Chapter 25: Quasars and active galaxies

• Features of quasars

• Quasars and distant galaxies

• Seyfert and radio galaxies

• Active galactic nuclei

• Supermassive black holes

• Properties of active galaxies

Strange radio sources

• Two “stars” were discovered in 1960s that appeared to strongly emit radio waves.– 3C 48 and 3C 273– Odd since stars don’t

typically emit radio.

• Beyond that, no one could explain spectra of these stars.

Clues from spectra

• Here is the spectrum of 3C 273.• A few years after discovery

astronomers realized that the lines were Balmer lines that were greatly redshifted.

• Redshift of 3C 273 is z=0.158 meaning the object is 2 billion ly away.

• Redshift of 3C 48 was 0.367 placing it almost 5 billion ly from Earth.– These objects must be incredibly

bright to be seen at such a distance.

Quasars

• These objects were called quasi-stellar radio sources which was soon shortened to quasars.– Soon many starlike objects with large redshifts

were discovered that emitted no radio waves.– Called “radio-quiet” quasars and comprise 90%

of all known quasars.

• All quasars have large redshifts meaning they are very distant.

Quasar distribution

• There are no quasars with small redshifts.– This means there are

no nearby quasars.– Nearest is about 800

million ly from Earth.

• Quasars were common in the distant past, but there a none in present-day universe.

Quasars are ultraluminous centers of distant galaxies

Luminosity of 3C 273 is ~1040 W, thousands of times brighter than the entire Milky Way galaxy! Are they

really as far away as we first thought?

Seyfert and radio galaxies

• “Missing links” between quasars and normal galaxies were actually discovered before quasars.– Seyfert galaxies are like dim, radio-quiet

quasars.– Radio galaxies are like dim, radio-loud quasars.

Radio galaxies

Radio galaxies

Blazars

• Giant elliptical galaxies with bright, starlike nuclei with many quasar-like properties.

• Spectra, however, are featureless.

• It is believed that blazars are radio galaxies with their jets aimed directly at the Earth.

Active galactic nuclei (AGNs)

• Quasars, blazars, Seyfert galaxies and radio galaxies are called active galaxies.

• Energy comes from source at center of galaxy meaning these galaxies possess active galactic nuclei (AGNs).

Brightness variations of an AGN

Limit on speed of variations

Supermassive black holes power AGNs

• Rapid brightness variations of active galaxies means that energy source must be very small.– A region no larger than our solar system can emit more

energy than 1000 galaxies!– How can we explain the production of so much energy

in such a small volume?

• In 1968 it was theorized that material falling into a black hole at the center of a galaxy could power an active galaxy.

How large would a black hole need to be to power an AGN?

• As gas falls into a black hole it releases energy, but there is a limit to the luminosity radiated by accretion onto a black hole.– Called the Eddington limit.– LEdd=30,000(M/M)L

• If luminosity exceeds Eddington limit the radiation pushes the infalling material away from the black hole.– Less material falls into black hole and luminosity decreases until a

balance is reached between radiation moving out and material moving in.

• Eddington limit says that 3C 273 must have a mass of 109 M - must be a black hole.

Evidence for supermassive black holes at centers of galaxies

Accretion disks

Unified model of AGNs