When radio galaxies collide, supermassive black holes form tightly bound pairs

A study using multiple radio telescopes confirms that supermassive black holes found in the centers of galaxies can form gravitationally bound pairs when galaxies merge.

The two compact radio sources separated by less than a light year at the center of the galaxyNGC7674. The two sources 
correspond to the location of the two active supermassive blackholes which form a binary and orbit around each other 
[Credit: TIFR-NCRA and RIT, USA]

The paper published in Nature Astronomy sheds light on a class of black holes having a mass upwards of one million times the mass of the sun. Supermassive black holes are expected to form tightly bound pairs following the merger of two galaxies.

"The dual black hole we found has the smallest separation of any so far detected through direct imaging," said David Merritt, professor of physics at Rochester Institute of Technology, a co-author on the paper.

The supermassive black holes are located in the spiral galaxy NGC 7674, approximately 400 million light years from earth, and are separated by a distance less than one light year. The study was led by Preeti Kharb, from the National Center for Radio Astrophysics at Pune University in India and co-authored by Dharam Vir Lal, also at Pune University and Merritt at RIT.

"The combined mass of the two black holes is roughly 40 million times the mass of the Sun, and the orbital period of the binary is about 100,000 years," Merritt said.

A class of smaller black holes form when massive stars explode as supernovae. A collision of stellar mass black holes led to the landmark discovery of gravitational waves in 2015 using the Laser Interferometer Gravitational-wave Observatory. The black holes were approximately 29 and 36 times the mass of the sun and collided 1.3 billion light years away

"A supermassive binary generates gravitational waves with much lower frequency than the characteristic frequency of stellar-mass binaries and its signal is undetectable by LIGO," Merritt said.

To simulate a highly sensitive detector, the researchers used a method to make radio telescopes around the world work together as a single large telescope and achieve a resolution roughly 10 million times the angular resolution of the human eye.

"Using very long baseline interferometry techniques, two compact sources of radio emission were detected at the center of NGC 7674; the two radio sources have properties that are known to be associated with massive black holes that are accreting gas, implying the presence of two black holes," Merritt said.

The galaxy hosting the binary supermassive black hole loudly emits radio waves. The detection confirms a theory predicting the presence of a compact binary in a radio galaxies bearing a "Z" shape.

"This morphology is thought to result from the combined effects of the galaxy merger followed by the formation of the massive binary," Merritt said.

Author: Susan Gawlowicz | Source: Rochester Institute of Technology [September 18, 2017]

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Two stars, three dimensions, and oodles of energy

For decades, astronomers have known about irregular outbursts from the double star system V745 Sco, which is located about 25,000 light years from Earth. Astronomers were caught by surprise when previous outbursts from this system were seen in 1937 and 1989. When the system erupted on February 6, 2014, however, scientists were ready to observe the event with a suite of telescopes including NASA’s Chandra X-ray Observatory.

Figure 1 [Credit: Chandra X-ray Center]

V745 Sco is a binary star system that consists of a red giant star and a white dwarf locked together by gravity. These two stellar objects orbit so closely around one another that the outer layers of the red giant are pulled away by the intense gravitational force of the white dwarf. This material gradually falls onto the surface of the white dwarf. Over time, enough material may accumulate on the white dwarf to trigger a colossal thermonuclear explosion, causing a dramatic brightening of the binary called a nova. Astronomers saw V745 Sco fade by a factor of a thousand in optical light over the course of about 9 days.

Astronomers observed V745 Sco with Chandra a little over two weeks after the 2014 outburst. Their key finding was it appeared that most of the material ejected by the explosion was moving towards us. To explain this, a team of scientists from the INAF-Osservatorio Astronomico di Palermo, the University of Palermo, and the Harvard-Smithsonian Center for Astrophysics constructed a three-dimensional (3D) computer model of the explosion, and adjusted the model until it explained the observations. In this model they included a large disk of cool gas around the equator of the binary caused by the white dwarf pulling on a wind of gas streaming away from the red giant.

