Active Galaxies and Quasars
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
Activity originates in nucleus - Active Galactic Nuclei (AGN)
Don't all have all characteristics
Several classes, but probably all the same phenomenon
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
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
Nuclei of E galaxies
Blazars: BL Lacs and very variable quasars
Identified by Carl Seyfert (1943)
S or SB galaxies with bright, compact nucleus
Strong emission in ultraviolet, infrared, x-ray , not radio
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 - 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.
"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