Andromeda’s Unstable Black Hole

The Andromeda galaxy as seen in optical light, and Chandra's X-ray vision of the changing supermassive black hole in Andromeda's heart. Image Credit: X-Ray NASA/CXC/SAO/Li et al.), Optical (DSS)


The Andromeda galaxy, the closest spiral galaxy to our own Milky Way, has a supermassive blackhole at the center of it much like other galaxies. Because of its proximity to us, Andromeda – or M31 – is an excellent place to study just how the supermassive black holes in the centers of galaxies consume material to grow, and interact gravitationally with the surrounding material.

Over the course of the last ten years, NASA’s Chandra X-Ray observatory has monitored closely the supermassive black hole at Andromeda’s heart. This long-term data set gives astronomers a very nuanced picture of just how these monstrous black holes change over time. Zhiyuan Li of the Harvard-Smithsonian Center for Astrophysics (CfA) presented results of this decade-long observation of the black hole at the 216th American Astronomical Society meeting in Miami, Florida this week.

From 1999 to 2006, M31 was relatively quiet and dim. In January of 2006, though, the black hole in the center of Andromeda suddenly brightened by over 100 times, and has remained 10 times as bright since. This suggests that the black hole swallowed something massive, but the details of the outburst in 2006 remain unclear.

The black hole in M31, located in the Andromeda constellation, likely continues to feed off of the stellar winds of a nearby star or the material in a large gas cloud that is falling into the black hole. As material is consumed, it drives the productions of X-rays in a relativistic jet streaming out from the black hole, which are then picked up by Chandra’s X-ray eyes.

The black hole in M31 is 10 to 100,000 times dimmer than expected, given that it has a large reservoir of gas surrounding it.

“The black holes in both Andromeda and the Milky Way are incredibly feeble. These two ‘anti-quasars’ provide special laboratories for us to study some of the dimmest type of accretion even seen onto a supermassive black hole,” Li said.

Accretion of matter into supermassive black holes is important to study because the evolution of galaxies is influenced by this process, Li said. The gravitational interplay of the black hole with the surrounding material in a galaxy, as well as the energy released when such supermassive black holes consume material in their surrounding accretion disks, change the structure of the galaxy as it forms. A better understanding of just how these supermassive black holes act in the later stages of spiral galaxy life may give clues as to what astronomers can expect to see in other galaxies.

M31 is readily seen with the naked eye in the constellation Andromeda, and is breathtaking to see through a telescope or binoculars. You won’t be able to see the black hole at its center, however! For more information on observing Andromeda, see our Guide to Space article on M31.

Source: Eurekalert

Black Hole in M87 Wanders using Jetpack

Hubble Space Telescope Images of M87. At right, a large scale image taken with the Wide-Field/Planetary Camera-2 from 1998. The zoom-in images on the left are of the central portion of M87. HST-1 is a knot in the jet from the SMBH. (NASA and the Hubble Heritage Team (STScI/AURA), J. A. Biretta, W. B. Sparks, F. D. Macchetto, E. S. Perlman)


The elliptical galaxy M87 is known for a jet of radiation that is streaming from the supermassive black hole (SMBH) that the galaxy houses. This jet, which is visible through large-aperture telescopes, may have functioned as a black hole ‘jetpack’, moving the SMBH from the center of mass of the galaxy – where most SMBHs are thought to reside.

Observations taken with the Hubble Space Telescope by a collaboration of astronomy researchers at Rochester Institute of Technology, Florida Institute of Technology and University of Sussex in the United Kingdom show the SMBH in M87 to be displaced from the center of the galaxy by as much as 7 parsecs (22.82 light years). This contradicts the long-held theory that supermassive black holes reside at the center of the galaxies they inhabit, and may give astronomers one way to trace the history of galaxies that have grown through merging.

