A Single Wave, Bigger Than the Milky Way, is Rolling Through the Perseus Galaxy Cluster

NASA has discovered a wave of hot gas larger than the Milky Way rolling through the Perseus galaxy cluster. This X-ray image is the result of 16 days of observing with the Chandra X-ray Observatory. The image was filtered to make details easier to see.Credit: NASA's Goddard Space Flight Center/Stephen Walker et al.
NASA has discovered a wave of hot gas larger than the Milky Way  rolling through the Perseus galaxy cluster. This X-ray image is the result of 16 days of observing with the Chandra X-ray Observatory. The image was filtered to make details easier to see. Credits: NASA's Goddard Space Flight Center/Stephen Walker et al.
NASA has discovered a wave of hot gas larger than the Milky Way rolling through the Perseus galaxy cluster. This X-ray image is the result of 16 days of observing with the Chandra X-ray Observatory. The image was filtered to make details easier to see. Credits: NASA’s Goddard Space Flight Center/Stephen Walker et al.

An international team of scientists has discovered an enormous wave of hot gas rolling its way through the Perseus galaxy cluster. The wave is a giant version of what’s called a Kelvin-Helmholtz wave. They’re created when two fluids intersect at different velocities: for example, when wind blows over water.

In this instance, the wave was caused by a small galaxy cluster grazing the Perseus cluster, and setting off a chain of events lasting billions of years. The findings appear in a paper in the June 2017 issue of the journal Monthly Notices of the Royal Astronomical Society.

“The wave we’ve identified is associated with the flyby of a smaller cluster, which shows that the merger activity that produced these giant structures is still ongoing.” – Stephen Walker, NASA’s Goddard Space Flight Center.

“Perseus is one of the most massive nearby clusters and the brightest one in X-rays, so Chandra data provide us with unparalleled detail,” said lead scientist Stephen Walker at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “The wave we’ve identified is associated with the flyby of a smaller cluster, which shows that the merger activity that produced these giant structures is still ongoing.”

The Perseus galaxy cluster, also known as Abell 426, is 240 million light years away, and is about 11 million light years across. It’s one of the most massive objects we know of, and it’s named after the Perseus constellation, which appears in the same part of the sky.

Galaxy clusters are the largest gravitationally-bound objects in the Universe. Most of the observable matter in galaxy clusters is gas. But the gas is super hot—tens of millions of degrees hot—which means it emits x-rays.

X-Ray observations of Perseus have revealed several features and structures in the gas structure of the cluster. Some of them are bubble-like features caused by the super-massive black hole (SMBH) in NGC 1275, the Perseus cluster’s central galaxy. Another of these features is known as “the bay.” The bay is a concave feature which couldn’t have been formed by the SMBH.

This Hubble image shows NGC 1275, the Super-Massive Black Hole at the center of the Perseus cluster. NGC 1275 could not have been responsible for the "bay" feature found in Perseus. Image: By NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration - http://hubblesite.org/newscenter/archive/releases/2008/28/image/a/, Public Domain, https://commons.wikimedia.org/w/index.php?curid=4634173
This Hubble image shows NGC 1275, the Super-Massive Black Hole at the center of the Perseus cluster. NGC 1275 could not have been responsible for the “bay” feature found in Perseus. Image: By NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration – http://hubblesite.org/newscenter/archive/releases/2008/28/image/a/, Public Domain, https://commons.wikimedia.org/w/index.php?curid=4634173

The bay is a puzzle because it doesn’t produce any emissions, which would be expected of something formed by a SMBH. The bay also doesn’t conform to models of how gas should behave in this situation.

The lead scientist behind the study is Stephen Walker at NASA’s Goddard Space Flight Center. Walker turned to the Chandra X-ray Observatory to help solve this puzzle. Existing Chandra images of the Perseus cluster were filtered in order to highlight the edges of structures, and to make any subtle details more visible.

These filtered and processed images were then compared to computer simulations of galaxy clusters merging. John ZuHone, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics, has created an online catalog of these simulations.

“Galaxy cluster mergers represent the latest stage of structure formation in the cosmos.” -John ZuHone, Harvard-Smithsonian Center for Astrophysics.

“Galaxy cluster mergers represent the latest stage of structure formation in the cosmos. Hydrodynamic simulations of merging clusters allow us to produce features in the hot gas and tune physical parameters, such as the magnetic field. Then we can attempt to match the detailed characteristics of the structures we observe in X-rays.” -John ZuHone, Harvard-Smithsonian Center for Astrophysics.

