XMM-Newton reveals cosmic collision in the Bullet Group

June 9, 2014
Composite image of the Bullet Group showing galaxies, hot gas (shown in pink) and dark matter (indicated in blue). Credit: ESA / XMM-Newton / F. Gastaldello (INAF/IASF, Milano, Italy) / CFHTLS

(Phys.org) —Despite the large distances between them, galaxies rarely exist in isolation. They are mostly found in large assemblies known as groups and clusters. Groups are the smallest gatherings, containing 50 or so galaxies bound together by gravity, whereas clusters are somewhat larger, consisting of hundreds or thousands more. These structures also contain large amounts of hot gas that fills the space between galaxies and shines brightly in the X-ray part of the spectrum, and even larger amounts of dark matter, which does not emit light but can be detected via its gravitational effect on other objects.

This invisible provides scaffolding for and hot gas, and its gravity affects the build-up of large cosmic structures. In most cases, galaxies and hot gas are found in the pockets of the Universe where the dark matter is densest, but when groups or clusters of galaxies collide with one another, their different constituents do not mix well. These cosmic clashes give rise to curious configurations where hot gas, which comprises the bulk of ordinary (baryonic) matter in a group or cluster, may lie in one region, while galaxies and dark matter lie elsewhere.

This is certainly the case in this image of SL2S J08544-0121, an object now nicknamed the Bullet Group. This group was created by such a collision, and what we see is the aftermath of this cosmic tussle. The group's diffuse gas is so hot that it strongly radiates X-rays, detected by ESA's XMM-Newton X-ray Observatory and shown here in pink. While the hot gas is concentrated in one large bubble, the rest of the group's mass – consisting of dark matter (shown here in blue) and galaxies – appears to be split into two distinct parts.

Astronomers believe that the blob to the right of the image centre acted as a "bullet", travelling from the lower left towards the upper right of the image. In the process, it impacted the other sub-structure of the group, and passed through it.

Mergers mix the contents of galaxy groups and clusters, but each component behaves differently. While the galaxies and dark matter from both colliding parties did take part in the Bullet Group merger, they were almost unaffected by this event and remained confined in the original sub-structures, as shown in this image. However, particles in the two colliding clouds of hot gas did interact with one another via the electromagnetic force, and the resulting friction caused the gas from the two merging parties to mix, creating a single billowing cloud.

This curious separation of gas, galaxies, and dark matter has been observed so far in a handful of massive galaxy clusters, including the famous Bullet Cluster. However, it had never been seen before in lower-mass objects such as , making the Bullet Group the smallest structure in which this effect has been detected.

Although it is not visible to the eye – nor to any type of telescope – astronomers were able to map the extent of the Bullet Group's dark matter by tracing the effect it has on the light from distant galaxies lying behind the group. This distortion, called gravitational lensing, is caused when light passes by a massive object such as a galaxy, which gravitationally affects the space around it, causing the space to bend and curve. The paths of light from the more distant object are also bent and curved, sometimes creating bizarre optical effects.

If the lensing object is very massive and favourably aligned with the background source of light, this effect becomes even more dramatic and striking – a phenomenon known as "strong gravitational lensing". It can turn galaxies into rings or bright arcs smeared across the sky, and it even creates multiple images of the same galaxy.

This can be seen in the centre-right part of this image, where a round, bright galaxy that belongs to the Bullet Group is circled by curious arcs of light – the distorted image of another galaxy lying much farther away.

By exploring the contents of these cosmic wrecks, astronomers can learn more about the properties of dark matter. In particular, from the split between the dark matter and the hot gas, they can constrain how much dark matter does – or does not – interact with normal matter. The possibility of observing this effect in smaller objects like the Bullet Group, which are much more numerous than the more massive galaxy clusters, opens up new perspectives to study the role of dark matter across the Universe.

This image is a composite of an X-ray image (shown in pink) from ESA's XMM-Newton observatory, a three-colour (red, green, blue) optical image from the Canada-France-Hawaii Telescope (CFHT), and a dark matter overlay (shown in blue) based on data from CFHT, the NASA/ESA Hubble Space Telescope, and the W. M. Keck Observatory. Bright foreground stars that belong to our Galaxy are also visible scattered across the frame.

Explore further: Dark matter core defies explanation

More information: "Dark matter-baryons separation at the lowest mass scale: the Bullet Group." F. Gastaldello et al. Monthly Notices of the Royal Astronomical Society. arxiv.org/abs/1404.5633

"Strong Lensing as a Probe of the Mass Distribution Beyond the Einstein Radius. Mass & Light in SL2S J08544-0121, a Galaxy Group at z=0.35." Marceau Limousin, et al. Astronomy and Astrophysics. arxiv.org/abs/0906.4118

Related Stories

Dark matter core defies explanation

March 2, 2012

(PhysOrg.com) -- Astronomers using data from NASA's Hubble Telescope have observed what appears to be a clump of dark matter left behind from a wreck between massive clusters of galaxies. The result could challenge current ...

Discovery of the Musket Ball Cluster

April 12, 2012

(Phys.org) -- Using a combination of powerful observatories in space and on the ground, astronomers have observed a violent collision between two galaxy clusters in which so-called normal matter has been wrenched apart from ...

Image: Hot gas sloshing in a galactic cauldron

November 19, 2013

(Phys.org) —Galaxies are social beasts that are mostly found in groups or clusters – large assemblies of galaxies that are permeated by even larger amounts of diffuse gas. With temperatures of 10 million degrees or more, ...

Image: Magnifying the distant universe

April 1, 2014

(Phys.org) —Galaxy clusters are some of the most massive structures that can be found in the Universe—large groups of galaxies bound together by gravity. This image from the NASA/ESA Hubble Space Telescope reveals one ...

Image: X-raying the cosmos

April 22, 2014

When we gaze up at the night sky, we are only seeing part of the story. Unfortunately, some of the most powerful and energetic events in the Universe are invisible to our eyes – and to even the best optical telescopes.

Recommended for you

New Horizons team selects potential Kuiper Belt flyby target

August 29, 2015

NASA has selected the potential next destination for the New Horizons mission to visit after its historic July 14 flyby of the Pluto system. The destination is a small Kuiper Belt object (KBO) known as 2014 MU69 that orbits ...

Prawn Nebula: Cosmic recycling

September 2, 2015

Dominating this image is part of the nebula Gum 56, illuminated by the hot bright young stars that were born within it. For millions of years stars have been created out of the gas in this nebula, material which is later ...

Image: Hubble sees a youthful cluster

August 31, 2015

Shown here in a new image taken with the Advanced Camera for Surveys (ACS) on board the NASA/ESA Hubble Space Telescope is the globular cluster NGC 1783. This is one of the biggest globular clusters in the Large Magellanic ...

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

cantdrive85
1.4 / 5 (5) Jun 09, 2014
This invisible dark matter provides scaffolding for galaxies and hot gas, and its gravity affects the build-up of large cosmic structures

The "dark matter" does form a "scaffolding" or "skeleton" for cosmological phenomena, but there is no need to conjure up a theoretical gravitational only non-baryonic dark matter to account for it.
Electrodynamical baryonic nanodust already observed in Tokamaks and other plasma discharge is more than capable to explain the "anomalous" effects of the so called "dark matter".

http://www.cosmol...hkin.pdf

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.