What caused M87’s SMBH to wander off so far from the center of the galaxy? The most likely cause is a merger between two smaller supermassive black holes sometime in the past. This merger could have created gravitational waves that gave the engorged black hole a swift kick. Elliptical galaxies like M87 are thought to become the size they are through the merger of smaller galaxies.

Another theory is that the jet of radiation that sprays out of the SMBH has pushed with enough energy to essentially propel the black hole away from the center of M87. Okay, so it’s not really a ‘black hole jetpack’, but you have to admit that the combination of black holes – which are cool – and jetpacks, also cool, is too good to pass up. The motion of the SMBH happens to be in the opposite direction of the jet that we can see streaming from the object. For this scenario to be true, however, the jet would have to have been much more energetic millions of years ago, the researchers concluded.

There also does exist evidence for another jet of material that is streaming out of the other side of the SMBH, which would cancel the pushing motion of the jet that we can see, making the merger scenario much more likely. If the two jets were asymmetric to a high degree, however, this scenario still may be the case. More information on the structure and history of the jets would better clarify the cause of the black hole’s displacement.

This study of M87 is part of a wider project aimed at constraining the placement of supermassive black holes, also known as Active Galactic Nuclei or quasars, in their home galaxies. David Axon, dean of mathematical and physical sciences at Sussex, said in a press release, “In current galaxy formation scenarios galaxies are thought to be assembled by a process of merging. We should therefore expect that binary black holes and post coalescence recoiling black holes, like that in M87, are very common in the cosmos.”

The displacement of such black holes would be apparent in archived Hubble Space Telescope images, and the researchers that discovered this phenomenon in M87 used the HST archives to pinpoint the location of the SMBH. Further analysis of these archives could yield many, many more ‘wandering’ black holes.

These findings were presented on May 25th at the American Astronomical Society meeting in Miami, Florida. The team of researchers that collaborated on the finding include Daniel Batcheldor and Eric Perlman of the Florida Institute of Technology, Andrew Robinson and David Merritt of the Rochester Institute of Technology and David Axon of the University of Sussex. Their results were accepted for publication in Astrophysical Journal Letters, and the original paper, A Displaced Supermassive Black Hole in M87, is available on Arxiv right here.

Source: Eurekalert, Arxiv, Eric Perlman’s website

Andromeda’s Double Nucleus – Explained at Last?

M31's nucleus (Credit: WF/PC, Hubble Space Telescope)

In 1993, the Hubble Space Telescope snapped a close-up of the nucleus of the Andromeda galaxy, M31, and found that it is double.

In the 15+ years since, dozens of papers have been written about it, with titles like The stellar population of the decoupled nucleus in M 31, Accretion Processes in the Nucleus of M31, and The Origin of the Young Stars in the Nucleus of M31.

And now there’s a paper which seems, at last, to explain the observations; the cause is, apparently, a complex interplay of gravity, angular motion, and star formation.

It is now reasonably well-understood how supermassive black holes (SMBHs), found in the nuclei of all normal galaxies, can snack on stars, gas, and dust which comes within about a third of a light-year (magnetic fields do a great job of shedding the angular momentum of this ordinary, baryonic matter).

Also, disturbances from collisions with other galaxies and the gravitational interactions of matter within the galaxy can easily bring gas to distances of about 10 to 100 parsecs (30 to 300 light years) from a SMBH.

However, how does the SMBH snare baryonic matter that’s between a tenth of a parsec and ~10 parsecs away? Why doesn’t matter just form more-or-less stable orbits at these distances? After all, the local magnetic fields are too weak to make changes (except over very long timescales), and collisions and close encounters too rare (these certainly work over timescales of ~billions of years, as evidenced by the distributions of stars in globular clusters).

That’s where new simulations by Philip Hopkins and Eliot Quataert, both of the University of California, Berkeley, come into play. Their computer models show that at these intermediate distances, gas and stars form separate, lopsided disks that are off-center with respect to the black hole. The two disks are tilted with respect to one another, allowing the stars to exert a drag on the gas that slows its swirling motion and brings it closer to the black hole.

