Seeing Double: Scientists Find Elusive Giant Black Hole Pairs

Astronomers have identified a bumper crop of dual supermassive black holes in the centers of galaxies. This discovery could help astronomers better understand how giant black holes grow and how they may produce the strongest gravitational wave signals in the Universe.

The new evidence reveals five pairs of supermassive black holes, each containing millions of times the mass of the Sun. These black hole couples formed when two galaxies collided and merged with each other, forcing their supermassive black holes close together.

The black hole pairs were uncovered by combining data from a suite of different observatories including NASA’s Chandra X-ray Observatory, the Wide-Field Infrared Sky Explorer Survey (WISE), and the ground-based Large Binocular Telescope in Arizona.

“Astronomers find single supermassive black holes all over the universe,” said Shobita Satyapal, from George Mason University in Fairfax, Virginia, who led one of two papers describing these results. “But even though we’ve predicted they grow rapidly when they are interacting, growing dual supermassive black holes have been difficult to find.”

Before this study fewer than ten confirmed pairs of growing black holes were known from X-ray studies, based mostly on chance detections. To carry out a systematic search, the team had to carefully sift through data from telescopes that detect different wavelengths of light.

Starting with the Galaxy Zoo project, researchers used optical data from the Sloan Digital Sky Survey (SDSS) to identify galaxies where it appeared that a merger between two smaller galaxies was underway. From this set, they selected objects where the separation between the centers of the two galaxies in the SDSS data is less than 30,000 light years, and the infrared colors from WISE data match those predicted for a rapidly growing supermassive black hole.

Seven merging systems containing at least one supermassive black hole were found with this technique. Because strong X-ray emission is a hallmark of growing supermassive black holes, Satyapal and her colleagues then observed these systems with Chandra. Closely-separated pairs of X-ray sources were found in five systems, providing compelling evidence that they contain two growing (or feeding) supermassive black holes.

Both the X-ray data from Chandra and the infrared observations suggest that the supermassive black holes are buried in large amounts of dust and gas.

“Our work shows that combining the infrared selection with X-ray follow-up is a very effective way to find these black hole pairs,” said Sara Ellison of the University of Victoria in Canada, who led the other paper describing these results. “X-rays and infrared radiation are able to penetrate the obscuring clouds of gas and dust surrounding these black hole pairs, and Chandra’s sharp vision is needed to separate them”.

The paper led by Ellison used additional optical data from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey to pinpoint one of the new black hole pairs. One member of this black hole pair is particularly powerful, having the highest X-ray luminosity in a black hole pair observed by Chandra to date.

This work has implications for the burgeoning field of gravitational wave astrophysics. While scientists using the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the VIRGO interferometer have detected the signals of merging black holes, these black holes have been of the smaller variety weighing between about eight and 36 times the mass of the Sun.

The merging black holes in the centers of galaxies are much larger. When these supermassive black holes draw even closer together, they should start producing gravitational waves. The eventual merger of the dual supermassive black holes in hundreds of millions of years would forge an even bigger black hole. This process would produce an astonishing amount of energy when some of the mass is converted into gravitational waves.
“It is important to understand how common supermassive black hole pairs are, to help in predicting the signals for gravitational wave observatories,” said Satyapal. “With experiments already in place and future ones coming online, this is an exciting time to be researching merging black holes. We are in the early stages of a new era in exploring the universe.”

LIGO/VIRGO is not able to detect gravitational waves from supermassive black hole pairs. Instead, pulsar timing arrays such as the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) are currently performing this search. In the future, the Laser Interferometer Space Antenna (LISA) project could also search for these gravitational waves.

Four of the dual black hole candidates were reported in a paper by Satyapal et al. that was recently accepted for publication in The Astrophysical Journal, and appears online. The other dual black hole candidate was reported in a paper by Ellison et al., which was published in the September 2017 issue of the Monthly Notices of the Royal Astronomical Society and appears online.

Astronomers Reveal Evidence Of Dynamical Dark Energy

An international research team, including astronomers from the University of Portsmouth, has revealed evidence of dynamical dark energy.

The discovery, recently published in the journal Nature Astronomy, found that the nature of dark energy may not be the cosmological constant introduced by Albert Einstein 100 years ago, which is crucial for the study of dark energy.

