What are Active Galactic Nuclei?

An artist's impression of the accretion disc around the supermassive black hole that powers an active galaxy. Astronomers want to know if the energy radiated from a black hole is caused by jets of material shooting away from the hole, or by the accretion disk of swirling material near the hole. Credit: NASA/Dana Berry, SkyWorks Digital

In the 1970s, astronomers became aware of a compact radio source at the center of the Milky Way Galaxy – which they named Sagittarius A. After many decades of observation and mounting evidence, it was theorized that the source of these radio emissions was in fact a supermassive black hole (SMBH). Since that time, astronomers have come to theorize that SMBHs at the heart of every large galaxy in the Universe.

Most of the time, these black holes are quiet and invisible, thus being impossible to observe directly. But during the times when material is falling into their massive maws, they blaze with radiation, putting out more light than the rest of the galaxy combined. These bright centers are what is known as Active Galactic Nuclei, and are the strongest proof for the existence of SMBHs.


It should be noted that the enormous bursts in luminosity observed from Active Galactic Nuclei (AGNs) are not coming from the supermassive black holes themselves. For some time, scientists have understood that nothing, not even light, can escape the Event Horizon of a black hole.

Instead, the massive burst of radiations – which includes emissions in the radio, microwave, infrared, optical, ultra-violet (UV), X-ray and gamma ray wavebands – are coming from cold matter (gas and dust) that surround the black holes. These form accretion disks that orbit the supermassive black holes, and gradually feeding them matter.

The incredible force of gravity in this region compresses the disk’s material until it reaches millions of degrees kelvin. This generates bright radiation, producing electromagnetic energy that peaks in the optical-UV waveband. A corona of hot material forms above the accretion disc as well, and can scatter photons up to X-ray energies.

A large fraction of the AGN’s radiation may be obscured by interstellar gas and dust close to the accretion disc, but this will likely be re-radiated at the infrared waveband. As such, most (if not all) of the electromagnetic spectrum is produced through the interaction of cold matter with SMBHs.

The interaction between the supermassive black hole’s rotating magnetic field and the accretion disk also creates powerful magnetic jets that fire material above and below the black hole at relativistic speeds (i.e. a significant fraction of the speed of light). These jets can extend for hundreds of thousands of light-years, and are a second potential source of observed radiation.

Types of AGN:

Typically, scientists divide AGN into two categories, which are referred to as “radio-quiet” and “radio-loud” nuclei. The radio-loud category corresponds to AGNs that have radio emissions produced by both the accretion disk and the jets. Radio-quiet AGNs are simpler, in that any jet or jet-related emission are negligible.

Carl Seyfert discovered the first class of AGN in 1943,  which is why they now bear his name. “Seyfert galaxies” are a type of radio-quiet AGN that are known for their emission lines, and are subdivided into two categories based on them. Type 1 Seyfert galaxies have both narrow and broadened optical emissions lines, which imply the existence of clouds of high density gas, as well as gas velocities of between 1000 – 5000 km/s near the nucleus.

Type 2 Seyferts, in contrast, have narrow emissions lines only. These narrow lines are caused by low density gas clouds that are at greater distances from the nucleus, and gas velocities of about 500 to 1000 km/s. As well as Seyferts, other sub classes of radio-quiet galaxies include radio-quiet quasars and LINERs.

Low Ionisation Nuclear Emission-line Region galaxies (LINERs) are very similar to Seyfert 2 galaxies, except for their low ionization lines (as the name suggests), which are quite strong. They are the lowest-luminosity AGN in existence, and it is often wondered if they are in fact powered by accretion on to a supermassive black hole.

Artist's representation of an active galactic nucleus (AGN) at the center of a galaxy. Credit: NASA/CXC/M.Weiss
Artist’s representation of an active galactic nucleus (AGN) at the center of a galaxy. Credit: NASA/CXC/M.Weiss

Radio-loud galaxies can also be subdivded into categories like radio galaxies, quasars, and blazars. As the name suggests, radio galaxies are elliptical galaxies that are strong emitters of radiowaves. Quasars are the most luminous type of AGN, which have spectra similar to Seyferts.