The new work is theoretical; however, Hopkins and Quataert note that several galaxies seem to have lopsided disks of elderly stars, lopsided with respect to the SMBH. And the best-studied of these is in M31.

Hopkins and Quataert now suggest that these old, off-center disks are the fossils of the stellar disks generated by their models. In their youth, such disks helped drive gas into black holes, they say.

The new study “is interesting in that it may explain such oddball [stellar disks] by a common mechanism which has larger implications, such as fueling supermassive black holes,” says Tod Lauer of the National Optical Astronomy Observatory in Tucson. “The fun part of their work,” he adds, is that it unifies “the very large-scale black hole energetics and fueling with the small scale.” Off-center stellar disks are difficult to observe because they lie relatively close to the brilliant fireworks generated by supermassive black holes. But searching for such disks could become a new strategy for hunting supermassive black holes in galaxies not known to house them, Hopkins says.

Sources: ScienceNews, “The Nuclear Stellar Disk in Andromeda: A Fossil from the Era of Black Hole Growth”, Hopkins, Quataert, to be published in MNRAS (arXiv preprint), AGN Fueling: Movies.

World-wide Campaign Sheds New Light on Nature’s “LHC”

Recent observations of blazar jets require researchers to look deeper into whether current theories about jet formation and motion require refinement. This simulation, courtesy of Jonathan McKinney (KIPAC), shows a black hole pulling in nearby matter (yellow) and spraying energy back out into the universe in a jet (blue and red) that is held together by magnetic field lines (green).

In a manner somewhat like the formation of an alliance to defeat Darth Vader’s Death Star, more than a decade ago astronomers formed the Whole Earth Blazar Telescope consortium to understand Nature’s Death Ray Gun (a.k.a. blazars). And contrary to its at-death’s-door sounding name, the GASP has proved crucial to unraveling the secrets of how Nature’s “LHC” works.

“As the universe’s biggest accelerators, blazar jets are important to understand,” said Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) Research Fellow Masaaki Hayashida, corresponding author on the recent paper presenting the new results with KIPAC Astrophysicist Greg Madejski. “But how they are produced and how they are structured is not well understood. We’re still looking to understand the basics.”

Blazars dominate the gamma-ray sky, discrete spots on the dark backdrop of the universe. As nearby matter falls into the supermassive black hole at the center of a blazar, “feeding” the black hole, it sprays some of this energy back out into the universe as a jet of particles.

Researchers had previously theorized that such jets are held together by strong magnetic field tendrils, while the jet’s light is created by particles spiraling around these wisp-thin magnetic field “lines”.

Yet, until now, the details have been relatively poorly understood. The recent study upsets the prevailing understanding of the jet’s structure, revealing new insight into these mysterious yet mighty beasts.

“This work is a significant step toward understanding the physics of these jets,” said KIPAC Director Roger Blandford. “It’s this type of observation that is going to make it possible for us to figure out their anatomy.”

Over a full year of observations, the researchers focused on one particular blazar jet, 3C279, located in the constellation Virgo, monitoring it in many different wavebands: gamma-ray, X-ray, optical, infrared and radio. Blazars flicker continuously, and researchers expected continual changes in all wavebands. Midway through the year, however, researchers observed a spectacular change in the jet’s optical and gamma-ray emission: a 20-day-long flare in gamma rays was accompanied by a dramatic change in the jet’s optical light.

Although most optical light is unpolarized – consisting of light with an equal mix of all polarizations – the extreme bending of energetic particles around a magnetic field line can polarize light. During the 20-day gamma-ray flare, optical light from the jet changed its polarization. This temporal connection between changes in the gamma-ray light and changes in the optical polarization suggests that light in both wavebands is created in the same part of the jet; during those 20 days, something in the local environment changed to cause both the optical and gamma-ray light to vary.

