Active Galactic Nuclei Chapter 25 Revised 2012

Active Galactic Nuclei Come in several varieties; Starburst Nuclei – Nearby normal galaxies with unusually high star formation rates in the nucleus. Seyfert Galaxies – identified by Carl Seyfert in Essentially normal, nearby galaxies, with unusually bright nuclei Quasars – quasi-stellar objects, distant, high luminosity first recognized as galaxies by Martin Scmidt in 1963.

NGC 7742 – A Seyfert Galaxy

3C 48 – A Quasar

To learn more we have to understand the emission spectrum

A typical quasar spectrum

But, of course, it’s redshifted…

Quasar Luminosities are very high. Quasars are observed with redshifts as high as v = 0.8c The fact that they are so far away and so bright must mean that they have very high luminosities ~ L o which is 1000 times more luminous than a normal spiral galaxy.

The most distant quasars have the largest redshifts and very broad emission lines.

We also see evidence for Jets

Jets power lobes of radio emission

A closer look at the nucleus reveals disks

All of which leads to a standard model for AGN’s

Time Variability Some quasars are observed to change in brightness on timescales of days, which can be used to set a limit on the size of the emitting region.

Correlating the continuum variations with the emission line variations provides the light travel time  t which yields the size of the emitting region r ~ c t r Variability observed on a timescale of a day leads to the following size; r = 3 x 10 8 m/s 24 hrs/day 60min/hr 60s/min r ~ – m or 100 – 1000 AU !

and the broad emission lines are radiated from the disk which allows an estimate of the black hole mass v M BH r m Balancing forces; mv 2 /r = G M BH m /r 2 which re-arranges to M BH = 2.32 x 10 5 v 2 (km/s) r(kpc) M o

If the width of the broad lines, typically 10,000 km/s wide, reflects the rotational velocity of an accretion disk around a super massive black hole, then the line width together with a size for the emitting region leads to an estimate for the mass of the black hole, typically, M BH ~ 10 8 M  The large mass inferred from the broad lines combined with the small size inferred from the rapid time variability combined with the high luminosities inferred from the large redshifts all point to an exotic object at the heart of a quasar - a Massive Black Hole

Are the broad lines caused by a disk or an inflow? Modeling the shape of the broad line also provides a measure of the size for the ionized region emitting the H emission lines.

Inflow – the Movie!

Basic Black Hole Physics Escape Velocity In the case of a Black Hole, the maximum escape speed is c, the speed of light, so c 2 = 2 GM BH /R which can be re-arranged to make R the subject of the formula R = 2GM BH / c 2 also known as the Schwarzschild radius or “the event horizon”

The Schwarzschild radius for a typical quasar is R = 2GM BH / c 2 R = 2. (6.67 x ) 10 8 (1.99 x )/ (3 x 10 8 ) 2 R = 2.94 x m or ~ 2 AU for a 10 8 M o Black Hole

Energy Source It is the release of gravitational potential energy as matter falls into a black hole that drives the high luminosities observed from quasars

Quasar Evolution Quasars are observed only at high redshifts – at large lookback times, so they existed in the distant past. There are no nearby quasars. The lower luminosity AGN’s, the Seyfert and starburst galaxies, bridge the gap between us and the more distant quasars

Galactic Center- Black Hole Ghez, A, 1998, ApJ M BH = (2.6 +/- 0.2) x 10 6 M 

Tidal disruption Stars passing close to a BH are torn apart by the enormous gradient in the gravitational field strength between the front-side of the star facing the BH and the back-side of the star furthest from the BH.