Active Galaxies


This image from Hubble’s Wide Field and Planetary Camera 2 (WFPC2) is likely the best view of ancient and brilliant quasar 3C 273, which resides in a giant elliptical galaxy in the constellation of Virgo, about 2.5 billion light years from Earth. Despite this great distance, it is still one of the closest quasars to our home. It was the first quasar ever to be identified, and was discovered in the early 1960s by astronomer Allan Sandage.

The term quasar is an abbreviation of the phrase “quasi-stellar radio source”, as they appear to be star-like on the sky. In fact, quasars are the intensely powerful centers of distant, active galaxies, powered by a huge disk of particles surrounding a supermassive black hole. As material from this disk falls inwards, some quasars - including 3C 273 - have been observed to fire off super-fast jets into the surrounding space. In this picture, one of these jets appears as a cloudy streak (at lower right), measuring some 200,000 light years in length.

Quasars are capable of emitting hundreds or even thousands of times the entire energy output of our galaxy, making them some of the most luminous and energetic objects in the entire Universe. Of these very bright objects, 3C 273 is the brightest in our skies. If it was located 30 light-years from our own planet — roughly seven times the distance between Earth and Proxima Centauri, the nearest star to us after the Sun — it would still appear as bright as the Sun in the sky.

Credit (image and some text): ESA/Hubble & NASA [link]

The NASA Hubble Space Telescope's Advanced Camera for Surveys (ACS) has provided the clearest visible-light view yet of the nearby quasar 3C 273. The ACS coronagraph was used to block the light from the brilliant central quasar, revealing that the quasar's host galaxy is significantly more complex than had been suggested in previous observations. Features in the surrounding galaxy normally drowned out by the quasar's glow now show up clearly. The ACS reveals a spiral plume wound around the quasar, a red dust lane, and a blue arc and clump in the path of the jet blasted from the quasar. These details had never been seen before. Previously known clumps of hot gas and the inner blue optical jet are now resolved more clearly.

The power of the ACS coronagraph is demonstrated in this picture. The Hubble image on the left, taken with the Wide Field Planetary Camera 2, shows the brilliant quasar but little else. The diffraction spikes demonstrate the quasar is truly a point-source of light (like a star) because the black hole's "central engine" is so compact. Once the blinding "headlight beam" of the quasar is blocked by the ACS coronagraph (right), the host galaxy pops into view. Note that the coronagraph's occulting "finger" and other coronagraphic spot are seen in black near the top of the ACS image.

Credit (image and some text): WFPC2 image: NASA and J. Bahcall (IAS); ACS image: NASA, A. Martel (JHU), H. Ford (JHU), M. Clampin (STScI), G. Hartig (STScI), G. Illingworth (UCO, Lick Observatory), the ACS Science Team and ESA [link]

Active galaxies - many names, but one phenomenon

Active galaxy, active galactic nucleus (AGN), (radio loud/radio quiet) quasar, blazar, Seyfert galaxy, radio galaxy

  • An active galaxy is a galaxy that is producing more energy than normal, in radio, visible, X-rays, and/or gamma rays.

  • They sometimes have enormous, long jets.

  • All forms of activity are ultimately caused by accretion of matter onto a supermassive black hole at the center of the galaxy (the black hole has a mass of almost a billion Msun in 3C273).

  • Different appearances of active galaxies are caused by the viewing angle from Earth (random!).

This artist's concept illustrates a supermassive black hole with millions to billions times the mass of our Sun. Supermassive black holes are enormously dense objects buried at the hearts of galaxies. In this illustration, the supermassive black hole at the center is surrounded by matter flowing onto the black hole in what is termed an accretion disk. This disk forms as the dust and gas in the galaxy falls onto the hole, attracted by its gravity. Also shown is an outflowing jet of energetic particles, believed to be powered by the black hole's spin.

Credit (image and some text): NASA/JPL-Caltech [link]

On 18 April 2019, the Event Horizon Telescope (EHT) Collaboration released an image of the immediate region of the supermassive black hole at the center of the elliptical galaxy M87 (located at a distance of 55 million light years). The background image shown here is a wide-field view of M87 in infrared light from NASA's Spitzer Space Telescope. At the galaxy's center is a supermassive black hole that spews two jets of material out into space. The top inset image shows a close-up of two shockwaves, created by jets emanating from the galaxy’s supermassive black hole. The Event Horizon Telescope obtained an image of the silhouette of that black hole, shown in the lower inset.

The EHT image (lower inset) shows a bright ring formed as light bends in the intense gravity around a black hole that is 6.5 billion times more massive than the Sun. This long-sought image provides the strongest evidence to date for the existence of supermassive black holes and opens a new window onto the study of black holes, their event horizons, and gravity.

Credit (image and some text): NASA/JPL-Caltech/IPAC/Event Horizon Telescope Collaboration [link]

This illustration shows the different features of an active galactic nucleus (AGN), and how our viewing angle determines what type of AGN we observe. The extreme luminosity of an AGN is powered by a supermassive black hole at the center. Some AGN have jets, while others do not.

