Orphan cloud discovered in galaxy cluster

Abell 1367
Hi-res image

New observations made with ESA’s X-ray XMM Newton telescope have revealed an “orphan cloud” – an isolated cloud in a galaxy cluster that is the first discovery of its kind.

A lot goes on in a galaxy cluster. There can be anything from tens to thousands of galaxies bound together by gravity. The galaxies themselves have a range of different properties, but typically contain systems with stars and planets, along with the material in between the stars – the interstellar medium. In between the galaxies is more material – tenuous hot gas known as the intercluster medium. And sometimes in all the chaos, some of the interstellar medium can get ripped out of a galaxy and get stranded in an isolated region of the cluster, as this new study reveals.

Unexpected discovery

Abell 1367, also known as the Leo Cluster, is a young cluster that contains around 70 galaxies and is located around 300 million light-years from Earth. In 2017, a small warm gas cloud of unknown origin was discovered in A1367 by the Subaru telescope in Japan. A follow-up X-ray survey to study other aspects of A1367 unexpectedly discovered X-rays emanating from this cloud, revealing that the cloud is actually bigger than the Milky Way.

This is the first time an intercluster clump has been observed in both X-rays and the light that comes from the warm gas. Since the orphan cloud is isolated and not associated with any galaxy, it has likely been floating in the space between galaxies for a long time, making its mere survival surprising.

The discovery of this orphan cloud was made by Chong Ge at the University of Alabama in Huntsville, and colleagues, and the study has been published in Monthly Notices of the Royal Astronomical Society.

Along with data from XMM-Newton and Subaru, Chong and colleagues also used the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope (VLT) to observe the cluster in visible light.

The orphan cloud is the blue umbrella-shaped part of the image. It has been colour-coded to show the X-ray part of the cloud in blue, the warm gas in red, and the visible region in white shows some of the galaxies in the cluster. The part of the cloud that had been discovered in 2017 (in red) overlaps with the X-ray at the bottom of the cloud.

How the cloud became an orphan

It was previously thought that the distribution of material between galaxies is smooth, however more recent X-ray studies have revealed the presence of clumps in clusters. It was theorised that clumps of gas in the clusters were originally the gas that exists between stars in individual galaxies. The intercluster gas acts as a wind that is strong enough to pull the interstellar gas out of the galaxy as the galaxy is moving through the cluster. However, observations showing that intercluster clumps are originally stripped interstellar material have never been made until now. The observation of the warm gas in the clump provides the evidence to show that this orphan cloud originated within a galaxy. Interstellar material is much cooler than intercluster material, and the temperature of the orphan cloud matches that of interstellar gas. The researchers were also able to determine why the orphan cloud has survived for as long as it has. An isolated cloud would be expected to be ripped apart by instabilities caused by velocity and density differences. However, they found that a magnetic field in the cloud would be able to suppress these instabilities.

Searching for the parent galaxy

It is likely that the parent galaxy of the orphan cloud is a massive one as the mass of the X-ray gas in the orphan is substantial. It is possible that the parent might one day be discovered with future observations by following some breadcrumbs. For example, there are traces of the warm gas that extend beyond the orphan cloud that could be used to identify the parent with more data. There are other unsolved mysteries regarding the cloud that could be deciphered with more observations, such as mysterious offset between the brightest X-rays and the brightest light from the warm gas.

A closer inspection of this orphan will also further our understanding of the evolution of stripped interstellar medium at such a great distance from its parent galaxy and will provide a rare laboratory to study other things such as turbulence and heat conduction. This study paves the way for research on intercluster clumps, as future warm gas surveys can now be targeted to search for other orphan clouds.

Source: ESA/Science Exploration

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A Tale of Tails

RSCG 42 group

A visible light image of the RSCG 42 group of galaxies (CGCG 097−026, CGCG 097−027 and Mrk 0182) in Abell 1367. This is based on a composite of images taken from the Sloan Digital Sky Survey through three colour filters. The white contours show the neutral hydrogen distribution, with the highest contour coinciding with the spiral galaxy GCGC 97-026 and a huge gas tail emanating from this galaxy in a north-easterly direction. Four other galaxies in the RSCG 42 group have associated neutral hydrogen here marked by contour lines. http://amiga.iaa.es/files/97026_ugr300-2.jpg

FGC 1287 group

A visible light image of the FGC 1287 group of galaxies in Abell 1367. This is based on a composite of images taken from the Sloan Digital Sky Survey through three colour filters. The white contours show the neutral hydrogen distribution. The huge gas tail emanates from the edge on spiral galaxy FGC 1287. Two other members of the group have associated neutral hydrogen here marked by contour lines. http://amiga.iaa.es/files/fgc1287_ugr300-1.jpg