The computer calculations showed that the nova explosion’s blast wave and ejected material were likely concentrated along the north and south poles of the binary system. This shape was caused by the blast wave slamming into the disk of cool gas around the binary. This interaction caused the blast wave and ejected material to slow down along the direction of this disk and produce an expanding ring of hot, X-ray emitting gas. X-rays from the material moving away from us were mostly absorbed and blocked by the material moving towards Earth, explaining why it appeared that most of the material was moving towards us.

Figure 2 [Credit: Chandra X-ray Center]

In Figure 1 which shows the new 3D model of the explosion, the blast wave is yellow, the mass ejected by the explosion is purple, and the disk of cooler material, which is mostly untouched by the effects of the blast wave, is blue. The cavity visible on the left side of the ejected material is the result of the debris from the white dwarf's surface being slowed down as it strikes the red giant. Below is an optical image from Siding Springs Observatory in Australia.

An extraordinary amount of energy was released during the explosion, equivalent to about 10 million trillion hydrogen bombs. The authors estimate that material weighing about one tenth of the Earth’s mass was ejected.

While this stellar-sized belch was impressive, the amount of mass ejected was still far smaller than the amount what scientists calculate is needed to trigger the explosion. This means that despite the recurrent explosions, a substantial amount of material is accumulating on the surface of the white dwarf. If enough material accumulates, the white dwarf could undergo a thermonuclear explosion and be completely destroyed. Astronomers use these so-called Type Ia supernovas as cosmic distance markers to measure the expansion of the Universe.

The scientists were also able to determine the chemical composition of the material expelled by the nova. Their analysis of this data implies that the white dwarf is mainly composed of carbon and oxygen.

A 3D print of the model was also created (Figure 2). This 3D print was simplified and printed in two parts, the blast wave (shown here in grey) and the ejected material (shown here in yellow).

A paper describing these results was published in the Monthly Notices of the Royal Astronomical Society and is available online.

Source: Chandra X-ray Center [September 18, 2017]

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3D supernova simulations reveal mysteries of dying stars

An international team of astronomers have created the longest consistent 3D model of a neutrino-driven supernova explosion to date, helping scientists to better understand the violent deaths of massive stars. The research, conducted using supercomputers in Australia, Germany, and the DiRAC facility in the UK, is published in the journal Monthly Notices of the Royal Astronomical Society.

Snapshot of the expansion of the neutrino-heated matter and the supernova shock wave during the 
explosion of an 18 solar mass star [Credit: Bernhard Müller]

The largest explosions in the Universe, so-called ’supernovae’, occur when stars many times larger than our own Sun reach the end of their lives and exhaust the nuclear fuel at their centres. At this point the innermost part of the star, an iron core itself about 1.5 times as massive as the Sun, succumbs to gravity and collapses to an ultra-dense neutron star within a fraction of a second.

In the process, the outer layers of the star are expelled in a gigantic supernova explosion, which ejects material at velocities of thousands of kilometres per second. Such supernovae are regularly observed in distant galaxies, and within the Milky Way we can still see the debris of many of them thousands of years later.

But a puzzle remains: how is the collapse of the star turned into an explosion? The team, from Monash University, Queen’s University Belfast, and the Max Planck Institute for Astrophysics, have worked on a solution to this problem, and the most promising theory suggests that extremely light and weakly interacting particles called neutrinos are the key to this process.

Animation of the expansion of the neutrino-heated matter (yellow/red) and the supernova shock wave 

(translucent cyan surface) during the explosion of an 18 solar mass star [Credit: B. Mueller]

Vast numbers of neutrinos are emitted from the surface of the young neutron star, and if the heating caused by the initial collapse is sufficiently strong, the neutrino-heated matter drives an expanding shock wave through the star and the collapse is reversed. Scientists have long attempted to show that this idea works with the help of computer simulations, but the computer models often still fail to explode, and can’t be run long enough to reproduce observed supernovae.

“What is crucial for success in three dimensions is the violent churning of hot and cold material behind the shock wave, which develops naturally due to the neutrino heating,” explains Dr Tobias Melson, a co-author of the study at the Max Planck Institute for Astrophysics in Germany. “But it often seems we need to stir these churning motions a bit more to obtain an explosion.”

To explore this possibility, the team simulated the fusion of oxygen to silicon in a star 18 times the size of our Sun, for the last 6 minutes before the supernova. They found that they could obtain a successful explosion because the collapsing silicon-oxygen shell was strongly stirred already.

