Active Galactic Nuclei

Types of Active Galaxies

• 4 main types– Seyfert galaxies– Radio galaxies– Quasars– Blazars

What are they?

In general,

• They are small astronomical objects with a large energy output.

• They are powered by the energy released when matter is dropped into a massive black hole in the center of the galaxy.

Seyferts

The brightest Seyfert known is NGC 1068

NGC 1068 and the X-Ray Flashlight Credit: X-ray: P. Ogle (UCSB) et al.; Optical: A.Capetti (INAF) et al.; CXO, STScI, NASA

A 10 minute exposure taken with an SBIG ST-9E CCD camera thru our 20-inch telescope working at F/5.5. The field of view is about 15x15 arc minutes.

Seyferts

• Has properties that lie in between Normal Galaxies and Quasars

• Physical Appearance: Spiral galaxies• Most interesting feature: its Galactic Nuclei

• Galactic Nuclei is compact ~ 1 light year across

• Galactic Nuclei is powerful ~ output energy more than several million supernovae

M77 is a type Sb spiral galaxy in the constellation Cetus. NGC1068 is the nearest and brightest example of a type II Seyfert galaxy, showing broad and strong emission lines due to high velocity gas in the galaxy's inner regions. A strong radio source known as Cetus A sits in the nucleus.

Seyferts• Comparison between Seyfect and the

NGC 1232

``True-color'' stellar continuum image of the Seyfert 2 Galaxy NGC 1068 (Messier 77), composed of UV, green, and red narrowband filters chosen to reject bright emission lines and only admit starlight. Prime Focus Camera + Loral 512x512 CCD, 3.6-meter Canada-France-Hawaii Telescope, Mauna Kea, Hawaii

The impressive spiral galaxy NGC 1232, which ressembles the Milky Way galaxy in which we live. The distance is about 100 million light-years. The photo is a composite of three exposures in difference wavebands (colours). (ESO VLT FORS2 multi-mode instrument; 1998).

ImpressiveNot so

impressive

• Sub-classes of Seyferts:

Seyferts

• Seyfert 1:

• Broad and Narrow emission lines

• Seyferts 2:

• Only observe narrow emission lines

• Emit radiation of all wave lengths, but emission lines mostly observed in the infrared region of the spectrum

• Fluctuations are apparent in Seyferts because.. HERMAN FILL THIS IN!

Seyferts

Chaisson and McMillan Chaisson and McMillan

• Unification of Seyferts with other type AGN types

Seyferts

Seyferts

Wind from Accretion Disk around a Black Hole This illustration depicts a massive black hole at the center of a galaxy. Around it is a swirling disk of gas, which gradually pours down into the black hole. As the gas falls inward, it heats up and glows brightly, getting hotter and hotter the closer it is to the event horizon. Some of the gas is blown away from the disk like steam from a kettle. As this gas streams off the disk, the intense radiation generated by the very hot gas near the event horizon forces the escaping gas into a cone and accelerates it to speeds as high as a tenth the speed of light.(Illustration: NASA/CXC/M.Weiss)

Seyferts

Illustration of Black Hole with Accretion Disk and Torus An artist's conception shows a black hole surrounded by a disk of hot gas, and a large doughnut or torus of cooler gas and dust. The light blue ring on the back of the torus is due to the fluorescence of iron atoms excited by X-rays from the hot gas disk.(Illustration: NASA/CXC/M.Weiss)

Radio Galaxies

• Has a very strong radio output ~ 1034W or 108Lsol – Compared to our Milky Way which has radio output of

~ 1030W or 2500Lsol

• Radio output not seen in the visible spectrum– When viewed in the radio spectrum, can see one or

two jets emerging

• Typically elliptical in shaped when viewed in visible spectrum

A few Radio Galaxies…

Centaurus A, radio image is superimposed on the visible image

Cygnus A in the visible spectrum

And in the Radio spectrum

Virgo A, radio image (left) and optical image (right)

Structure of Radio GalaxiesWhen viewed in the radio spectrum, one can notice the following:

• The nucleus – the centre of the galaxy• Jets - bright lines where strong radio emission

streams out from the nucleus• Lobes – region around the jets • Plumes – similar to lobes, yet they have a much

more elongated structure, replaces lobes

• Hotspots – bright spots typically at the end of a jet

Quasars

• Brightest objects in the known universe

• Very far away with high redshift– We see them as they were when the Universe

was only 1/6 of its present age.

• > 1012 Lsol

• Most emit in infrared

• Have jets and lobes like radio galaxies

Example of quasars

This is a classic double-lobed radio source with radio emission from relativistic streams of high energy particles generated by the quasar

Quasar 3C 273

This is the most luminous quasar known with an absolute magnitude of -26.7 and approximately 2.2 billion years away.

Interesting fact

• Quasars show evidence of elements heavier than helium – This is taken to mean that galaxies underwent

a massive phase of star formation creating hypothetical population III stars between the time of the Big Bang and the first observed quasars.

– Necessary in explaining the beginning of the Universe

Blazars• BL Lac Objects

– Named for their prototype BL Lacertae(1926)• Was thought to have been an extragalactic

variable star, until they realized the redshift

– Weak emission lines– Strong luminosities– High rate of fluctuation

• Periods of fluctuation in days, even hours

– ‘Loud’ in radio spectrum (like all quasars)

• Optically Violently Variable (OVV) Similar to BL Lacs w/ strong emission lines

BL Lac Object 0735+178, imaged in the microwave range by VLBA

Blazars

An artist’s conception of a quasar. Counter jets are not visible in observation due to the luminosity of the forward jet and accretion disk

M87

Nearby quasar

3C 273Optical image by Hubble.

This can be considered a blazar because it appears that the jet is pointed at us.

Summary of Active Galactic Nuclei Classification

Orientation Luminosity

Low High

“on – Angle” Seyfert 1 Quasar

“Face – on” --- Blazar

“Edge – on” Seyfert 2 Radio