Lead author of the study Professor Gong-Bo Zhao, from the Institute of Cosmology and Gravitation (ICG) at the University of Portsmouth and the National Astronomical Observatories of China (NAOC), said: “We are excited to see that current observations are able to probe the dynamics of dark energy at this level, and we hope that future observations will confirm what we see today.”

Co-author Professor Bob Nichol, Director of the ICG, said: “Since its discovery at the end of last century, dark energy has been a riddle wrapped in an enigma. We are all desperate to gain some greater insight into its characteristics and origin. Such work helps us make progress in solving this 21st Century mystery.”

Revealing the nature of dark energy is one of key goals of modern sciences. The physical property of dark energy is represented by its Equation of State (EoS), which is the ratio of pressure and energy density of dark energy.

In the traditional Lambda-Cold Dark Matter (LCDM) model, dark energy is essentially the cosmological constant, i.e., the vacuum energy, with a constant EoS of -1. In this model, dark energy has no dynamical features.

In 2016, a team within the SDSS-III (BOSS) collaboration led by Professor Zhao performed a successful measurement of the Baryonic Acoustic Oscillations (BAO) at multiple cosmic epochs with a high precision.

Based on this measurement and a method developed by Professor Zhao for dark energy studies, the team found an evidence of dynamical dark energy at a significance level of 3.5 sigma. This suggests that the nature of dark energy may not be the vacuum energy, but some kind of dynamical field, especially for the quintom model of dark energy whose EoS varies with time and crosses the -1 boundary during evolution, according to NAOC.

The dynamics of dark energy needs to be confirmed by next-generation astronomical surveys. The team points to the upcoming Dark Energy Spectroscopic Instrument (DESI) survey, which aims to begin creating a 3D cosmic map in 2018.

In the next five to ten years, the world largest galaxy surveys will provide observables which may be key to unveil the mystery of dark energy.

The new study was supported by the National Natural Science Foundation of China (NSFC), Chinese Academy of Sciences and a Royal Society Newton Advanced Fellowship.

Professor Nichol added: “This work is the culmination of many years of work in collaboration between scientists in China and the UK. Gong-Bo is one of our brightest stars holding a joint position between NAOC and here at the ICG.”

The Material That Obscures Supermassive Black Holes

Cristina Ramos Almeida, researcher at the IAC, and Claudio Ricci, from the Institute of Astronomy of the Universidad Católica de Chile, have published a review in Nature Astronomy on the material that obscures active galactic nuclei obtained from infrared and X-ray observations.

Black holes appear to play a fundamental role in how galaxies evolve during a phase in which they are active and consuming material from the galaxy itself. During this phase, the galaxy hosts an active galactic nucleus (AGN), and the effect that this nuclear activity produces in the galaxy is known as AGN feedback. For instance, the AGN can heat, disrupt, consume and remove the gas available to form new stars, preventing further galaxy growth. AGN feedback is now required by simulations of galaxy formation to explain the observations of massive galaxies at cosmological distances. “If AGN feedback is not accounted for in the simulations,” explains Cristina Ramos, “the predicted number of massive galaxies when the universe was younger is much higher than those that are observed.”

Directly studying the influence of nuclear activity on galaxy evolution is challenging because of the different spatial scales and timescales involved in the two processes. Massive galaxies host extremely compact supermassive black holes of millions or even billions of solar masses in their nuclei. It is estimated that the phases of nuclear activity last for a short period of time, between 1 and 100 million years, whereas galaxy evolution processes, such as bulge growth or bar formation, last much longer. “In order to study the connection between the AGN and the host galaxy, we need to look at the nucleus of galaxies, where the material that links them is found. This material consists mainly of gas and dust, which are normally studied in the infrared and X-ray band,” explains Claudio Ricci.

The astrophysicists offer a comprehensive view of the current understanding derived from infrared and X-ray studies. These have greatly improved in the last decade thanks to observing facilities such as CanariCam on the Gran Telescopio CANARIAS (GTC), located at the Roque de los Muchachos Observatory (Garafía, La Palma) and the Very Large Array Interferometer (VLTI) in the infrared range, as well as X-ray satellites like NuSTAR, Swift/BAT and Suzaku.

Cristina Ramos says, “We now know that this nuclear material is more complex and dynamic than we thought a few years ago: It is very compact, formed by gas and dusty clouds orbiting the black hole, and its properties depend on the AGN luminosity and accretion rate. Moreover, it is not an isolated structure, but appears connected with the galaxy via outflows and inflows of gas, like streams of material flowing as part of a cycle. This gas flow cycle keeps feeding the black hole and regulates the formation of new stars in the galaxy.”

