2. Lecture 7 Continuum Emission in AGN UV-Optical Continuum
Infrared Continuum
High Energy Continuum
Radio Continuum - Jets and superluminal motion

3. Goal: The foundation of all astrophysical observations is the photon
Goal: The foundation of all astrophysical observations is the photon. All morphological and spectral information about astrophysical sources is derived from the emitted radiation. We learned about the power of line emission (spectroscopy) Continuum radiation is a natural consequence of the principle that accelerating charges radiate.
Can have : thermal or nonthermal emission

4. Spectral Energy Distribution
AGN show emission lines in all astrophysically relevant wavelength regimes

5. Actual SED is a function of the AGN Class
Power Law Continuum
Emission observed from 108 Hz to 1027Hz:
α=energy index now know to differ in different bands
Actual SED is a function of the AGN Class

6. From last class:AGN Taxonomy
Seyfert galaxies 1 and 2
Quasars (QSOs and QSRs)
Radio Galaxies
LINERs
Blazars
Related phenomena

7. Definition: radio-loud if
is larger than 10 (Kellermann et al. 1989)
RL AGN have prominent radio features
10% of AGN population
RL: BLRGs, NLRGs, QSRs, Blazars
RQ: Seyferts, most QSOs
Deep radio surveys show intermediate sources

8. The Continuum A phenomenological approach: Power law continuum
Thermal features
Spectral Energy Distributions of
Radio-loud and Radio-quiet AGN

9. Observing the SEDs of AGN

10. Types of Continuum Spectra
Blazars: non-thermal emission from radio to gamma-rays (2 components)
Seyferts, QSOs, BLRGs:
IR and UV bumps (thermal)
radio, X-rays (non-thermal)
Spectral Energy Distributions (SEDs): plots of power per decade versus frequency (log-log)

11. Spectral Energy Distributions
EUV gap
Big Blue Bump
IR bump
Sanders et al. 1989

12. The radio and IR bands
Radio emission is two orders of magnitude or more larger in radio-loud than in radio-quiet
Radio and IR are disconnected, implying different origins

13. The IR and Blue bumps LIR contains up to 1/3 of Lbol
LBBB contains a significant fraction of Lbol
IR bump due to dust reradiation, BBB due to blackbody from an accretion disk
The 3000 A bump in A:
Balmer Continuum
Blended Balmer lines
Forest of FeII lines

14. The highest energies Typically α=0.7-0.9 in 2-10 keV
Radio-loud AGN (BLRGs, QSRs) have flatter X-ray continua than radio-quiet
Soft X-ray excess is also observed, often smoothly connected to UV bump
The only AGN emitting at gamma-rays
( MeV) are blazars

15. Blue blazars: PKS 2155-398 Red blazars: 3C279 Blazars’ SEDs
Wehrle et al. 1999
Bertone et al. 2001

16. Blazar SEDs main features
Two main components:
Radio to UV/X-rays
X-rays to gamma-rays
Component 1 is polarized and variable
Synchrotron emission from jet
Component 2: possibly inverse Compton
scattering

17. A fundamental question
How much of the AGN radiation is primary and how much is secondary?
Primary: due to particles powered directly by the central engine (e.g., synchrotron, accretion disk)
Secondary: due to gas illuminated by primary and re-radiating

18. An important issue Isotropy of emitted radiation
Thermal radiation is usually isotropic
Non-thermal radiation can be highly directed (“beamed”). In this case:
We can not obtain the true luminosity of the AGN
We will not have a true picture of various AGN emission processes

19. BBB=thermal disk emission?!
1. UV-Optical Continuum
Interpreting the BBB
From accretion disk theory (last class),
And the maximum emission frequency is at
i.e., in the EUV/soft X-ray emission region.
BBB=thermal disk emission?!

20. Model Spectrum of an Accretion Disk

21. Spectrum from an accretion disk
Optically thick, geometrically thin accretion disk radiates locally as a blackbody due to sheer viscosity
Total integrated spectrum goes like ~ν2 at low frequencies, decays exponentially at high frequencies
For intermediate frequencies spectrum goes as ~ ν1/3
T=T(R) and T is max in the inner regions in correspondence of UV emission

22. Observations of optical-to-UV continuum
After removing the small blue bump, the observed continuum goes as ν-0.3
Removing the extrapolation of the IR power law gives ν-1/3 - but is the IR really described by a power law??
More complex models predict Polarization and Lyman edge – neither convincingly observed
Disk interpretation is controversial!