Credit: D. W. Hoard (2018) based on a public domain NASA diagram; acknowledgement: A. Simonnet (Sonoma State University) [link]

Active galaxy Hercules A (3C348): a radio galaxy

Spectacular jets powered by the gravitational energy of a supermassive black hole in the core of the elliptical galaxy Hercules A illustrate the combined imaging power of two of astronomy's cutting-edge tools, the Hubble Space Telescope's Wide Field Camera 3, and the recently upgraded Karl G. Jansky Very Large Array (VLA) radio telescope in New Mexico.

Some two billion light years away, the yellowish elliptical galaxy in the center of the image appears quite ordinary as seen by Hubble in visible wavelengths of light. The galaxy is roughly 1,000 times more massive than the Milky Way and harbors a 2.5-billion-solar-mass central black hole that is 1,000 times more massive than the black hole in the Milky Way. But the innocuous-looking galaxy, also known as 3C 348, has long been known as the brightest radio-emitting object in the constellation Hercules. Emitting nearly a billion times more power in radio wavelengths than our Sun, the galaxy is one of the brightest extragalactic radio sources in the entire sky.

Credit (image and some text): NASA, ESA, S. Baum and C. O'Dea (RIT), R. Perley and W. Cotton (NRAO/AUI/NSF), and the Hubble Heritage Team/STScI/AURA [link]

Active galaxy Centaurus A: an elliptical galaxy that swallowed a spiral galaxy

The peculiar galaxy Centaurus A (NGC 5128; distance of 10 million light years) is pictured in this image taken with the Wide Field Imager attached to the MPG/ESO 2.2-metre telescope at the La Silla Observatory in Chile. It looks like an elliptical galaxy, but displays a prominent central dust lane, likely the result of a head-on collision and merger with a spiral galaxy.

Credit (image and some text): ESO, CC BY 4.0 [link]

Centaurus A is one of the foremost examples of a radio-loud active galactic nucleus (AGN). On images obtained at optical wavelengths, thick dust layers almost completely obscure the galaxy's center. There is strong evidence that Centaurus A is a merger of an elliptical with a spiral galaxy, since elliptical galaxies would not have had enough dust and gas to form the young, blue stars seen along the edges of the dust lane. The core of Centaurus A is the smallest known extragalactic radio source, only 10 light days across. The core probably contains a supermassive black hole with a mass of about 100 million solar masses.

Credit (image and some text): ESO, CC BY 4.0 [link]

This image obtained by NASA's Spitzer Space Telescope shows in unprecedented detail the galaxy Centaurus A's last big meal: a spiral galaxy seemingly twisted into a parallelogram-shaped structure of dust. Spitzer's ability to both see dust and also see through dust allowed the telescope to peer into the center of Centaurus A and capture this galactic remnant as never before.

Astronomers have created a model that explains how such a strangely geometric structure could arise. In this model, a spiral galaxy falls into an elliptical galaxy, becoming warped and twisted in the process. The folds in the warped disk create the parallelogram-shaped illusion.

Credit (image and some text): NASA/JPL-Caltech/J. Keene (SSC/Caltech) [link]

Color composite image of Centaurus A, revealing the lobes and jets emanating from the active galaxy’s central black hole. This is a composite of images obtained with three instruments, operating at very different wavelengths. The 870-micron submillimetre data, from LABOCA on APEX, are shown in orange. X-ray data from the Chandra X-ray Observatory are shown in blue. Visible light data from the Wide Field Imager (WFI) on the MPG/ESO 2.2 m telescope located at La Silla, Chile, show the stars and the galaxy’s characteristic dust lane in close to "true color".

Credit (image and some text): ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimeter); NASA/CXC/CfA/R.Kraft et al. (X-ray); CC BY 4.0 [link]

Active galaxy M77: a Seyfert galaxy

M77 is one of the largest galaxies in the Messier catalog. It is located in the constellation Cetus at a distance of 45 million light years from Earth. The Hubble Space Telescope captured this vivid image of M77’s center using visible and infrared observations. The streaks of red and blue in the image highlight pockets of star formation along the pinwheeling arms, with dark dust lanes wrapping around the galaxy’s starry center.

M77 is a prime example of a Seyfert galaxy, or a galaxy with an intensely active center that is obscured by gas and dust in visible light.

Credit (image and some text): NASA, ESA & A. van der Hoeven [link]

This ultraviolet image from the Hubble Space Telescope probes the bright, active core of M77. Brightness fluctuations in this region imply that an enormous amount of energy is being released from a hidden source in the galaxy’s nucleus. The most likely candidate is a supermassive black hole.

Credit: NASA, ESA [link]

Active galaxy M82: a starburst galaxy

Messier 82 (M82) is located 12 million light-years away, and appears high in the northern spring sky in the direction of the constellation Ursa Major, the Great Bear. This active galaxy is remarkable for its bright blue disk, webs of shredded clouds, and fiery-looking plumes of glowing hydrogen blasting out of its central regions.

Throughout the galaxy's center, young stars are being born 10 times faster than they are inside our entire Milky Way Galaxy. The resulting huge concentration of young stars carved into the gas and dust at the galaxy's center. The fierce galactic superwind generated from these stars compresses enough gas to make millions of more stars.