Recently, the Atacama Large Millimeter/submillimeter Array (ALMA) has imaged the nuclear obscuring material in an active galaxy for the first time. ALMA operates in the millimiter and sub-millimeter range, and the latter traces the coolest dust and gas surrounding AGN. In the case of the galaxy NGC 1068, ALMA has shown that this material is distributed in a very compact disc-like shape of seven to10 parsecs (pc) in diameter, and in addition to the regular rotation of the disk, there are non-circular motions that correspond to high-velocity gas outflowing from the galaxy nucleus. “Over the next decade, the new generation of infrared and X-ray facilities will contribute greatly to our understanding of the structure and physical properties of the nuclear material,” concludes Claudio Ricci.

Is The Milky Way An ‘Outlier’ Galaxy? Studying Its ‘Siblings’ For Clues

The most-studied galaxy in the universe — the Milky Way — might not be as “typical” as previously thought, according to a new study.

The Milky Way, which is home to Earth and its solar system, is host to several dozen smaller galaxy satellites. These smaller galaxies orbit around the Milky Way and are useful in understanding the Milky Way itself.

Early results from the Satellites Around Galactic Analogs (SAGA) Survey indicate that the Milky Way’s satellites are much more tranquil than other systems of comparable luminosity and environment. Many satellites of those “sibling” galaxies are actively pumping out new stars, but the Milky Way’s satellites are mostly inert, the researchers found.

This is significant, according to the researchers, because many models for what we know about the universe rely on galaxies behaving in a fashion similar to the Milky Way.

“We use the Milky Way and its surroundings to study absolutely everything,” said Yale astrophysicist Marla Geha, lead author of the paper, which appears in the Astrophysical Journal. “Hundreds of studies come out every year about dark matter, cosmology, star formation, and galaxy formation, using the Milky Way as a guide. But it’s possible that the Milky Way is an outlier.”

The SAGA Survey began five years ago with a goal of studying the satellite galaxies around 100 Milky Way siblings. Thus far it has studied eight other Milky Way sibling systems, which the researchers say is too small of a sample to come to any definitive conclusions. SAGA expects to have studied 25 Milky Way siblings in the next two years.

Yet the survey already has people talking. At a recent conference where Geha presented some of SAGA’s initial findings, another researcher told her, “You’ve just thrown a monkey wrench into what we know about how small galaxies form.”

“Our work puts the Milky Way into a broader context,” said SAGA researcher Risa Wechsler, an astrophysicist at the Kavli Institute at Stanford University. “The SAGA Survey will provide a critical new understanding of galaxy formation and of the nature of dark matter.”

Wechsler, Geha, and their team said they will continue to improve the efficiency of finding satellites around Milky Way siblings. “I really want to know the answer to whether the Milky Way is unique, or totally normal,” Geha said. “By studying our siblings, we learn more about ourselves.”

Cosmic Velocity Web: Motions Of Thousands Of Galaxies Mapped

The cosmic web — the distribution of matter on the largest scales in the universe — has usually been defined through the distribution of galaxies. Now, a new study by a team of astronomers from France, Israel and Hawaii demonstrates a novel approach. Instead of using galaxy positions, they mapped the motions of thousands of galaxies. Because galaxies are pulled toward gravitational attractors and move away from empty regions, these motions allowed the team to locate the denser matter in clusters and filaments and the absence of matter in regions called voids.

Matter was distributed almost homogeneously in the very early universe, with only miniscule variations in density. Over the 14-billion-year history of the universe, gravity has been acting to pull matter together in some places and leave other places more and more empty. Today, the matter forms a network of knots and connecting filaments referred to as the cosmic web. Most of this matter is in a mysterious form, the so-called “dark matter.” Galaxies have formed at the highest concentrations of matter and act as lighthouses illuminating the underlying cosmic structure.

The newly defined cosmic velocity web defines the structure of the universe from velocity information alone. In those regions with abundant observations, the structure of the velocity web and the web inferred from the locations of the galaxy lighthouses are similar. This agreement provides strong confirmation of the fundamental idea that structure developed from the growth of initially tiny fluctuations through gravitational attraction.

