Complete and Total Mayhem in a Distant Galaxy Collision

A cluster of galaxies is nothing trivial. The shocks, the turbulence, the energy, as all of that matter and energy merges and interacts. And we can watch all the chaos and mayhem as it happens.

A team of astronomers are looking at the galaxy cluster Abell 2255 with the European Low-Frequency Array (LOFAR) radio telescope, and their images are showing some never-before-seen details in this actively merging cluster.

A new study based largely on LOFAR of observations of Abell 2255 has revealed the scope of the galaxy cluster and its emissions. The title of the paper is “The beautiful mess in Abell 2255.” The lead author is Andrea Botteon, from Leiden University in the Netherlands. The paper will be published in The Astrophysical Journal.

“The picture emerging from our work is that of one of the most intricate diffuse radio sources known to date.”

From the paper “The beautiful mess in Abell 2255.”

Abell 2255 is considered one of the most intricate objects in the sky, when it comes to radio emissions, an ideal target for LOFAR. And these new LOFAR images only strengthens that conclusion. In their paper the authors write that “The picture emerging from our work is that of one of the most intricate diffuse radio sources known to date.”

The LOFAR radio telescope is a geographically spread out array of about 20,000 small radio antennae in nearly 50 nations. LOFAR’s total signal collecting area can be up to 300,000 square meters (3.2 million sq. ft.) depending on how the array is configured and used. All of the collected signals are sent to one super-computer for processing, but not in real-time like other interferometers.

The LOFAR observations have revealed some details that researchers have never seen before.

“We discovered the existence of numerous filaments within the halo emission that have not been seen previously.”

Andrea Botteon, Lead Author, Leiden University.

Like other galaxy clusters, Abell 2255 contains hundreds of galaxies. The galaxies are spread out over a vast area, spanning hundreds millions of light years, but are still bound together gravitationally. And there’s more to the cluster than just galaxies.

The cluster also has a vast magnetic field, and particles moving at relativistic speeds. When these particles interact with the magnetic field, they emit radiation in the radio band. LOFAR tunes into those emissions.

A spectral index map of the central radio emission in Abell 2255. These maps show how dependent radiation flux density is on radiation frequency. Image Credit: Botteon et al, 2020.

Astrophysicists call those emissions a radio halo because they’re spherical and smooth. Scientists think the halos are created when two galaxy clusters collide, and the emissions come from the central region of the collision. That’s the most accepted hypothesis, anyway, though there’s still some uncertainty and some unknowns.

But Abell 2255’s halo is different. It lacks the smoothness of other radio halos.

In a press release, lead author Botteon said, “We discovered the existence of numerous filaments within the halo emission that have not been seen previously. This was possible thanks to LOFAR, which has a sensitivity and angular resolution much higher than the radio telescopes that have observed galaxy clusters in the past, and also because the discovered filaments emit most of their radiation in long radio wavelengths, precisely those detected by the LOFAR antennas.”

The filamentary structures observed by LOFAR at the center of Abell 2255, here reported in red. These radio emissions are due to trails of particles and magnetic fields released by the galaxies during their motion inside the cluster (credits: Botteon et al. (2020) – LOFAR – SDSS).

This new study didn’t only use LOFAR observations. They also used observations from another radio telescope, the Westerbork Synthesis Radio Telescope (WSRT). A comparison of images from both ‘scopes shows how powerful LOFAR is, and the new details it uncovered in Abell 2255.

The WSRT image of Abell 225 on the left, and the LOFAR image of the same on the right. Image Credit: Pizzo et al. (2009) – WSRT; Botteon et al. (2020) – LOFAR.

Researchers already knew that Abell 2255 bucks the trend of smooth, spherical, radio halos. But this work just emphasizes Abell 2255’s unusual emissions morphology. In their paper the authors write, “The new LOFAR data add further complexity to our picture of A2255, where a plethora of structures on various scales are embedded within the central diffuse emission.”

So what’s unique in Abell 2255 that creates the filamentary nature of its radio emission, as opposed to the smooth and spherical emissions of other galaxy clusters?

All of the radio halos come from the turbulence generated in the cluster’s gas by the merging clusters. That’s the hypothesis, anyway. It’s possible that these new observations will shed some light on the entire process.

“The filaments discovered by LOFAR could form exactly as a consequence of these turbulent motions,” says Gianfranco Brunetti of INAF-Bologna (Italy) and second author of the study. “Another possibility that we are considering is that the filaments originate from the interaction between the galaxies, which move at speeds of many hundreds of km/s inside the cluster and the plasma that produces the radio emission of the halo.”

The LOFAR image of Abell 2255 with features named. The features with black labels were previously known, but the features with blue labels were newly-identified in this study. Image Credit: Botteon et al, 2020.

“The filamentary nature of the emission shows the importance of turbulent magnetic fields, as the bands of emission are likely to follow the threads of magnetic fields,” adds team member Marcus Brüggen from the University of Hamburg, Germany.

The team of researchers think that some of the radio features don’t stem from the center of the cluster. Some of them come from more distant regions of Abell 2255. And only a radio telescope like LOFAR can help untangle that.

The emissions coming from the distant regions, rather than the cluster center, have their own story to tell.

According to astrophysicists, these emissions trace shock waves that propagate outwards at great distances. The shock waves can accelerate energetic particles, and they can amplify magnetic fields. In the outer regions where these emissions come from, the hot intergalactic gas emits only weak radiation. So weak that instruments that sense gamma rays and x-rays can’t really see them. But LOFAR is ideally suited to observe them.

A Chandra X-ray image of Abell 2255 shows much less detail than the LOFAR images, and can’t detect the radiation from the cluster’s distant regions. Image Credit: NASA/Chandra/Botteon et al, 2020.

And these detailed LOFAR observations are bringing up another question for astrophysicists: do these emissions trace the cosmic web that connects galaxy clusters?

“In order to study how far the radio emission extends in the cluster, in the past few months LOFAR has carried out an even deeper observation of Abell 2255,” says co-author Reinout van Weeren from Leiden University, the Netherlands. “One of the goals is to understand if the radio emission extends also beyond Abell 2255, tracing the gigantic cosmic web that connects clusters of galaxies in the Universe.”

But that question will have to wait for future studies to address. And LOFAR will likely have a role to play in those inquiries as well.

As the authors write in the conclusion of their paper, “The analysis of
other features in the system, such as the extended emission at large distance from the cluster center, will be performed in forthcoming works which will exploit deep (75 hr), pointed LOFAR observations of the cluster.”

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Evan Gough

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