Category: galaxies

Paired black holes are theorized to be common, but have escaped detection — until now.

Astronomers Todd Boroson and Tod Lauer, from the National Optical Astronomy Observatory (NOAO) in Tucson, Arizona, have found what looks like two massive black holes orbiting each other in the center of one galaxy. Their discovery appears in this week’s issue of Nature.

Astronomers have long suspected that most large galaxies harbor black holes at their center, and that most galaxies have undergone some kind of merger in their lifetime. But while binary black hole systems should be common, they have proved hard to find.  Boroson and Lauer believe they’ve found a galaxy that contains two black holes, which orbit each other every 100 years or so. They appear to be separated by only 1/10 of a parsec, a tenth of the distance from Earth to the nearest star. 

After a galaxy forms, it is likely that a massive black hole can also form at its center. Since many galaxies are found in cluster of galaxies, individual galaxies can collide with each other as they orbit in the cluster. The mystery is what happens to these central black holes when galaxies collide and ultimately merge together. Theory predicts that they will orbit each other and eventually merge into an even larger black hole.

“Previous work has identified potential examples of black holes on their way to merging, but the case presented by Boroson and Lauer is special because the pairing is tighter and the evidence much stronger,” wrote Jon Miller, a University of Michigan astronomer, in an accompanying editorial.

The material falling into a black hole emits light in narrow wavelength regions, forming emission lines which can be seen when the light is dispersed into a spectrum. The emission lines carry the information about the speed and direction of the black hole and the material falling into it. If two black holes are present, they would orbit each other before merging and would have a characteristic dual signature in their emission lines. This signature has now been found.

The smaller black hole has a mass 20 million times that of the sun; the larger one is 50 times bigger, as determined by the their orbital velocities.

Boroson and Lauer used data from the Sloan Digital Sky Survey, a 2.5-meter (8-foot) diameter telescope at Apache Point in southern New Mexico to look for this characteristic dual black hole signature among 17,500 quasars. 

Quasars are the most luminous versions of the general class of objects known as active galaxies, which can be a hundred times brighter than our Milky Way galaxy, and powered by the accretion of material into supermassive black holes in their nuclei. Astronomers have found more than 100,000 quasars.

Boroson and Lauer had to eliminate the possibility that they were seeing two galaxies, each with its own black hole, superimposed on each other. To try to eliminate this superposition possibility, they determined that the quasars were at the same red-shift determined distance and that there was a signature of only one host galaxy.

“The double set of broad emission lines is pretty conclusive evidence of two black holes,” Boroson said. “If in fact this were a chance superposition, one of the objects must be quite peculiar.  One nice thing about this binary black hole system is that we predict that we will see observable velocity changes within a few years at most.  We can test our explanation that the binary black hole system is embedded in a galaxy that is itself the result of a merger of two smaller galaxies, each of which contained one of the two black holes.”  

LEAD IMAGE CAPTION (more): Artist’s conception of the binary supermassive black hole system. Each black hole is surrounded by a disk of material gradually spiraling into its grasp, releasing radiation from x-rays to radio waves.  The two black holes complete an orbit around their center of mass every 100 years, traveling with a relative velocity of 6000 kilometers (3,728 miles) per second.  (Credit P. Marenfeld, NOAO)

Source: NOAO

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There are some interesting dynamics going on with Centaurus A, an elliptical galaxy about 13 million light-years away. This is a very active and luminous region of space and a great disturbance is going on as another spiral galaxy is trying to get in on the action by merging with Centaurus A. But astronomers now have new insight on what causing all the ruckus: a supermassive black hole at the core of Centaurus A. Jets and lobes emanating from the central black hole have been imaged at submillimeter wavelengths for the first time by using the 12-meter Atacama Pathfinder Experiment (APEX) telescope in Chile. By using a combination of visible and X-ray wavelengths, astronomers were able to produce this striking new image. Help me APEX, you are our only hope!

Centaurus A (NGC 5128) is one of our closest galactic neighbors, and is located in the southern constellation of Centaurus. The supermassive black hole is the source of the force: strong radio and X-ray emissions. Visible in the image is a dust ring encircling the giant galaxy, and the fast-moving radio jets ejected from the galaxy center. In submillimeter light, the heat glow from the central dust disc can be seen and also the emission from the central radio source.

APEX was also able to discern – for the first time in the submillimeter – the inner radio lobes north and south of the disc. Measurements of this emission, which occurs when fast-moving electrons spiral around the lines of a magnetic field, reveal that the material in the jet is travelling at approximately half the speed of light. In the X-ray emission, we see the jets emerging from the centre of Centaurus A and, to the lower right of the galaxy, the glow where the expanding lobe collides with the surrounding gas, creating a shockwave.