This alternate image of the Perseus galaxy cluster shows the wave at the 7 o'clock position. Image: NASA's Goddard Space Flight Center/Stephen Walker et al.
This alternate image of the Perseus galaxy cluster shows the wave at the 7 o’clock position. Image: NASA’s Goddard Space Flight Center/Stephen Walker et al.

One of the simulations matched what astronomers were seeing in Perseus. In it, a large cluster like Perseus had settled itself into two regions: a colder region of gas around 30 million degrees Celsius, and a hotter region of gas at almost 100 million degrees Celsius. In this model, a cluster smaller than Perseus, but about a thousand times more massive than the Milky Way passes close to Perseus, missing its center by about 650,000 light years.

That happened about 2.5 billion years ago, and it set off a chain of events still playing itself out.

The near miss caused a gravitational disturbance that created an expanding spiral of the colder gas. An enormous wave of gas has formed at the edge of the spiral of colder gas, where it intersects with the hotter gas. This is the Kelvin-Helmholtz wave seen in the images.

“We think the bay feature we see in Perseus is part of a Kelvin-Helmholtz wave, perhaps the largest one yet identified, that formed in much the same way as the simulation shows,” Walker said. “We have also identified similar features in two other galaxy clusters, Centaurus and Abell 1795.”

The study provided another benefit besides just spotting an impossibly enormous wave. It allowed the team to measure the magnetic properties of the Perseus cluster. The researchers discovered that the strength of the magnetic field in the cluster affected the size of the wave of gas. It the field is too strong, the waves don’t form at all, and if the magnetic field is too weak, then the waves would be even larger.

According to the team, there is no other known way to measure the magnetic field.

Source: Scientists Find Giant Wave Rolling Through the Perseus Galaxy Cluster

A Cosmic Intruder Grabbed Hot Gas From This Galaxy Group

So galaxy group NGC 5044 was just sitting quietly by itself a few million years ago when galaxy NGC 5054 decided to pass right through it. That close encounter finished long ago, but the ricochet is still visible in telescopes as astronomers spotted hot gas rippling through the host galaxy.

“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, the gas in galaxy groups and clusters is hot enough to shine brightly in X-rays and be detected by ESA’s XMM-Newton X-ray observatory,” the European Space Agency stated.

“As galaxies speed through these gigantic cauldrons, they occasionally jumble the gas and forge it into lop-sided shapes. An example is revealed in this composite image of the galaxy group NGC 5044, the brightest group in X-rays in the entire sky.”

Fresh observations from XMM-Newton (in blue) are visible in this composite image with other pictures from the Wide-field Infrared Survey Explorer, the Digitized Sky Survey (optical) and Galex (near-ultraviolet).

Publication of this research was accepted in MNRAS and is currently available on prepublishing site Arxiv. The lead author is Ewan O’Sullivan, a visiting scientist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Hot Gas Bridge Discovered Connecting Galaxy Clusters

An “bridge” of hot gas stretches between galaxy clusters Abell 401 and Abell 399

It may not be good practice to burn bridges but this is one super-heated bridge that astronomers were happy to find: an enormous swath of hot gas connecting two galaxy clusters 10 million light-years apart, and nearly a billion light-years away.

Using ESA’s Planck space telescope, astronomers have identified leftover light from the Big Bang interacting with a filament of hot gas stretching between Abell 401 and Abell 399, two galactic clusters each containing hundreds of individual galaxies.

Launched in May 2009, Planck is designed to study the Cosmic Microwave Background (CMB) — the leftover light from the Big Bang. When this radiation interacts with large-scale cosmic structures, like the hot gas bridging clusters of galaxies, its energy is modified in a specific way. This is referred to as the Sunyaev–Zel’dovich Effect (SZE), and Planck is specifically attuned to finding it.

This, however, is Planck’s first discovery of inter-cluster gas found using the SZ technique.

The temperature of the gas is estimated to be around 80 million degrees C, similar to the temperature of the gas found within the clusters themselves. It’s thought that the gas may be a combination of cosmic web filaments left over from the early Universe mixed with gas from the clusters.

The image above shows the clusters Abell 401 and Abell 399 as seen at optical wavelengths with ground-based telescopes overlaid with the SZE from Planck. The entire bridge spans a distance about the size of two full Moons in the sky.

Read more on ESA’s news page here.

Top image: Sunyaev–Zel’dovich effect: ESA Planck Collaboration; optical image: STScI Digitized Sky Survey. Inset image: Artist’s impression of Planck against the CMB. (ESA and the HFI Consortium, IRAS)