.

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. Image credit: P. Korngut.

One subcluster moves so quickly — 3,000 km/s — that the speedy electrons push background light to higher energies: the first direct observation of the kinetic Sunyaev-Zel’dovich effect.

A reconstruction of overall mass (in blue) and the X-ray data from colliding, hot gas (pink) are overlaid atop an optical/infrared view of the galaxies within the cluster MACS J0717. Image credit: X-ray: NASA/CXC/Ecole Polytechnique Federale de Lausanne, Switzerland/D.Harvey & NASA/CXC/Durham Univ/R.Massey; Optical & Lensing Map: NASA, ESA, D. Harvey (Ecole Polytechnique Federale de Lausanne, Switzerland) and R. Massey (Durham University, UK).

Finally, weak gravitational lensing reveals separations between mass and X-rays: further indisputable proof of dark matter.

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