However, they are different in that their stellar absorption features are weak or absent (meaning they are likely less dense in terms of gas) and the narrow emission lines are weaker than the broad lines seen in Seyferts.  Blazars are a highly variable class of AGN that are radio sources, but do not display emission lines in their spectra.


Historically speaking, a number of features have been observed within the centers of galaxies that have allowed for them to be identified as AGNs. For instance, whenever the accretion disk can be seen directly, nuclear-optical emissions can be seen. Whenever the accretion disk is obscured by gas and dust close to the nucleus, an AGN can be detected by its infra-red emissions.

Then there are the broad and narrow optical emission lines that are associated with different types of AGN. In the former case, they are produced whenever cold material is close to the black hole, and are the result of the emitting material revolving around the black hole with high speeds (causing a range of Doppler shifts of the emitted photons). In the former case, more distant cold material is the culprit, resulting in narrower emission lines.

Image taken by the Hubble Space Telescope of a 5000-light-year-long jet ejected from the active galaxy M87. The blue synchrotron radiation contrasts with the yellow starlight from the host galaxy. Credit: NASA/The Hubble Heritage Team (STScI/AURA)
Image taken by the Hubble Space Telescope of a 5000-light-year-long jet ejected from the active galaxy M87. Credit: NASA/The Hubble Heritage Team (STScI/AURA)

Next up, there are radio continuum and x-ray continuum emissions. Whereas radio emissions are always the result of the jet, x-ray emissions can arise from either the jet or the hot corona, where electromagnetic radiation is scattered. Last, there are x-ray line emissions, which occur when x-ray emissions illuminate the cold heavy material that lies between it and the nucleus.

These signs, alone or in combination, have led astronomers to make numerous detections at the center of galaxies, as well as to discern the different types of active nuclei out there.

The Milky Way Galaxy:

In the case of the Milky Way, ongoing observation has revealed that the amount of material accreted onto Sagitarrius A is consistent with an inactive galactic nucleus. It has been theorized that it had an active nucleus in the past, but has since transitioned into a radio-quiet phase. However, it has also been theorized that it might become active again in a few million (or billion) years.

When the Andromeda Galaxy merges with our own in a few billion years, the supermassive black hole that is at its center will merge with our own, producing a much more massive and powerful one. At this point, the nucleus of the resulting galaxy – the Milkdromeda (Andrilky) Galaxy, perhaps? – will certainly have enough material for it to be active.

The discovery of active galactic nuclei has allowed astronomers to group together several different classes of galaxies. It’s also allowed astronomers to understand how a galaxy’s size can be discerned by the behavior at its core. And last, it has also helped astronomers to understand which galaxies have undergone mergers in the past, and what could be coming for our own someday.

We have written many articles about galaxies for Universe Today. Here’s What Fuels the Engine of a Supermassive Black Hole?, Could the Milky Way Become a Black Hole?, What is a Supermassive Black Hole?, Turning on a Supermassive Black Hole, What Happens when Supermassive Black Holes Collide?.

For more information, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.

Astronomy Cast also has episodes about galactic nuclei and supermassive black holes. Here’s Episode 97: Galaxies and Episode 213: Supermassive Black Holes.


Black Holes, Fermi Bubbles and the Milky Way

Deep at the heart of our galaxy lurks a black hole. This isn’t exciting news, but neither is it a very exciting place. Or is it? While all might be quiet on the western front now, there may be evidence that our galactic center was once home to some pretty impressive activity – activity which may have included multiple collision events and mergers of black holes as it gorged on a satellite galaxies. Thanks to new insights from a pair of assistant professors, Kelly Holley-Bockelmann at Vanderbilt and Tamara Bogdanovic at Georgia Institute of Technology, we have more evidence which points to the Milky Way’s incredibly active past.