“We have a fairly good idea of where in the jet optical light is created; now that we know the gamma rays and optical light are created in the same place, we can for the first time determine where the gamma rays come from,” said Hayashida.

This knowledge has far-reaching implications about how a supermassive black hole produces polar jets. The great majority of energy released in a jet escapes in the form of gamma rays, and researchers previously thought that all of this energy must be released near the black hole, close to where the matter flowing into the black hole gives up its energy in the first place. Yet the new results suggest that – like optical light – the gamma rays are emitted relatively far from the black hole. This, Hayashida and Madejski said, in turn suggests that the magnetic field lines must somehow help the energy travel far from the black hole before it is released in the form of gamma rays.

“What we found was very different from what we were expecting,” said Madejski. “The data suggest that gamma rays are produced not one or two light days from the black hole [as was expected] but closer to one light year. That’s surprising.”

In addition to revealing where in the jet light is produced, the gradual change of the optical light’s polarization also reveals something unexpected about the overall shape of the jet: the jet appears to curve as it travels away from the black hole.

“At one point during a gamma-ray flare, the polarization rotated about 180 degrees as the intensity of the light changed,” said Hayashida. “This suggests that the whole jet curves.”

This new understanding of the inner workings and construction of a blazar jet requires a new working model of the jet’s structure, one in which the jet curves dramatically and the most energetic light originates far from the black hole. This, Madejski said, is where theorists come in. “Our study poses a very important challenge to theorists: how would you construct a jet that could potentially be carrying energy so far from the black hole? And how could we then detect that? Taking the magnetic field lines into account is not simple. Related calculations are difficult to do analytically, and must be solved with extremely complex numerical schemes.”

Theorist Jonathan McKinney, a Stanford University Einstein Fellow and expert on the formation of magnetized jets, agrees that the results pose as many questions as they answer. “There’s been a long-time controversy about these jets – about exactly where the gamma-ray emission is coming from. This work constrains the types of jet models that are possible,” said McKinney, who is unassociated with the recent study. “From a theoretician’s point of view, I’m excited because it means we need to rethink our models.”

As theorists consider how the new observations fit models of how jets work, Hayashida, Madejski and other members of the research team will continue to gather more data. “There’s a clear need to conduct such observations across all types of light to understand this better,” said Madejski. “It takes a massive amount of coordination to accomplish this type of study, which included more than 250 scientists and data from about 20 telescopes. But it’s worth it.”

With this and future multi-wavelength studies, theorists will have new insight with which to craft models of how the universe’s biggest accelerators work. Darth Vader has been denied all access to these research results.

Sources: DOE/SLAC National Accelerator Laboratory Press Release, a paper in the 18 February, 2010 issue of Nature.

What is Sagittarius A*?

Detection of an unusually bright X-Ray flare from Sagittarius A*, a supermassive black hole in the center of the Milky Way galaxy. Credit: NASA/CXC/Stanford/I. Zhuravleva et al.

At the very heart of the Milky Way is a region known as Sagittarius A*. This region is known the be the home of a supermassive black hole with millions of times the mass of our own Sun. And with the discovery of this object, astronomers have turned up evidence that there are supermassive black holes at the centers most most spiral and elliptical galaxies.

The best observations of Sagittarius A*, using Very Long Baseline Interferometry (VLBI) radio astronomy have determined that it’s approximately 44 million km across (that’s just the distance of Mercury to the Sun). Astronomers have estimated that it contains 4.31 million solar masses.

Of course, astronomers haven’t actually seen the supermassive black hole itself. Instead, they have observed the motion of stars in the vicinity of Sagittarius A*. After 10 years of observations, astronomers detected the motion of a star that came within 17 light-hours distance from the supermassive black hole; that’s only 3 times the distance from the Sun to Pluto. Only a compact object with the mass of millions of stars would be able to make a high mass object like a star move in that trajectory.