The rapid rate of star formation in this galaxy eventually will be self-limiting. When star formation becomes too vigorous, it will consume or destroy the material needed to make more stars. The starburst then will subside, probably in a few tens of millions of years.

Credit (image and some text): NASA, ESA, and The Hubble Heritage Team/STScI/AURA; acknowledgment: J. Gallagher (University of Wisconsin), M. Mountain (STScI), and P. Puxley (National Science Foundation) [link

Why isn’t the Milky Way an active galaxy?

The supermassive black hole at the center of our galaxy emits only a (relatively!) feeble amount of energy.


Best guess: Nothing is feeding it!

There is evidence from other galaxies that being an active galaxy requires a significant recent or ongoing collision or interaction with another galaxy to stir up the core.

So maybe after we collide with the Andromeda Galaxy?

The beating heart of our galaxy. This three color animation created from infrared observations of the supermassive black hole at the center of the Milky Way (called Sgr A*) shows, for the first time ever, the broadband color of Sgr A* throughout an outburst. The image is 1 arcsec on a side and covers about two hours of observations. The outburst resulted form energy released during the accretion of a large “blob” of matter (asteroid to planet size) onto the event horizon of the supermassive black hole.

Credit (image and some text): These images/animations were created by Prof. Andrea Ghez and her research team at UCLA and are from data sets obtained with the W. M. Keck Telescopes [link]

There seems to have been a wave of star formation close to Sgr A* about 10 Myr ago. But the available gas went into new stars instead of feeding the black hole? (Why???)

An extremely high resolution image of the region around the galactic center (circled in red) obtained in July and August 2000 using the Gemini-North 8-meter telescope with the Hokupa'a/QUIRC Adaptive Optics system. This is a near-infrared (JHK') color-composite image. Young stars around the galactic center region appear as bright blue objects.

Credit: Gemini Observatory/AURA [link]

Not quite dead yet?

There is some evidence that the Milky Way's core is still lit up by expanding (and fading) “X-ray echoes”.

This might indicate that Sgr A* was active about 500 years ago.

Recent (2013, 2015) X-ray flares have been observed from Sgr A* (>400x normal X-ray emission)

  • Coincidental accretion of an asteroid?

  • Reconfiguration of the magnetic field around the black hole?

  • Or is the Milky Way getting ready to turn on as an active galaxy?

The recent (2015) discovery of “changing look” quasars suggests that the black hole can turn off or on in a matter of months…

Astronomers have detected the largest X-ray flare ever from the supermassive black hole at the center of the Milky Way, known as Sagittarius A* (Sgr A*), using NASA's Chandra X-ray Observatory. This event was 400 times brighter than the usual X-ray output from Sgr A*. Shown in this series of images is the area around Sgr A* in a Chandra image where low, medium, and high-energy X-rays are red, green, and blue respectively. The inset image contains an X-ray movie of the region close to Sgr A* and shows the giant flare, along with much steadier X-ray emission from a nearby magnetar, to the lower left. A magnetar is a neutron star with a strong magnetic field. The bottom panels compare still frames of the quiescent X-ray emission (showing only the magnetar's normal brightness; at left) and the Sgr A* X-ray flare, which is mush brighter than the magnetar (at right).

Credit (image and some text): NASA/CXC/Northwestern Univ/D.Haggard et al. [link]


X-ray echoes

  • Nobukawa, M., Ryu, S. G., Tsuru, T. G., et al. 2011, The Astrophysical Journal Letters, 739, L52 – “New Evidence for High Activity of the Supermassive Black Hole in our Galaxy” (2011ApJ...739L..52N)

  • Also see: "Milky Way's black hole flared during the Renaissance" [link]

Changing Look Quasars

  • LaMassa, S. M., Cales, S., Moran, E. C., et al. 2015, The Astrophysical Journal, 800, 144 – “The Discovery of the First “Changing Look” Quasar: New Insights Into the Physics and Phenomenology of Active Galactic Nucleus” (2015ApJ...800..144L)

  • Also see:

    • "Black hole diet quasar" [link]

    • "Unexpectedly vanishing quasars are mystifying scientists" [link]

    • "Why changing look quasars appear to vanish" [link]

Sgr A* X-ray flare

  • Haggard, D., Baganoff, F. K., Ponti, G., et al. 2015, American Astronomical Society Meeting Abstracts #225 225, 102.09 – “An Update on Chandra/VLA Galactic Center Campaigns Targeting Sgr A* and G2” (2015AAS...22510209H)

  • Also see: "NASA's Chandra Detects Record-Breaking Outburst from Milky Way's Black Hole" [link]

Increasing rate of X-ray flaring from Sgr A*

  • Ponti, G., De Marco, B., Morris, M. R., et al. 2015, Monthly Notices of the Royal Astronomical Society, 454, 1525 – “Fifteen years of XMM-Newton and Chandra monitoring of Sgr A: evidence for a recent increase in the bright flaring rate” (2015MNRAS.454.1525P)