“Tamara and I had just attended an astronomy conference in Aspen, Colorado, where several of these new observations were announced,” said Holley-Bockelmann. “It was January 2010 and a snow storm had closed the airport. We decided to rent a car to drive to Denver. As we drove through the storm, we pieced together the clues from the conference and realized that a single catastrophic event – the collision between two black holes about 10 million years ago – could explain all the new evidence.”

Now, imagine a night sky illuminated by a a huge nebula, one that covers half the celestial sphere. This isn’t a dream, it’s a reality. These massive lobes of high-energy radiation are known as Fermi bubbles and they cover a region some 30,000 light years on either side of the Milky Way’s core. While we can’t observe them directly in visible light, these particles are moving along at close to 186,000 miles per second and glowing in x-ray and gamma ray wavelengths.

According to Fulai Guo and William G. Mathews of the University of California at Santa Cruz: “The Fermi bubbles provide plausible evidence for a recent powerful AGN jet activity in our Galaxy, shedding new insights into the origin of the halo CR population and the channel through which massive black holes in disk galaxies release feedback energy during their growth.”

However, our galactic center is home to more than just some incredible bubbles – it’s the location of three of the most massive clusters of young stars within the Milky Way’s realm. Known as the Central, Arches and Quintuplet clusters, each grouping houses several hundred hot, young stars which dwarf the Sun. They will live short, bright, violent lives… burning out in a scant few million years. Because they live fast and die young, these cluster stars must have formed within recent years during a eruption of star formation near the galactic center – another clue to this cosmic puzzle.

“Because of their high mass, and apparent top-heavy IMF, the Galactic Center clusters contain some of the most massive stars in the Galaxy. This is important, as massive stars are key ingredients and probes of astrophysical phenomena on all size and distance scales, from individual star formation sites, such as Orion, to the early Universe during the age of reionization when the first stars were born. As ingredients, they control the dynamical and chemical evolution of their local environs and individual galaxies through their influence on the energetics and composition of the interstellar medium.” says Donald F. Figer. “They likely play an important role in the early evolution of the first galaxies, and there is evidence that they are the progenitors of the most energetic explosions in the Universe, seen as gamma ray bursts. As probes, they define the upper limits of the star formation process and their presence likely ends further formation of nearby lower mass stars. They are also prominent output products of galactic mergers, starburst galaxies, and active galactic nuclei.”

To deepen the mystery, take a closer look at our central black hole. It spans about 40 light seconds in diameter and weighs about four million solar masses. According to what we know, this should produce intensive gravitational tides – ones that should be sucking in the surroundings. So how is it that astronomers have uncovered groups of new, bright stars closer than 3 light years from the event horizon? Of course, they could be on their way to oblivion, but the data shows these stars seem to have formed there. That’s quite a feat considering it would require a molecular cloud 10,000 times more dense than the one located at our galactic center! Shouldn’t there also be old stars located there as well? The answer is yes, there should be… but there are far fewer than what we can observe and what current theoretical models predict.

Holley-Bockelmann wasn’t about to let the problem rest. When she returned home, she enlisted the aid of Vanderbilt graduate student Meagan Lang to help solve the riddle. Then they recruited Pau Amaro-Seoane from the Max Planck Institute for Gravitational Physics in Germany, Alberto Sesana from the Institut de Ciències de l’Espai in Spain, and Vanderbilt Research Assistant Professor Manodeep Sinha to help. With so many bright minds to help solve this riddle, they soon arrived at a plausible explanation – one which matches observations and allows for testable predictions.

According to their theory, a Milky Way satellite galaxy began migrating towards our core. As it merged with our galaxy, its mass was torn away, leaving only its black hole and a small collection of gravitationally bound stars. After several million years, this “leftover” eventually reached the galactic center and the black holes began to merge. As the smaller black hole was swirled around the larger, it plowed up huge furrows of gas and dust, pushing it into the larger black hole and created the Fermi bubbles. The dueling gravitational forces weren’t gentle… these intense tides were quite capable of compressing the molecular clouds surrounding the core into the density required to produce fresh, young stars. Perhaps the very young stars we now observe at the galactic center?

