Supermassive Black Holes

Supermassive black holes undoubtedly play a key role in the formation and evolution of galaxies. But how is the development of a galaxy tied to the making of stars and to the growth of the black hole at its center? Urry and her team use radio, infrared, optical and X-ray wavelength observations to study how black holes and their host galaxies evolve over time.

Unlike the Milky Way, about a tenth of all galaxies have unusually bright centers powered by matter falling into their supermassive black holes. Known as “active galactic nuclei,” the centers of these galaxies can be seen from vast distances. Yale astrophysicist Meg Urry and her colleagues study the energetics, structure and evolution of active galaxies, especially the effect black holes have on the evolution of the galaxies around them. Black holes grow in essentially every galaxy, so having a bright, active galactic nucleus must be a common phase in galaxy evolution.

Co-evolution of Galaxies and their Supermassive Black Holes

Supermassive black holes undoubtedly play a key role in the formation and evolution of galaxies. But how is the development of a galaxy tied to the making of stars and to the growth of the black hole at its center? Urry and her team use radio, infrared, optical and X-ray wavelength observations to study how black holes and their host galaxies co-evolve over time.

Observational data suggest that there is a very close connection between the formation of a galaxy and the growth of a black hole. As matter from the inner galaxy nears the event horizon, it is captured by gravitational attraction. The black hole grows more and more massive, feeding on its host galaxy this way. The outflows and light released as matter falls toward the black hole should suppress the formation of new stars. Instead Urry's team, led by postdoctoral researcher Kevin Schawinski, discovered in 2009 that nearby galaxies stop forming stars long before their central supermassive black holes reach their most powerful stage. In these galaxies, at least, it appears the black holes do not shut down star formation. But subsequent work by the Urry team, led by graduate student Carolin Cardamone, showed that black hole growth in more distant galaxies probably does stop star formation. Although the "feedback" between a galaxy and a black hole is not completely understood, this research at Yale implies it is more important in the distant Universe, where star formation and black hole growth were most vigorous

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Meg Urry

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Artist’s conception of the innermost region of an active galactic nucleus. A hot accretion disk feeds the central supermassive black hole and jets of highly energetic particles are launched from near the black hole’s event horizon. Clouds of dust and gas only light-days to light-months away are illuminated by the intense radiation given off by the accretion disk.

NASA/CXC/M.Weiss

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The supermassive black hole at the center of the Milky Way is known as Sagittarius A* (or Sgr A* for short). It is about 26,000 light years away from Earth.

NASA/CXC/MIT/F.K. Baganoff et al./E. Slawik.K. Baganoff et al.

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An active galaxy in turmoil. This composite image shows the galaxy Centaurus A in X-ray (blue), radio (pink and green), and optical (orange and yellow) wavelengths. A broad band of
dust and cold gas across the Centaurus A galaxy is bisected at an angle by opposing jets of high-energy particles blasting away from the supermassive black hole in the nucleus. Two large arcs of X-ray emitting hot gas, ultimately heated by this outflow, fill the outskirts of the galaxy. The active accretion onto the black hole may be due to the merger of a small spiral galaxy and Centaurus A about 100 million years ago.

NASA

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This Hubble Ultra Deep Field picture is a million-second-long exposure with the Hubble Space Telescope's most sensitive camera. It is the deepest portrait of the visible universe ever achieved, and it reveals the  youngest galaxies ever imaged. These formed a few hundred million years after the Universe emerged from the so-called “dark ages,” the time lasting from a few hundred thousand years after the big bang, when the Universe became very cold and inert, to a few hundred million years later, when the first stars reheated the cold, dark universe.

NASA/ESA/S. Beckwith(STScI) and The HUDF Team.

Fermi Gamma Ray Space Telescope

The Fermi Gamma-ray Space Telescope, launched on June 11, 2008, detects extremely energetic radiation called gamma rays. Ground-based telescopes can only see optical and radio wavelengths. Most infrared radiation
and all ultraviolet, X-ray and gamma-ray radiation are absorbed by Earth’s atmosphere. A series of NASA orbiting observatories study the Universe by observing radiation that never reaches the ground. Hundreds of times more sensitive than previous gamma-ray observatories, the Fermi telescope maps the entire sky several times a day. This means that sudden events, like the accretion of a chunk of matter by a black hole, can be identified and studied.

What are Gamma Rays?

Visible light makes up only a tiny fraction of the broad range of the electromagnetic radiation spectrum. This spectrum stretches from very low-energy radiowaves through infrared radiation, visible light, ultraviolet light and X-rays, and finally to very high-energy gamma rays. The processes that produce the photons (packets of electromagnetic radiation) of each type of radiation differ, as does their energy, but all forms of radiation are part of the electromagnetic spectrum family. The only real difference between a gamma-ray photon and a visible light photon is its energy. Gamma rays can have more than a billion times the energy of visible light. Gamma rays are so energetic that they are harmful to life on Earth. Luckily, Earth’s atmosphere absorbs gamma rays.

Blazars

Gamma rays are emitted by very hot, energetic cosmic sources, especially a special class of active galaxies called blazars. Blazars have relativistic jets — outflows of matter moving at nearly the speed of light — aimed directly toward us. Gamma rays from blazars, along with emissions at other wavelengths, reveal the speed and energy content of a jet, which is often much greater than the light of the blazar. Yale astronomers are using the SMARTS ground-based telescopes and the Fermi telescope simultaneously to monitor the highly variable light emission from blazars. In one of the first published reports using data from Fermi, Yale postdoctoral associate Erin Bonning, together with Charles Bailyn, Meg Urry and other Yale astronomers, showed that the gamma-ray and optical emissions from blazar 3C454.3 varied together: when the blazar shines brightly with gamma rays, the optical light is also at its brightest. This confirmed the basic picture scientists had worked out about how blazar jets shine and refuted several competing theories. Without the sensitivity of
the Fermi space telescope and the frequent monitoring provided by the SMARTS telescopes, such simultaneous observations would not have been possible.

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The Fermi Gamma-ray Space Telescope.

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The Fermi Gamma-ray Space Telescope was named in honor of Enrico Fermi, a Nobel Prize-winning physicist and one of the leading scientists of the 20th century.

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An artist's conception of the blazar PKS 2155-304, which is located 1.5 billion light-years away in the southern constellation of Piscis Austrinus.

NASA/Goddard Space Flight Center Conceptual Image Lab

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Blazar 3C 454.3’s Record Flare

Unprecedented flares from the blazar 3C 454.3 (in the constellation Pegasus) make it the brightest persistent gamma-ray source in the sky. That title usually goes to the Vela pulsar in our galaxy, which is millions of times closer. These all-sky images, which show the numbers of high-energy gamma-rays captured by Fermi on November 3 and December 2, 2009, clearly show the change.

NASA/DOE/Fermi LAT Collaboration

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The blazar 3C 454.3 as seen by the SMARTS 1.3 meter telescope.

Alberto Miranda