The cosmic velocity web analysis was led by Daniel Pomarede, Atomic Energy Center, France, with the collaboration of Helene Courtois at the University of Lyon, France; Yehuda Hoffman at the Hebrew University, Israel; and Brent Tully at the University of Hawaii’s Institute for Astronomy.

“With the motions of the galaxies, we can infer where all of the mass is located: the galaxies and the 5 times more abundant transparent matter (usually wrongly called dark matter). This total gravitating mass, together with the expansion of the universe, is responsible for the motions that create the architecture of the universe. The gravity from galaxies alone cannot create this network we see,” said Dr. Courtois.

Dr. Tully adds, “Moreover, a wide swath of the universe is hidden behind the obscuring disk of our own Milky Way galaxy. Our reconstruction of structure with the velocity web is revealing for the first time filaments of matter that stretch all the way around the sky and are easily followed through these regions of obscuration.”

This definition of the cosmic velocity web was made possible by the large and coherent collection of galaxy distances and velocities in the Cosmicflows series. The current analysis is based on a study of 8,000 galaxies in the second release of Cosmicflows. The third release, with over twice as many galaxy distances and velocities is already available, and will reveal the cosmic velocity web in increasingly rich detail.

The key element of the program is the acquisition of good distances to galaxies. Several methods are used, such as exploiting the known luminosities of old stars that are just beginning to burn Helium in their cores, and the relationship between the rotation speed of galaxies and the number of stars they possess. The observations have involved dozens of telescopes around the world and in space and at wavelengths from visible light through the infrared to radio.

“The velocity web method for mapping the cosmos is analogous to using plate tectonics in geology. It helps understand not just the current layout of the universe, but also the movement of the invisible underlying masses responsible for that topology,” said Dr. Courtois.

The team has produced an extensive video demonstrating the cosmic velocity web. It first explains the concepts underlying the cosmic velocity web reconstruction, followed by a description of its major elements. The video then shows how cosmic flows are organized within its structure, and how the basin of attraction of the recently mapped Laniakea Supercluster resides within its elements. In the final sequence, the viewer enters an immersive exploration of the filamentary structure of the local universe, navigating inside the filaments and visiting the major nodes such as the Great Attractor.

Saturn’s Rings Viewed In The Mid-Infrared Show Bright Cassini Division

A team of researchers has succeeded in measuring the brightnesses and temperatures of Saturn’s rings using the mid-infrared images taken by the Subaru Telescope in 2008. The images are the highest resolution ground-based views ever made. They reveal that, at that time, the Cassini Division and the C ring were brighter than the other rings in the mid-infrared light and that the brightness contrast appeared to be the inverse of that seen in the visible light. The data give important insights into the nature of Saturn’s rings.

The beautiful appearance of Saturn and its rings has always fascinated people. The rings consist of countless numbers of ice particles orbiting above Saturn’s equator. However, their detailed origin and nature remain unknown. Spacecraft- and ground-based telescopes have tackled that mystery with many observations at various wavelengths and methods. The international Cassini mission led by NASA has been observing Saturn and its rings for more than 10 years, and has released a huge number of beautiful images.

Subaru Views Saturn

The Subaru Telescope also has observed Saturn several times over the years. Dr. Hideaki Fujiwara, Subaru Public Information Officer/Scientist, analyzed data taken in January 2008 using the Cooled Mid-Infrared Camera and Spectrometer (COMICS) on the telescope to produce a beautiful image of Saturn for public information purposes. During the analysis, he noticed that the appearance of Saturn’s rings in the mid-infrared part of the spectrum was totally different from what is seen in the visible light

Saturn’s main rings consist of the C, B, and A rings, each with different populations of particles. The Cassini Division separates the B and A rings. The 2008 image shows that the Cassini Division and the C ring are brighter in the mid-infrared wavelengths than the B and A rings appear to be. This brightness contrast is the inverse of how they appear in the visible light, where the B and A rings are always brighter than the Cassini Division and the C ring.

“Thermal emission” from ring particles is observed in the mid-infrared, where warmer particles are brighter. The team measured the temperatures of the rings from the images, which revealed that the Cassini Division and the C ring are warmer than the B and A rings. The team concluded that this was because the particles in the Cassini Division and C ring are more easily heated by solar light due to their sparser populations and darker surfaces.

