Chapter 14

Active Galaxies and Quasars

Active Galaxies:

Galaxies with unusual characteristics

Unusually bright in radio, infrared, ultraviolet, x-rays

Total emitted energy greater than 'normal galaxy'

Synchrotron emission strong

Bright, small, variable nucleus

Jets of material being ejected

Disturbed appearance

Activity originates in nucleus - Active Galactic Nuclei (AGN)

Don't all have all characteristics

Several classes, but probably all the same phenomenon

Radio Galaxies

Strong source of radio emission - up to 107 times radio emission of "normal" galaxies

E.g. Cygnus A - unusual E galaxy

Most radio emission from radio lobes on either side of visible galaxy

Lobes can be several times larger than visible galaxy

Some radio galaxies contain central, compact source emitting 1 or 2 jets of material, coincides with visible nucleus.

Lobes - energetic charged particles ejected from galaxy nucleus

Jets - charged particles accelerated in nucleus and ejected towards lobes.

Spectrum - relatively smooth, some emission lines - hot gas

Dominated by synchrotron radiation - relativistic, charged particles spiraling around magnetic field lines (mostly electrons)

Synchrotron radiation polarized

Radio galaxies usually ellipticals

Some have optical, x-ray jets as well, e.g. M87

All jet emission due to synchrotron radiation


Quasi-stellar radio sources

Quasi-stellar objects (QSOs)


Strong emission lines - large redshifts due to Doppler effect

λ0: rest wavelength

λ: wavelength observed

z: redshift

For v greater than 0.1 c, must use relativistic expression for velocity dependence.

Quasar redshifts large, greater than ~0.1

Radial velocities large

Hubble law: Bigger radial velocity, more distant the galaxy

Quasars very distant - billions of l. y.

Most distant quasar, z greater than 5

Quasars must be very luminous

Some doubted quasars were so distant, luminous

Spectra of underlying galaxies supported large distances

Quasars are AGN's, but so bright overwhelm underlying galaxy

Share characteristics of other AGN's: spectra, variability, jets, etc.

Quasar activity more common billions of years ago than now

BL Lacertae Objects and Blazars

Similar to quasars, but no strong emission lines in spectra

Star-like, radio sources, emit synchrotron radiation

Very variable

Nuclei of E galaxies

Blazars: BL Lacs and very variable quasars

Seyfert Galaxies

Identified by Carl Seyfert (1943)

S or SB galaxies with bright, compact nucleus

Strong emission in ultraviolet, infrared, x-ray , not radio

Nucleus variable

Luminosity between normal galaxies and quasars

Brightest similar to least luminous QSO's

Seyfert I - broad and narrow emission lines

Seyfert II - narrow emission lines only

Emission lines arise in clouds in nucleus

The Energy Machine

Black hole - 10 x 106 - 109 solar masses at center

Accretion disk - material spirals into black hole

Energy released by in-falling matter and friction within disc

Somehow - accretion disc produces jets

Size and Mass of Central Source

Rapid variability suggests central engine very small

Size must be less than speed of light times duration of variation in brightness

Suggests sizes of a few light days or less for some AGNs - maximum size of black hole

Estimate maximum mass of black hole from

R (km) = 3 x M (solar masses)

M = R/3

Estimate minimum size, mass from Eddington limit

Eddington limit: in order for gravity to hold body together against radiation pressure,

L < 10 31 M watts, for L, luminosity, M, mass in solar masses

Therefore M > 10-31 L solar masses

=> AGN's have black holes with masses 10 x 106 - 109 solar masses

Spectra suggest gas, stars in centers of AGN's move rapidly

Velocities from Doppler shifts or widths of lines, calculate central mass

Observations of several galaxies provide evidence for supermassive black holes

'Superluminal' Motion

Superluminal - faster than the speed of light

QSOs, radio galaxies apparently emitting blobs moving faster than the speed of light.

Discovered using VLBI.

Relativistic jet model:

Matter ejected close to speed of light, almost directly towards us.

Appears to travel at velocity greater than the speed of light.

Optical illusion

"Unified Model" for AGN's

One theory to explain all AGNs.

What we see depends on viewing angle.

Black hole, gas inflow via accretion disk.

Jets ejected along axis of disk.

Intensity of jet beamed towards observer relativistically enhanced compared to jet moving away from observer.

Explains why many AGNs appear to have single jets

Central 3 l.y., dense clouds, high velocity.

Produce broad emission lines, seen in QSOs, Seyfert I, radio galaxies

30 - 300 l.y., narrow line region

Low density gas, lower velocities.

Narrow emission lines produced, seen in Seyfert I & II, QSO's, radio galaxies

10 l.y. from center - opaque torus of gas and dust.

Looking down jet - blazar.

Edge-wise, through torus, narrow line region => Seyfert II, radio galaxies.

Disk tipped - see compact source, broad, narrow line regions => Seyfert I, radio galaxies, QSOs.

Model supported by observations of Seyfert II in scattered light - Seyfert I spectra

Model accounts for many observations

Recent Chandra observations - 'normal' looking galaxies have bright x-ray emission from nucleus - may contradict model but too early to say

Many galaxies may have dormant black holes at center

Could revive if matter available, e.g. from an interaction

Prof. Donna Weistrop

University of Nevada, Las Vegas