They then followed the explosion for more than 2 seconds. Although it still takes about a day for the shock to reach the surface, they could tell that the explosion and the left-over neutron star were starting to look like the ones that we observe in nature.

“It’s reassuring that we now get plausible explosion models without having to tweak them by hand,” comments Dr Bernhard Mueller of the Monash Centre for Astrophysics in Australia, the lead author of the study.

Source: Monash University [September 15, 2017]

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Explosive birth of stars swells galactic cores

Astronomers found that active star formation upswells galaxies, like yeast helps bread rise. Using three powerful telescopes on the ground and in orbit, they observed galaxies from 11 billion years ago and found explosive formation of stars in the cores of galaxies. This suggests that galaxies can change their own shape without interaction with other galaxies.

Submillimeter waves detected with ALMA are shown in the left, indicating the location of dense dust and gas where stars 
are being formed. Optical and infrared light seen with the Hubble Space Telescope are shown in the middle and right, 
respectively. A large galactic disk is seen in infrared, while three young star clusters are seen in optical light 
[Credit: ALMA (ESO/NAOJ/NRAO), NASA/ESA Hubble Space Telescope, Tadaki et al.]

"Massive elliptical galaxies are believed to be formed from collisions of disk galaxies," said Ken-ichi Tadaki, the lead author of two research papers and a postdoctoral researcher at the National Astronomical Observatory of Japan (NAOJ). "But, it is uncertain whether all the elliptical galaxies have experienced galaxy collision. There may be an alternative path."

Aiming to understand galactic metamorphosis, the international team explored distant galaxies 11 billion light-years away. Because it takes time for the light from distant objects to reach us, by observing galaxies 11 billion light-years away, the team can see what the Universe looked like 11 billion years ago, 3 billion years after the Big Bang. This corresponds the peak epoch of galaxy formation; the foundations of most galaxies were formed in this epoch.

Stars are actively formed in the massive reservoir of dust and gas at the center of the galaxy 
[Credit: NAOJ]

Receiving faint light which has travelled 11 billion years is tough work. The team harnessed the power of three telescopes to anatomize the ancient galaxies. First, they used NAOJ's 8.2-m Subaru Telescope in Hawai`i and picked out 25 galaxies in this epoch. Then they targeted the galaxies for observations with NASA/ESA's Hubble Space Telescope (HST) and the Atacama Large Millimeter/submillimeter Array (ALMA). The astronomers used HST to capture the light from stars which tells us the "current" (as of when the light was emitted, 11 billion years ago) shape of the galaxies, while ALMA observed submillimeter waves from cold clouds of gas and dust, where new stars are being formed. By combining the two, we know the shapes of the galaxies 11 billion years ago and how they are evolving.

Thanks to their high resolution, HST and ALMA could illustrate the metamorphosis of the galaxies. With HST images the team found that a disk component dominates the galaxies. Meanwhile, the ALMA images show that there is a massive reservoir of gas and dust, the material of stars, so that stars are forming very actively. The star formation activity is so high that huge numbers of stars will be formed at the centers of the galaxies. This leads the astronomers to think that ultimately the galaxies will be dominated by the stellar bulge and become elliptical or lenticular galaxies.

First the galaxy is dominated by the disk component (left) but active star formation occurs in the huge dust and gas 
cloud at the center of the galaxy (center). Then the galaxy is dominated by the stellar bulge and becomes 
an elliptical or lenticular galaxy [Credit: NAOJ]

"Here, we obtained firm evidence that dense galactic cores can be formed without galaxy collisions. They can also be formed by intense star formation in the heart of the galaxy." said Tadaki. The team used the European Southern Observatory's Very Large Telescope to observe the target galaxies and confirmed that there are no indications of massive galaxy collisions.

Almost 100 years ago, American astronomer Edwin Hubble invented the morphological classification scheme for galaxies. Since then, many astronomers have devoted considerable effort to understanding the origin of the variety in galaxy shapes. Utilizing the most advanced telescopes, modern astronomers have come one step closer to solving the mysteries of galaxies.

The findings are published in The Astrophysical Journal and Astrophysical Journal Letters.

Source: National Institutes of Natural Sciences [September 10, 2017]

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