The discovery of a supermassive black hole at the heart of the Milky Way helped astronomers puzzle out a different mystery: quasars. These are objects that shine with the brightness of millions of stars. We now know that quasars come from the radiation generated by the disks of material surrounding actively feeding supermassive black holes. Our own black hole is quiet today, but it could have been active in the past, and might be active again in the future.

Some astronomers have suggested other objects that could have the same density and gravity to explain Sagittarius A, but anything would quickly collapse down into a supermassive black hole within the lifetime of the Milky Way.

We have written many articles about Sagittarius A. Here’s an article about how the Milky Way’s black hole is sending out flares, and even more conclusive evidence after 16 years of observations.

Here’s an article from NASA back in 1996 showing how astronomers already suspected it was a supermassive black hole, and the original ESO press release announcing the discovery.

We have recorded an episode of Astronomy Cast all about the Milky Way. Give it a listen: Episode: 99 – The Milky Way

Source: Wikipedia

Magnetic Fields Shape the Jets Pouring Out of Supermassive Black Holes (with video)

Artist's impression of a supermassive black hole. Credit: NRAO

The cores of galaxies contain supermassive black holes, containing hundreds of millions of times the mass of Sun. As matter falls in, it chokes up, forming a super hot accretion disk around the black hole. From this extreme environment, the black hole-powered region spews out powerful jets of particles moving at the speed of light. Astronomers have recently gotten one of the best views at the innermost portion of the jet.

A team of astronomers led by Alan Marscher, of Boston University, used the National Radio Astronomy Observatory’s Very Long Baseline Array (VLBA) to peer at the central region of a galaxy called BL Lacertae.

“We have gotten the clearest look yet at the innermost portion of the jet, where the particles actually are accelerated, and everything we see supports the idea that twisted, coiled magnetic fields are propelling the material outward,” said Alan Marscher, of Boston University, leader of an international research team. “This is a major advance in our understanding of a remarkable process that occurs throughout the Universe,” he added.

Here’s how the theory goes. As material falls into the supermassive black hole faster than it can consume it, an accretion disk forms. This is a flattened, rotating disk that circles the black hole. The spinning interaction with the black hole creates powerful magnetic fields that twist and form into a tightly-coiled bundle. It’s these magnetic fields that blast out particles into focused beams.

The theorists expected that the region inside the acceleration region would follow a corkscrew-shaped path inside the twisting magnetic fields. Furthermore, researchers expected that light and material would brighten when it was pointed directly towards Earth. And finally, the astronomers expected that there should be a flare when material hits a stationary shock wave called the “core” after it comes out of the acceleration region.

And that’s just what the observations show. The VLBA was used to study how a knot of material was ejected out of the black hole’s environment. As the knot moved through the stationary shock wave, it flared just as the theorists had predicted.

Original Source: NRAO News Release

Why are there Black Holes in the Middle of Galaxies?

Question: Why are Black Holes in the Middle of Galaxies?

Answer: The black holes you’re thinking of are known as supermassive black holes. Stellar mass black holes are created when a star at least 5 times larger than the Suns out of fuel and collapses in on itself forming a black hole. The supermassive black holes, on the other hand, can contain hundreds of millions of times the mass of a star like our Sun.

Astronomers are now fairly certain that these supermassive black holes are at the heart of almost every galaxy in the Universe. Furthermore, the mass of these black holes is somehow tied to the mass of the rest of the galaxy. They grown in tandem with each other.

When large quantities of material falls into the black hole, it chokes up, unable to get consumed all at once. This “accretion disk” begins to heat up and blaze brightly in many different wavelengths, including X-rays. When supermassive black holes are actively feeding, astronomers call these quasars.

So how do these black holes get there in the first place? Astronomers aren’t sure, but it could be that the dark matter halo that surrounds every galaxy serves to focus and concentrate material as the galaxy was first forming. Some of this material became the supermassive black hole, while the rest became the stars of the galaxy. It’s also possible that the black hole formed first, and collected the rest of the galaxy around it.

Astronomers just don’t know.