However, there’s more to the picture than meets the eye. This same plowing of the cosmic turf would have also pushed out existing older stars from the vicinity of the massive central black hole. It’s a scene which fits current models where a black hole merger flings stars out into the galaxy at hyper velocities… a scene which fits the observation of a lack of old stars at the boundaries of our supermassive black hole.

“The gravitational pull of the satellite galaxy’s black hole could have carved nearly 1,000 stars out of the galactic centre,” said Bogdanovic. “Those stars should still be racing through space, about 10,000 light years away from their original orbits.”

Can any of this be proved? The answer is yes. Thanks to large scale surveys like the Sloan Digital Sky Survey, we should be able to pinpoint stars moving at a higher velocity than stars which haven’t been subjected to a similar interaction. If astronomers like Holley-Bockelmann and Bogdanovic look at the hard evidence, they are likely to discover a credible number of high velocity stars which will validate their Milky Way merger model.

Or are they just blowing bubbles?

Messier 106: Amateur and Professional Astronomers Join Together to Peer Into the Eyes of Creation

Nearly four million light years away in the direction of the constellation of Canes Venatici, a visage of creation awaited to be revealed. Now, thanks to the teamwork of the astronomical image processors at the Space Telescope Science Institute in Baltimore, Maryland, and world-renowned astrophotographers Robert Gendler and Jay GaBany, we’re able to see combined Hubble Space Telescope data with ground-based telescope imaging. Let’s look deep into spiral galaxy, Messier 106.

This wasn’t an overnight imaging project. “A few months ago the Hubble Heritage Team contacted me and asked if I’d be interested in making a large format image of M106 from the available data on the Hubble Legacy Archive,” says Gendler. “I agreed and went to work downloading a large number of data sets from the HLA. I realized this would be a massive project. The image would be a mosaic of more than 30 panels and would incorporate both wideband and narrowband data sets.”

With the cooperation of Jay GaBany, they combined their own observations/images of this magnificent structure and compiled it with Hubble data – filling in areas where no data was available. The resulting image is a portrait of such depth and beauty that it’s almost like looking into the eyes of creation itself.

Be swept away…

If you’re drawn to the core of Messier 106, there’s good reason. It isn’t just an ordinary spiral galaxy, it’s one that has a peculiar jet flow which can be detected in radio and in H-alpha wavelengths. “Due to the special geometry of the galaxy, the jets emerge from the nuclear region through the galactic disk,” says Marita Krause (et al). “Also the distribution of molecular gas looks different from that in other spiral galaxies.” It is just this difference that makes NGC 4258 (M106) stand out a bit from the crowd and so worthy of further processing. According to new modeling techniques the “concentration of CO along the ridges is due to interaction of the rotating gas clouds with the jet’s magnetic field by ambipolar diffusion. This magnetic interaction is thought to increase the time the molecular clouds reside near the jet thus leading to the quasi-static CO ridge.”

Knowing those jets are present and the hunger to reveal them through imaging became the driving force for R. Jay GaBany. “Since the early 1960s, M106, also known as NGC 4258, has been known to exhibit an extra pair of arms, located between the spiral arms comprised of stars, dust and gas. But an explanation for their existence remained elusive until earlier in this decade,” says Jay. “My contribution to the image came from my 2010 image of M-106 that revealed the full extent of its amazing jets. My image include 22 hours of white light exposures through clear, red, blue and green filters plus and other 15 hours of imaging through a 6nm narrow band h-alpha filter.”