On the other hand, in the visible light, observers see sunlight being reflected by the ring particles. Therefore, the B and A rings, with their dense populations of particles, always seem bright in the visible wavelengths, while the Cassini Division and the C ring appear faint. The difference in the emission process explains the inverse brightnesses of Saturn’s rings between the mid-infrared and the visible-light views.

Changing Angles Change the Brightnesses

It turns out that the Cassini Division and the C ring are not always brighter than the B and A rings, even in the mid-infrared. The team investigated images of Saturn’s rings taken in April 2005 with COMICS, and found that the Cassini Division and the C ring were fainter than the B and A rings at that time, which is the same contrast to what was seen in the visible light.

The team concluded that the “inversion” of the brightness of Saturn’s rings between 2005 and 2008 was caused by the seasonal change in the ring opening angle to the Sun and Earth. Since the rotation axis of Saturn inclines compared to its orbital plane around the Sun, the ring opening angle to the Sun changes over a 15-year cycle. This makes a seasonal variation in the solar heating of the ring particles. The change in the opening angle viewed from the Earth affects the apparent filling factor of the particles in the rings. These two variations — the temperature and the observed filling factor of the particles — led to the change in the mid-infrared appearance of Saturn’s rings.

The data taken with the Subaru Telescope revealed that the Cassini Division and the C ring are sometimes bright in the mid-infrared though they are always faint in visible light. “I am so happy that the public information activities of the Subaru Telescope, of which I am in charge, led to this scientific finding,” said Dr. Fujiwara. “We are going to observe Saturn again in May 2017 and hope to investigate the nature of Saturn’s rings further by taking advantages of observations with space missions and ground-based telescopes.”

Both Push and Pull Drive Our Galaxy’s Race Through Space

Although we can’t feel it, we’re in constant motion: the earth spins on its axis at about 1,600 km/h; it orbits around the Sun at about 100,000 km/h; the Sun orbits our Milky Way galaxy at about 850,000 km/h; and the Milky Way galaxy and its companion galaxy Andromeda are moving with respect to the expanding universe at roughly 2 million km/h (630 km per second). But what is propelling the Milky Way’s race through space?

Until now, scientists assumed that a dense region of the universe is pulling us toward it, in the same way that gravity made Newton’s apple fall to earth. The initial “prime suspect” was called the Great Attractor, a region of a half dozen rich clusters of galaxies 150 million lightyears from the Milky Way. Soon after, attention was drawn to an area of more than two dozen rich clusters, called the Shapley Concentration, which sits 600 million lightyears beyond the Great Attractor.

Now researchers led by Prof. Yehuda Hoffman at the Hebrew University of Jerusalem report that our galaxy is not only being pulled, but also pushed. In a new study in the forthcoming issue of Nature Astronomy, they describe a previously unknown, very large region in our extragalactic neighborhood. Largely devoid of galaxies, this void exerts a repelling force on our Local Group of galaxies.

“By 3-d mapping the flow of galaxies through space, we found that our Milky Way galaxy is speeding away from a large, previously unidentified region of low density. Because it repels rather than attracts, we call this region the Dipole Repeller,” said Prof. Yehuda Hoffman. “In addition to being pulled towards the known Shapley Concentration, we are also being pushed away from the newly discovered Dipole Repeller. Thus it has become apparent that push and pull are of comparable importance at our location.”

The presence of such a low density region has been suggested previously, but confirming the absence of galaxies by observation has proved challenging. But in this new study, Hoffman, at the Hebrew university’s Racah Institutes of Physics, working with colleagues in the USA and France, tried a different approach.

Using powerful telescopes, among them the Hubble Space Telescope, they constructed a 3-dimensional map of the galaxy flow field. Flows are direct responses to the distribution of matter, away from regions that are relatively empty and toward regions of mass concentration; the large scale structure of the universe is encoded in the ?ow ?eld of galaxies.

They studied the peculiar velocities – those in excess of the Universe’s rate of expansion – of galaxies around the Milky Way, combining different datasets of peculiar velocities with a rigorous statistical analysis of their properties. They thereby inferred the underlying mass distribution that consists of dark matter and luminous galaxies—over-dense regions that attract and under-dense ones that repel.

By identifying the Dipole Repeller, the researchers were able to reconcile both the direction of the Milky Way’s motion and its magnitude. They expect that future ultra-sensitive surveys at optical, near-infrared and radio wavelengths will directly identify the few galaxies expected to lie in this void, and directly confirm the void associated with the Dipole Repeller.