23. Alternative interpretation
Optical-UV could be due to Free-free (bremsstrahlung) emission from many small clouds Barvainis 1993
Slope consistent with observed (α~0.3), low polarization and weak Lyman edge predicted
Requires high T~106 K

24. Is an accretion disk really there?
Indirect evidence:
Fitting of SEDs
Double-peaked line profiles
Direct evidence:
Water maser in NGC 4258

25. Optical emission lines
Eracleous and Halpern 1984

26. Water Masers in NGC 4258
Within the innermost 0.7 ly, Doppler-shifted molecular clouds:
Obey Kepler’s Law
Massive object at center

27. 2. The IR emission
In most radio-quiet AGN, there is evidence that the IR emission is thermal and due to heated dust
However, in some radio-loud AGN and blazars the IR emission is non-thermal and due to synchrotron emission from a jet

28. Evidence for IR thermal emission
Obscuration :
Many IR-bright AGN are obscured (UV and optical radiation is strongly attenuated)
IR excess is due to re-radiation by dust

29. Radial dependence of dust temperature
From the balance between emission and absorption:
With R in pc, Leff in erg/s, T in Kelvin
Hotter dust lies closer to the AGN

30. Evidence for IR thermal emission
IR continuum variability :
IR continuum shows same variations as UV/optical but with significant delay
variations arise as dust emissivity changes in response to changes of UV/optical that heats it

31. Emerging picture
The 2μ-1mm region is dominated by thermal emission from dust (except in blazars and some other radio-loud AGN)
Different regions of the IR come from different distances because of the radial dependence of temperature

32. The 1μ minimum General feature of AGN
Consistent with the above picture: hottest dust has T~2000 K (sublimation temperature) and is at 0.1 pc
This temperature limit gives a natural explanation for constancy of the 1μ minimum flux

33. 3. Radio properties of AGN
I) Basic features of radio morphology
II) Observed phenomena
Superluminal motion
Beaming

34. Radio features
Lobes
Jet
Hotspot
Core

35. Speed of Jets
What is the speed of radio jets in AGN? Since this is non-thermal plasma where no spectral lines are seen, the Doppler-shift cannot be used to derive a jet velocity for the nucleus!

36. Radio Telescopes: VLA, VLBI
The Very Large Array has angular resolution
At z=0.5 this is ~2 kpc
For the Very Long Baseline Interferometry, R~1m.a.s.
At z=0.5 this is ~2 pc

37. The power of resolution
Energy is transported by jets from the cores to the outer regions

38. Superluminal Motion
VLBI observations of the inner jet of 3C273 shows ejected blobs moving at v~3-4c
This is called superluminal motion
How is this possible??

39. Historgram of observed v/c in 33 jets

40. Explanation of apparent superluminal motion
Explain apparent superluminal motion as an optical illusion caused by the finite speed of light. Consider a knot in the jet moving almost directly towards us at high speed:
The blobs are moving towards us at an
angle  measured from the
line of sight.
Photon emitted along the
line of sight at time t=0, travels
a distance d to us, taking a time
t1 to arrive: t1 = d/c
A second photon is emitted at a time te later, when
The blob is a distance d – vte cos away from us. The second photon
arrives at t2 = te + (d - vte cos)/c
The observed difference in the time of arrival from photon 1 & 2 is:
tobs = t2 - t1 = te (1 – vcos/c) < te

41. The apparent transverse velocity
is vT = vte sin / t
= v sin / (1 – v cos /c)
As v approaches c, vT can
appear > than c! Superluminal motion, typically 5-10c!
Let  = 1/(1- v2/c2)1/2, this is the Lorentz factor. Then:
vT  v (the maximum observed velocity) which occurs
when cos  = v/c. We will only observe superluminal motion when
the jets are pointed within an angle of 1/ towards the line of sight,
but this light will be beamed and brightened.

