Astronomers Find Confounding Cone Shape in Cluster Collision

Five cluster collisions show how the shapes evolve from bullet to cone to curled tongue
Galaxy cluster collisions evolve in form, with three major levels: first a blunt form as seen in the Bullet Cluster, then a pointy cone as in ZwCl 2341+0000, after which a tongue-like form as in Abell 168. (At high left and proper are the pre- and post-collision phases.) The cone form in the center is a transitory part that hasn’t been caught “on film” till now. These X-ray photos present the new gas in the cluster relatively than the galaxies themselves — this intracluster medium is what collides, the galaxies and attendant darkish matter principally move one another by.
SRON Netherlands Institute for Space Research

For the primary time, astronomers have seen a comparatively short-lived stage that happens in the course of the collision of huge galaxy clusters. Computer simulations of such dramatic smash-ups have efficiently reproduced the brand new observations.

A staff led by Xiaoyuan Zhang (Leiden University and SRON Netherlands Institute for Space Research) used NASA’s Chandra X-ray Observatory to map the distribution of scorching gas in the merging cluster pair named ZwCL 2341+0000, which is a few 3 billion light-years distant in the constellation Pisces.

The staff requested Chandra to stare on the distant cluster pair for 57 hours, accumulating a number of hundreds of X-ray photons. The observations revealed a pointy, cone-like construction of scorching gas in between the colliding clusters, certainly one of which is about 3 times extra huge than the opposite. “It was really unexpected,” says coauthor Aurora Simionescu (SRON and Leiden University). “I had never seen anything like this before.”

When two clusters collide, their particular person galaxies move “through” one another comparatively undisturbed. Just like the large quantities of darkish matter in the clusters, galaxies are “collisionless,” which means they’re solely affected by their mutual gravity. The scorching, X-ray-emitting intracluster gas, nonetheless, does collide, ensuing in bow shocks and so-called “cold fronts” on the interface between gas volumes of various temperatures.

This composite picture of the Bullet Cluster, which is definitely two clusters in the act of merging, exhibits scorching gas (pink, detected through X-rays) sloshes across the galaxies (seen in seen gentle, red, green, blue), that are in flip anchored in darkish matter (blue, visualized through gravitational lensing). The “bullet” form is seen solely in the X-rays; the galaxies and darkish matter of the 2 clusters have handed by way of one another largely undisturbed.
NASA / CXC / M. Weiss

In the early levels of a merger, these buildings have a relatively blunt look – a well-known instance is seen in the Bullet Cluster. In the ultimate levels, the buildings have a tendency to twist again on themselves like breaking waves, giving them the looks of a tongue or a slingshot. Astronomers have usually noticed each shapes – the blunt “bullet” and the wavy “tongue” — however the sharp cone form seen in ZwCl 2341+0000 was new.

Zhang and his colleagues bought in contact with John ZuHone (Center for Astrophysics, Harvard & Smithsonian), who has been finishing up laptop simulations of colliding and merging galaxy clusters since 2011. According to ZuHone, such simulations reveal that cluster properties similar to mass and density affect the distribution of X-ray-emitting gas, as do collision parameters similar to angle and velocity.

In 2019 ZuHone and Bryan Brzycki (University of California, Berkeley) printed extra elaborate simulations that additionally integrated the consequences of magnetic fields. “When magnetic field lines are draped around the cold fronts, they tend to suppress the development of velocity perturbations,” says ZuHone. With magnetic fields protecting gas in line, the result’s a a lot narrower construction, with comparatively sharp edges which may be over 1,000,000 light-years lengthy.

New magnetohydrodynamic simulations tailor-made to the case of ZwCl 2341+0000 efficiently reproduced the cone-shaped construction as a comparatively short-lived characteristic, lasting for only a few hundred million years. Eventually, the gas will fall again into darkish matter’s gravity properly, says ZuHone, “sloshing around a bit like wine in a glass.”

The new Chandra observations and the outcomes of the newest laptop simulations are printed in Astronomy & Astrophysics (preprint available here). The cluster “is likely in a short-lived phase that is rarely observed and offers an example of the complex transition between a bullet-like morphology and the development of a slingshot tail,” the authors write.

According to Simionescu, the brand new photos of ZwCl 2341+0000 present a sneak peek of how the well-known Bullet Cluster would possibly change its form in a number of hundred million years. Cluster collisions play themselves out on very gradual timescales, provides ZuHone, however the good factor about laptop simulations is that they allow you to speed up time and place one observational snapshot into context in a billion-year-long film.


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