Messier 106 Courtesy of R. Jay GaBany
Messier 106 Courtesy of R. Jay GaBany

“Seen in the light emitted by hydrogen molecules when they become ionized, these arms display an artificial red hue to make them visible in the image I produced. The extra arms are now believed to be caused by high energy jets emanating from an active 40 million solar mass super-massive black hole menacing the galaxy’s center,” explains GaBany. “Because the jets are tilted at a low inclination they pierce the disk and surrounding halo of this galaxy. So, as the jets pass through regions of gas, they create an expanding cocoon of shock waves that heats the surrounding material causing it to release radiation in optical wavelengths. The curvature and fraying seen at their extremities represents previous trajectories of the jet due to past precession. Precession is a change in the orientation of the rotation axis of a spinning object. For example, the wobble of a spinning top.”

Yet, that’s not all. This low luminosity Seyfert II galaxy is also hosting a maser – its warped disk of water molecules discovered in 1994. Through radio observations, M106 became the first of its kind to show the exact location of the core of an AGN (active galactic nucleus). According to a study done by JR Herrnstein (et al): “NGC 4258 is an exceptional laboratory for the study of AGN accretion processes. The nuclear maser reveals details about the kinematics and structure of the accretion disk on subparsec scales and permits the determination of the central mass with great precision.”

And there is still more…

Deep inside lurks that known supermassive black hole – one that’s extremely active and produces bright microwave radiation. But, don’t stop there. Ordinarily a spiral galaxy has two arms, but M106 has double. These ethereal “extras” can be seen as faint ribbons of gas at optical wavelengths, but become solidified when viewed in x-ray and radio. Here the structure is formed in hot gas rather than stars. While this process was once a mystery to astronomers, new information suggests they may arise from the black hole activity, making them a unique artifact. What could cause it? These “extra arms” could be the result of the violent turbulence at the core – where gases are superheated and interact with their denser counterparts causing them to illuminate. At the perimeter of the galactic structure, the gases are more loose and the arching formation could be the product of the movement of jet activity.

“One goal I had early on was to feature the well known ‘anomalous arms’ of M106,” said Gendler. “This feature, peculiar to M106, is thought to arise from superheated gases, energized by accretion of matter into the galaxy’s massive black hole. The anomalous arms emit light in the visual spectrum around 656nm (hydrogen alpha) and I found a fair amount of hydrogen alpha data sets for the arms in the HLA.”

Gendler was responsible for all the image assembly and processing. “Assembling the image required over two months,” he said. “The quality of the data ranged from good to very poor. The central galaxy had sufficient color data but away from the center the Hubble data was incomplete and in some areas did not exist. I then decided to use ground based data from my own image and Jay GaBany’s image of M106 to fill in areas of missing or incomplete Hubble data. I also used ground based data to boost the signal of the outer areas of the galaxy as the Hubble data was sparse and of short exposure for the more remote areas of the galaxy.”

All in all, Messier 106 is a galaxy that deserves attention – attention and a loving touch given by two of the very best amateur astronomers and dedicated astrophotographers to be found.

Original News Source: HubbleSite Image Release.

Astronomers Discover Ancient ‘Ultra-Red’ Galaxies

[/caption]A team of astronomers, led by Jiasheng Huang (Harvard-Smithsonian Center for Astrophysics) using the Spitzer Space Telescope, have discovered four ‘Ultra-Red’ galaxies that formed when our Universe was about a billion years old. Huang and his team used several computer models in an attempt to understand why these galaxies appear so red, stating, “We’ve had to go to extremes to get the models to match our observations.”

The results of Huang’s research were recently published in The Astrophysical Journal

Using the Spitzer Space Telescope helped make the discovery possible, as it is more sensitive to infrared light than other space telescopes such as the Hubble. The newly discovered galaxies are sixty times brighter in the infrared than they are at the longest/reddest wavelengths HST can detect.

What processes are at work to create these extremely red objects, and why are they of interest to astronomers?

There are several reasons a galaxy could be reddened. For starters, extremely distant galaxies can have their light “redshifted” due to the expansion of the universe. If a galaxy contains large amounts of dust, it will also appear redder than a galaxy with less dust. Lastly, older galaxies will tend to be redder, due to a higher concentration of old, red stars and less younger bluer stars.

According to the paper, Huang and his team created three models to determine why these galaxies appear so red. Of their models, the one which suggests an old stellar population is currently the best fit to the observations. Supporting this conclusion, co-author Giovanni Fazio stated, “Hubble has shown us some of the first protogalaxies that formed, but nothing that looks like this. In a sense, these galaxies might be a ‘missing link’ in galactic evolution”.

Studying these extremely distant galaxies helps provide astronomers with a better understanding of the early universe, specifically how early galaxies formed and what conditions were present when some of the first stars were created. The next step in understanding these “ERO” galaxies is to obtain an accurate redshift for the galaxies, by using more powerful telescopes such as the Large Millimeter Telescope or Atacama Large Millimeter Array.

Huang and his team have plans to search for more galaxies similar to the four recently discovered by his team. Huang’s co-author Giovanni Fazio adds, “There’s evidence for others in other regions of the sky. We’ll analyze more Spitzer and Hubble observations to track them down.”

If you’d like to learn more, you can access the full paper (via arXiv.org) at: http://arxiv.org/pdf/1110.4129v1

Source: Harvard-Smithsonian Center for Astrophysics press release , arxiv.org

Galaxy Interactions Could Cause Overweight Black Holes


Yep. It’s true. Almost all galaxies are guilty of having a supermassive black hole in their centers. Some even tip the scales at millions – or even billions – of times more mass than the Sun. However, how they came to be so weighty is a true enigma. Thanks to research done by Dr. John Silverman (IPMU) and the international COSMOS team, the Chandra X-Ray Observatory and the European Southern Observatory’s Very Large Telescope have revealed that galaxy interactions may be responsible for the growth of supermassive black holes – and they’ve left behind some very important clues…

If you’re big – you’re big. As a general rule, supermassive black holes like to hang out in massive galaxies. Their mass is usually directly related to the central bulge. Now the consensus is that massive galaxies gained their girth (at least in part) by mergers and interactions with smaller galaxies. This act of cannibalism in galactic evolution has been postulated to explain how matter gathers toward the middle, eventually resulting in a supermassive black hole.

How do we determine this? One way is to take a closer look at galaxies currently in merger as compared to ones in isolation. While the concept is easy, carrying out the test hasn’t been. A supermassive black hole leaves visual observations “blinded by the light” while a quasar can effectively “outshine” an entire host galaxy, leaving an interactor almost impossible to detect. But, like a bulging waistline, such interactions should distort the overall contours of the galaxy.

Now the COSMOS team might have an answer to the riddle.. by assuming a galaxy is interacting if it has a nearby neighbor. It’s a test that can happen without needing to know if distortion is present in optical images. What makes it possible are accurate distance measurements of about 20,000 galaxies in the COSMOS field as provided by the zCOSMOS redshift survey with the European Southern Observatory’s Very Large Telescope. Isolated galaxies are used to give a comparison sample to lay the foundation as to whether an active galactic nucleus is common to interacting galaxies. With help from NASA’s Chandra Observatory, X-ray observations pinpoint galaxies which host an AGN. The X-ray emission signature dominates in growing SMBHs and X-rays are capable of cutting through the gas and dust of star-forming regions.

In their report to The Astrophysical Journal the team states that galaxies in close pairs are twice as likely to harbor AGNs as compared to galaxies in isolation. This answer may prove that beginning galaxy interactions can lead to “enhanced black hole growth”. Because it’s not a drastically common occcurrance, it means that only about 20% of SMBHs that break the scale happen via a merger event and that “final coalescence” might also play a role.

One thing we do know is that galaxies and their black holes, like people and their waistlines, all get a little heavier with time.

Original Story Source: Institute for Physics and Mathematics of the Univserse.