Related paper.

Source: ESO

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Using data from the Very Large Telescope’s powerful near-infrared eyes, astronomers have created a movie that takes you across 13 million light-years to galaxy NGC 253, an active galaxy filled with young, massive and dusty stellar nurseries. “We now think that these are probably very active nurseries that contain many stars bursting from their dusty cocoons,” says Jose Antonio Acosta-Pulido, a member of the team from Instituto de Astrofísica de Canarias in Spain. NGC 253 is known as a starburst galaxy, after its very intense star formation activity. Each bright region could contain as many as one hundred thousand young, massive stars. And in the center of this galaxy appears a strikingly familiar sight: a virtual twin of our own Milky Way’s supermassive black hole.

Watch the movie. (For different viewing options, click here).

The astronomers used NACO, a sharp-eyed adaptive optics instrument on the VLT to study the fine detail in NGC 253, one of the brightest and dustiest spiral galaxies in the sky. Adaptive Optics (AO) corrects for the blurring effect introduced by the Earth’s atmosphere. This turbulence causes the stars to twinkle in a way that delights poets, but frustrates astronomers, since it smears out the images. With AO in action the telescope can produce images that are as sharp as is theoretically possible, as if the telescope were in space.

NACO revealed features in the galaxy that were only 11 light-years across. “Our observations provide us with so much spatially resolved detail that we can, for the first time, compare them with the finest radio maps for this galaxy — maps that have existed for more than a decade,” says Juan Antonio Fernández-Ontiveros, the lead author of the paper reporting the results.

In looking at all the data together, astronomers concluded that the center of NGC 253 hosts a scaled-up version of Sagittarius A*, the bright radio source that lies at the core of the Milky Way and which we know harbors a massive black hole. “We have thus discovered what could be a twin of our Galaxy’s Centre,” says co-author Almudena Prieto.

Source: ESO

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Do galaxies form first and then a black hole springs up in the center, or possibly, do galaxies form around an already existing black hole? That’s the cosmic chicken-and-the-egg problem astronomers have been trying to figure out. The answer? “It looks like the black holes form before the host galaxy, and somehow grow a galaxy around them. The evidence is piling up,” said Chris Carilli, of the National Radio Astronomy Observatory (NRAO), speaking at today’s press conference at the American Astronomical Society’s meeting. By observing with the Very Large Array radio telescope and the Plateau de Bure Interferometer in France at sub-kiloparsec resolution, the researchers have been “weighing” the earliest galaxies, ones that formed within a billion years of the Big Bang.

Previous studies of galaxies and their central black holes in the nearby Universe revealed an intriguing connection between the masses of the black holes and of the central “bulges” of stars and gas in the galaxies. The ratio of the black hole and the bulge mass is nearly the same for a wide range of galactic sizes and ages. For central black holes from a few million to many billions of times the mass of our Sun, the black hole’s mass is about one one-thousandth of the mass of the surrounding galactic bulge.

“This constant ratio indicates that the black hole and the bulge affect each others’ growth in some sort of interactive relationship,” said Dominik Riechers, of Caltech. “The big question has been whether one grows before the other or if they grow together, maintaining their mass ratio throughout the entire process.”

“We finally have been able to measure black-hole and bulge masses in several galaxies seen as they were in the first billion years after the Big Bang, and the evidence suggests that the constant ratio seen nearby may not hold in the early Universe. The black holes in these young galaxies are much more massive compared to the bulges than those seen in the nearby Universe,” said Fabian Walter of the Max-Planck Institute for Radioastronomy (MPIfR) in Germany.

“The implication is that the black holes started growing first.”

The next challenge is to figure out how the black hole and the bulge affect each others’ growth. “We don’t know what mechanism is at work here, and why, at some point in the process, the ‘standard’ ratio between the masses is established,” Riechers said.

New telescopes now under construction will be key tools for unraveling this mystery, Carilli explained. “The Expanded Very Large Array (EVLA) and the Atacama Large Millimeter/submillimeter Array (ALMA) will give us dramatic improvements in sensitivity and the resolving power to image the gas in these galaxies on the small scales required to make detailed studies of their dynamics,” he said.

“To understand how the Universe got to be the way it is today, we must understand how the first stars and galaxies were formed when the Universe was young. With the new observatories we’ll have in the next few years, we’ll have the opportunity to learn important details from the era when the Universe was only a toddler compared to today’s adult,” Carilli said.

Carilli, Riechers and Walter worked with Frank Bertoldi of Bonn University; Karl Menten of MPIfR; and Pierre Cox and Roberto Neri of the Insitute for Millimeter Radio Astronomy (IRAM) in France.

Source: NRAO, AAS Press Conference