Saturday, October 19, 2019

Stormy Cluster Weather Could Unleash Black Hole Power and Explain Lack of Cosmic Cooling



“Weather” in clusters of galaxies may explain a longstanding puzzle, according to a team of researchers at the University of Cambridge. The scientists used sophisticated simulations to show how powerful jets from supermassive black holes are disrupted by the motion of hot gas and galaxies, preventing gas from cooling, which could otherwise form stars. The team publish their work in the journal Monthly Notices of the Royal Astronomical Society.

Typical clusters of galaxies have several thousand member galaxies, which can be very different to our own Milky Way and vary in size and shape. These systems are embedded in very hot gas known as the intracluster medium (ICM), all of which live in an unseen halo of so-called ‘dark matter’.

An artist’s impression of the jet launched by a supermassive black hole, which inflates lobes of very hot gas that are distorted by the cluster weather. 
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Credit Institute of Astronomy, University of Cambridge

A large number of galaxies have supermassive black holes in their centres, and these often have high speed jets of material stretching over thousands of light years that can inflate very hot lobes in the ICM.

The researchers, based at the Kavli Institute for Cosmology and the Institute of Astronomy, performed state-of-the-art simulations looking at the jet lobes in fine detail and the X-rays emitted as a result. The model captures the birth and cosmological evolution of the galaxy cluster, and allowed the scientists to investigate with unprecedented realism how the jets and lobes they inflate interact with a dynamic ICM.

They found that the mock X-ray observations of the simulated cluster revealed the so-called “X-ray cavities” and “X-ray bright rims” generated by supermassive black hole-driven jets, which itself is distorted by motions in the cluster, remarkably resemble those found in observations of real galaxy clusters.

The left hand panel shows an actual observation of the galaxy cluster MS 0735.6+7421, while on the right the background Hubble image has instead been overlaid with a mock observation of the jet (pink) and X-ray emission (blue) made from the simulation. Both images show cavities excavated by the lobe inflation surround by X-ray bright rims of dense gas (blue), which are filled by distorted jet material (pink).


Credit: NASA, ESA, CXC, STScI, and B. McNamara (University of Waterloo); Very Large Array Telescope Image: NRAO, and L. Birzan and team (Ohio University); Simulated Data: M. A. Bourne (University of Cambridge)


Dr Martin Bourne of the Institute of Astronomy in Cambridge led the team. He commented: “We have developed new computational techniques, which harness the latest high-performance computing technology, to model for the first time the jet lobes with more than a million elements in fully realistic clusters. This allows us to place the physical processes that drive the liberation of the jet energy under the microscope.”

As galaxies move around in the cluster, the simulation shows they create a kind of ‘weather’, moving, deforming and destroying the hot lobes of gas found at the end of the black hole jets. The jet lobes are enormously powerful and if disrupted, deliver vast amounts of energy to the ICM.

The Cambridge team believe that this cluster weather disruption mechanism may solve an enduring problem: understanding why ICM gas does not cool and form stars in the cluster centre. This so-called “cooling flow” puzzle has plagued astrophysicists for more than 25 years.

The simulations performed provide a tantalising new solution that could solve this problem. Dr Bourne commented: “The combination of the huge energies pumped into the jet lobes by the supermassive black hole and the ability of cluster weather to disrupt the lobes and redistribute this energy to the ICM provides a simple and yet elegant mechanism to solve the cooling flow problem.”

A series of next generation X-ray space telescopes will launch into orbit over the next decade. These advanced instruments should help settle the debate – and if intergalactic weather really does stop the birth of stars.


Contacts and sources:
Dr Robert Massey
Royal Astronomical Society

Dr Martin Bourne
Institute of Astronomy
Cambridge








Mars Once Had Salt Lakes Similar To Earth



Mars once had salt lakes that are similar to those on Earth and has gone through wet and dry periods, according to an international team of scientists that includes a Texas A&M University College of Geosciences researcher.

Marion Nachon, a postdoctoral research associate in the Department of Geology and Geophysics at Texas A&M, and colleagues have had their work published in the current issue of Nature Geoscience.

The team examined Mars’ geological terrains from Gale Crater, an immense 95-mile-wide rocky basin that is being explored with the NASA Curiosity rover since 2012 as part of the MSL (Mars Science Laboratory) mission.

A color image from NASA’s Curiosity rover’s Mast Camera shows part of the wall of Gale Crater, the location on Mars where the rover landed August 5, 2012 on Mars.
Mars Gale Crater
Credit: NASA

The results show that the lake that was present in Gale Crater over 3 billion years ago underwent a drying episode, potentially linked to the global drying of Mars.

Gale Crater formed about 3.6 billion years ago when a meteor hit Mars and created its large impact crater.

“Since then, its geological terrains have recorded the history of Mars, and studies have shown Gale Crater reveals signs that liquid water was present over its history, which is a key ingredient of microbial life as we know it,” Nachon said. “During these drying periods, salt ponds eventually formed. It is difficult to say exactly how large these ponds were, but the lake in Gale Crater was present for long periods of time – from at least hundreds of years to perhaps tens of thousands of years,” Nachon said.

So what happened to these salt lakes?

Nachon said that Mars probably became dryer over time, and the planet lost its planetary magnetic field, which left the atmosphere exposed to be stripped by solar wind and radiation over millions of years.

“With an atmosphere becoming thinner, the pressure at the surface became lesser, and the conditions for liquid water to be stable at the surface were not fulfilled anymore,” Nachon said. “So liquid water became unsustainable and evaporated.”

The salt ponds on Mars are believed to be similar to some found on Earth, especially those in a region called Altiplano, which is near the Bolivia-Peru border.

Nachon said the Altiplano is an arid, high-altitude plateau where rivers and streams from mountain ranges “do not flow to the sea but lead to closed basins, similar to what used to happen at Gale Crater on Mars,” she said. “This hydrology creates lakes with water levels heavily influenced by climate. During the arid periods Altiplano lakes become shallow due to evaporation, and some even dry up entirely. The fact that the Atliplano is mostly vegetation free makes the region look even more like Mars,” she said.”

Nachon added that the study shows that the ancient lake in Gale Crater underwent at least one episode of drying before “recovering.” It’s also possible that the lake was segmented into separate ponds, where some of the ponds could have undergone more evaporation.

Because up to now only one location along the rover’s path shows such a drying history, Nachon said it might give clues about how many drying episodes the lake underwent before Mars’s climate became as dry as it is currently.

“It could indicate that Mars’s climate ‘dried out’ over the long term, on a way that still allowed for the cyclical presence of a lake,” Nachon said. “These results indicate a past Mars climate that fluctuated between wetter and drier periods. They also tell us about the types of chemical elements (in this case sulphur, a key ingredient for life) that were available in the liquid water present at the surface at the time, and about the type of environmental fluctuations Mars life would have had to cope with, if it ever existed.”



Contacts and sources:
Keith RandallTexas A&M University

Citation: