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Glimpsing the hearts of galaxies

Inside all huge galaxies lies a supermassive black hole. Some are shining brightly, while others are barely visible — but all of them can teach us something about how galaxies evolve.
CentaurusA
Centaurus A is a nearby active galaxy. It is a Seyfert galaxy: Its center hosts an actively feeding supermassive black hole, but that region’s light is not bright enough to drown out the galaxy around it.
ESO/WFI (Optical); MPIfR/ESO/APEX/A. Weiss et al. (Submillimeter); NASA/CXC/CfA/R. Kraft et al. (X-ray)

In a galaxy like the Milky Way, light comes entirely from a combination of shining stars and glowing gas. However, in an active galaxy, the energy output is too high to attribute to these factors alone. The excess energy is concentrated in the galaxy’s center — its active galactic nucleus. 

Active galactic nuclei (AGN) are found throughout the cosmos in many forms. Some hide within seemingly normal galaxies, while the brightest pump out so much energy they outshine their host galaxy entirely. AGN are manifestations of the supermassive black holes found in nearly every galaxy we see, and they have played an important role in shaping the universe.

Quasi-stellar objects

Observations of galactic centers had turned up odd results since the early 1900s, but initially received little attention. By the late 1950s, astronomers surveying the sky with radio telescopes were attempting to match radio sources with visible objects such as stars and galaxies. They discovered that while many optical counterparts were normal-looking galaxies, some appeared as bright blue stars often embedded in fuzzy halos barely discernible in the wash of light from the star.

These oddballs, initially dubbed “radio stars” and later “quasi-stellar radio sources,” remained mysterious until 1963, when Dutch astronomer Maarten Schmidt observed the starlike counterpart of radio source 3C 273 from Palomar Observatory in California. He examined the source’s spectra, spreading out the light by wavelength to identify features associated with the emission and absorption of energy by different atoms.

AGN
The unified model of AGN states that all AGN contain the same components, simply viewed at different angles. From the inside out, AGN contain a supermassive black hole; an accretion disk and a hot corona of gas; a fast-moving gas region; an obscuring torus of dust; and a slower-moving gas region. Some AGN have powerful jets, which may be pointed toward Earth.
Astronomy: Roen Kelly
Poring over the results, Schmidt recognized a series of features associated with hydrogen — as if the features had been shifted as a group to redder wavelengths. This phenomenon, called redshift, occurs when an object recedes at great speeds, causing the wavelength of its light to shift toward the red end of the spectrum. The hydrogen lines Schmidt observed had been shifted by an amount corresponding to a redshift of 0.158, placing 3C 273 roughly 2 billion light-years away. But if the 13th-magnitude “star” really was so distant, it must be shining at least 100 times brighter than a normal galaxy.

Shortly afterward, astronomers revisited the spectrum of a different radio star, 3C 48, and identified features associated with a redshift of 0.3679, corresponding to a distance of over 4 billion light-years. Measurements of more quasi-stellar objects followed, all extremely distant. Soon after, the term quasar was coined. By 1973, a paper by Jerome Kristian in The Astrophysical Journal concluded that “all quasars occur in the nuclei of giant galaxies.” They appear starlike because they are so bright that the galaxy around them cannot be easily seen. 

More classes pop up

Not all AGN are so dramatic. In 1943, Carl Seyfert reported several nearby, normal-looking spiral galaxies with unusually bright nuclei. Their centers displayed high-energy emission that could not come from stars. Galaxies like these are now called Seyfert galaxies; their AGN are only a fraction of the host galaxies’ total light.

Many AGN emit X-rays, showing up in surveys of those wavelengths. Astronomers also find AGN shining in infrared light, as their high-energy emission is absorbed by dust and re-emitted at longer wavelengths.

3C273
3C 273, the first identified quasar, is so bright that it appears as a blue star. Although the quasar resides in the center of a massive elliptical galaxy, it outshines its host, rendering it invisible. A jet from the quasar, stretching 200,000 light-years, can be seen to its upper left.

ESA/Hubble & NASA
Most active galaxies are variable, so astronomers can discover them by taking images of the same region of sky some time apart. Their visible light flickers over months or years, while their X-ray emission can vary over hours or days. Changes on these short timescales narrow down both the mechanism powering the AGN and the size of the region they can occupy, allowing researchers to answer one key question: What powers them?

Powering the engine

After the discovery of 3C 273, astronomers introduced ideas for power sources that included bursts of star formation or supernovae, and exotic options such as supermassive stars, huge pulsars, or supermassive black holes.

In 1969, Donald Lynden-Bell showed that the gravitational potential energy around a black hole with a mass of 10 billion Suns and squeezed into a space 10 light-hours across could more than account for the energy outputs of quasars. He argued that matter falling at varying rates into black holes with a range of masses could explain all AGN, from low-energy Seyfert galaxies to high-energy quasars.

Astronomers now believe supermassive black holes reside in the centers of nearly all galaxies. Accretion onto these black holes is the “central engine” powering AGN. Infalling matter forms a swirling accretion disk as it approaches the black hole. As material moves from the outer disk toward the event horizon, its gravitational potential energy is converted into radiation across the spectrum. Not all galaxies are considered active, though, even if the black hole is feeding. But if there’s enough accretion, we see AGN.

Blazarmap
The brightest AGN produce gamma rays and cosmic rays. Blazars  — quasars with jets pointed at Earth — are circled on this gamma-ray map. Brighter ones indicate higher energy. In 2018, researchers with the IceCube collaboration traced neutrinos and gamma rays to the blazar TXS 0506+056. The find confirmed the theory that AGN can produce high-energy neutrinos.
NASA/DOE/Fermi LAT Collaboration
What turns the engine on? As galaxies assemble and form stars, there is a wealth of material in the core available to feed the black hole, fueling a quasar. Over time, however, that fuel runs out, and the quasar shuts off. Compared with the lifetimes of galaxies, the “active” lifetime of a quasar is short and occurs early in the galaxy’s development. Even after being turned off, AGN can be reactivated if interactions — galaxy mergers or close flybys — funnel material inward toward the supermassive black hole, restarting accretion.

“The evolution of quasars and the evolution of galaxies look very similar, and they’re actually very closely linked,” says Patrick McCarthy, staff scientist at the Carnegie Institution for Science and vice president of the Giant Magellan Telescope Organization. Indeed, the largest number of quasars is found at the same time most galaxies in the universe were forming the bulk of their stars, between redshifts 2 and 3. There are no quasars closer than 600 million light-years, meaning none still exist today. Closer AGN are not quasars, but lower-luminosity Seyfert galaxies.

A unified theory

The unified theory of AGN explains their different properties through orientation effects. It states that all AGN are the same type of object viewed from different angles, and all share similar features, whether they are visible or not.

Every active galactic nucleus begins with a supermassive black hole, typically defined as an object with 1 million solar masses or more. Its event horizon is light-hours across. Just above it is the accretion disk and a hot, spherical corona of gas. These stretch a few light-days across. At a distance of about 100 light-days is a region of fast-moving gas. About 100 light-years out, the AGN is surrounded by a torus — a doughnut-shaped ring of dust and gas that can hide portions of the central engine from view, depending on the angle it tilts with respect to Earth. Beyond the torus, about 1,000 light-years out, is a region of smaller slower-moving gas clouds.

Quasar
Astronomers obtain a spectrum to look for features associated with emission or absorption of energy by atoms. This spectrum of the quasar CXOCDFS J033229.9-275106 has a redshift of 3.6, placing it about 12 billion light-years away. One of the quasar’s strongest features is the Lyman-alpha (Lyα) line, which is associated with hydrogen. In an object at rest, Lyα emission occurs at a wavelength of 121.567 nanometers; in this quasar, that wavelength has been redshifted to nearly 580 nm.
ESO
Some AGN have fast-moving jets, which are thought to arise from magnetic fields close to the black hole. The jets can stretch outward for hundreds or even thousands of light-years, spewing material at close to the speed of light.

The angle at which we see AGN determines their classification. Looking directly down the barrel of the jet reveals a blazar. The two major classes of Seyfert galaxies differ only by whether both the fast- and slow-moving gas clouds can be seen, or if the torus hides the former.

But astronomers believe brightness does stem from intrinsic properties, including the amount of fuel available and the rate at which the black hole consumes that fuel. Different accretion modes, or types of accretion, are believed to generate more or less radiation, accounting for the range observed. “There are accretion modes that produce a lot of luminosity at high energies in the visible, X-ray, ultraviolet, and then there are other accretion modes that can accrete a fair amount of matter but not have a strong radiative signature,” says McCarthy. “One of the areas of interest is trying to understand how those different accretion modes switch on and off … [and] when they’re producing a lot of external radiation, how long do those episodes last? Is there just one big flash, or are there multiple episodes?”

Evolving together

The discovery of supermassive black holes inside galaxies brought other revelations. The mass of a galaxy’s supermassive black hole is correlated with certain properties of the galaxy’s central regions, such as its total mass and the velocities of stars in the bulge. These links suggest that galaxies and their supermassive black holes form and evolve together, somehow affecting each other despite their vast difference in scales.

CL0542
Active galaxies often emit X-rays. This Chandra X-ray Observatory image of the galaxy cluster CL 0542-4100 shows hot, diffuse gas in the cluster’s center; circled points identify active galaxies within the cluster. Red colors correspond to lower-energy X-rays, green to intermediate energies, and blue to high-energy X-ray emissions. 
NASA/CXC/ Ohio State University/J. Eastman et al.
“One of the things we learned about the evolution of massive galaxies is in order to reproduce the properties we see — the colors, the stellar ages — the key is not so much getting the star formation to turn on, but turning it off, and turning it off fairly abruptly and fairly early so the galaxies age quickly enough and the ellipticals look essentially like dead sources,” says McCarthy. AGN feedback is one possible way to shut off star formation. Winds or jets from AGN inject energy into the galaxy’s center, heating the gas so it cannot collapse and form stars. This “can pretty rapidly and quite globally in a sense shut down star formation throughout a massive galaxy,” says McCarthy.

But how such massive black holes form in the first place is perhaps the biggest unanswered question surrounding AGN and galaxy formation to date. 

“I think, in a sense, one of the aha moments was the recognition that nearly all galaxies have massive black holes in their centers, and that there’s roughly a fixed fraction of the galactic bulge mass in the black hole mass,” McCarthy says. “And then it makes sense that galaxies and black holes, or galaxies and AGN, co-evolve. But it begs the question, then, of which came first: the black hole in the center of the galaxy, or the galaxy and then the black hole formed. So that’s one of the frontiers.”

Active galaxies have changed the way astronomers think about the universe and the way galaxies within it grow. Their bright beacons have shaped the cosmos and still serve as powerful tools for understanding its properties across time.

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