Wide angle view of Abell 1367

Image showing a wide angle view of Abell 1367 and the positions of the RSCG 42 and FGC 1287 groups. The cyan contours trace the X-ray emission from the hot intra-cluster gas at the centre of the cluster and the white contours show the cold neutral hydrogen gas detected in the two groups. http://amiga.iaa.es/files/figPR.jpg

Clusters of galaxies are the biggest structures in the Universe that are held together by gravity. They are like a huge metropolis populated by galaxies that interact with one another and with their environment, the hot gas trapped within the cluster's gravitational field. In the last few decades, astronomers have revealed that, in order to enjoy life in these big cities, galaxies have to pay an entrance fee: they are stripped of their cold hydrogen gas when they enter.

Without this gas, the fuel for future star formation, they age much more quickly than their counterparts who shun the big conurbations. Scientists believe that this is why clusters of galaxies have a significantly larger fraction of passive, quiescent objects where stars are no longer forming than is found in lower density environments. The research by Scott and his team might change this view, showing that galaxies can be robbed of their gas reservoir well before reaching the outskirts of the metropolis.

The astronomers made their discovery after using the Expanded Very Large Array (EVLA) of the National Radio Astronomy Observatory (NRAO) in the USA to study Abell 1367 in detail. "When we looked at the data, we were amazed to see these tail structures" says Tom Scott.

'The projected lengths of the gaseous tails are 9 to 10 times that of the size of the parent galaxies, i.e., 520,000 and 815,000 light years respectively. In both cases the amount of cold hydrogen gas in the tails is approximately the same as that remaining in the galaxy's disk. In other words, these galaxies have already left behind half of their fuel for star formation before entering the sphere of influence of the cluster."

The commonly accepted scenario invoked to explain the loss of gas in galaxies is based on the idea that when galaxies enter a big cluster, they lose their gas interacting with the hot intra-cluster medium. Like a meteor entering the Earth's atmosphere, the atomic hydrogen is stripped by the pressure that builds up from moving through the dense, hot gas that pervades the cluster, the stripped gas being dispersed within the intergalactic medium. Scientists call this mechanism ram-pressure stripping.

However, in this case, the perturbed galaxies are still well outside the sphere of influence of this cluster medium and it is not clear which mechanism is responsible for these unique gas tails.

"We considered the various physical processes proposed by theorists in the past to describe gas removal from galaxies, but no one seems to be able to explain our observations" says Luca Cortese, researcher at ESO-Garching, Germany, and co-author of this work. "Whereas in the case of CGCG97-026, the gravitational interaction between the various members of the group could explain what we see, FGC1287 is completely different from any case we have seen before."

Indeed, ram pressure stripping does not seem to be a viable explanation in this case. As this galaxy is located in the outskirts of the cluster, it lies beyond what is thought to be the sphere of influence of the hot and diffuse gas in Abell 1367. To make things even more complicated, gravitational interactions are apparently unable to explain the extraordinary length of the tail and the lack of any sign of disturbance in the stellar body of the galaxy.

The origin of these extraordinary tails still remains a puzzle for scientists, and they perhaps require some physical mechanism that has not been considered before.

"Although the mechanism responsible for this extraordinary gas tail remains to be determined, our discovery highlights how much there still is to learn about environmental effects in galaxy groups" says team member Elias Brinks, a scientist at the University of Hertfordshire. "This discovery might open a new chapter in our understanding of environmental effects on galaxy evolution."

SCIENCE CONTACTS

T. C. Scott
Instituto de Astrofísica de Andalucía (CSIC)
Granada
Spain
Tel: +34 958 121 311
Email: tom@iaa.es

L. Cortese
European Southern Observatory
Germany
Tel: +49 (0)89 3200 6610
Email: lcortese@eso.org

E. Brinks
Centre for Astrophysics Research
University of Hertfordshire
UK
Tel: +44 (0)1707 286072
Email: e.brinks@herts.ac.uk


MEDIA CONTACTS

Robert Massey
Royal Astronomical Society
Tel: +44 (0)20 7734 3307 x214
Mob: +44 (0)794 124 8035
Email: rm@ras.org.uk


FURTHER INFORMATION

The new research appears in "Two long tails in the outskirts of Abell 1367", Scott T., Cortese L., Brinks E., Bravo-Alfaro H., Auld R. and Minchin R., Monthly Notices of the Royal Astronomical Society, in press. A preprint of the paper can be seen at http://arxiv.org/abs/1111.7144

NOTES FOR EDITORS

The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

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