44. Relativistic motion of plasma
Relativistic bulk motion in radio sources has important consequences on the following observed quantities:
Frequency
Length and time
Intensity
Direction light is emitted

45. Relativistic Doppler Effect
Assume an emitting source moving at a speed v c at an angle q with respect to the observer.
Time-dilation tells us that dt in the observers rest frame for a periodic signal with frequency n’ in the co-moving (primed) frame is:
However, since the emitting source is moving almost as fast as the
emitted photon, the source will be catching up on the photon, and
travel a distance s = v Dtcos q . The time difference in the arrival
time of the two photons will therefore be reduced by s/c, i.e

46. and the observed frequency is
This is the relativistic Doppler effect which defines the Doppler factor
One can show (i.e. Rybicki & Lightman, chap. 4.9) that the ratio of the flux density Sn and the frequency cubed is invariant under Lorentz transormation:
Since the observed frequency is n=Dn’, = we find that also the observed flux has to be (S’n= flux density in co-moving frame)

47. Even for relatively modest relativistic velocities of v=0
Even for relatively modest relativistic velocities of v=0.97c, for example, the flux in the forward direction can be boosted by a factor 1000, while it is reduced by a factor 1000 in the backward direction!
The transformation from a spherical to an elliptical polar diagram shows that angles are also transformed by relativistic effects. The so-called relativistic aberration (see Rybicki & Lightman, chap. 4.1) is given by:
In the rest frame of the source, half of the radiation will be emitted from –p/2 to p/2, hence setting q’ = p/2 will give
thus for g>> 1 half of the radiation will be emitted in a cone with half-opening angle

48. Jet-sidedness
Since we expect jets to be two-sided, we always have two angles
under which the emission is seen by an observer: q and q+p . We
can now calculate the flux ratio R between jet and counter-jet under
the assumption of intrinsically symmetric jets:
Even for mildly relativistic jets one side will always be significantly brighter than the other

49. Most of the strong, compact radio cores seem to come from sources where the angle to the line of sight is small, these jets are always one-sided.
Even most of the large scale jets appear to be one- sided, even though 2 extended lobes are seen indicating that
really two jets are present.
Nearby FRI radio galaxy and LINER galaxy M87 - no counter-
Jet observed

50. Summary: evidence for relativistic motion in AGN
Superluminal motion
One-sided jets (pc and kpc scales)
Caveats
None of the above evidence proves that relativistic motion exists
Alternative explanation exist for each observed property (e.g., one-side jets)
But relativistic motion=beaming is the only and the simplest explanation for all of them at once

51. The Emission-Line Regions (BLR, NRL)
Physics of AGN
The Emission-Line Regions (BLR, NRL)
Reprocessed Radiation
An AGN produces a lot of ionizing radiation, most likely from the
accretion disk.
This emission is intercepted by gas and dust in the host galaxy.
Correspondingly an AGN spectrum shows reprocessed radiation from
this gas and dust. The respective features are:
Broad-Line Region (BLR)
Narrow-Line Region (NLR)
IR-bump from a molecular (dusty) “torus” (which we talked about last class)

52. BLR:Properties
Broad, permitted emission lines (e.g., Ha) in the optical spectrum:
FWHM several thousand km/sec up to km/sec FWZI (zero intensity)
derived gas temperatures are several 104 K
Doppler broadening through bulk motion of gas in gravitational field
with velocities as high as 0.1c, the distance from the Black Hole can be as close as 100 Rs
Comparison of continuum and BLR fluxes indicate that only 10% of the continuum radiation is absorbed by BLR clouds
The volume filling factor is very low - a few millionth of the central region is occupied by BLR 'clouds'
The necessary mass in the BLR to produce the observed luminosity is only a few solar masses
Broad-lines are very smooth - they are either made up of a huge number of small clouds or represent a coherent structure

56. Reverberation Mapping
Lines from highly ionized gas (He II 1640, C IV 1549) respond
faster than lines from lower ionization levels (e.g. Balmer lines)
ionization structure in BLR
more highly excited lines are further in

57. Reverberation Mapping
Size of BLR
For Keplerian rotation, the FWHM of the lines should correspond to the typical velocity dispersion at the radius where the line is produced.
More highly ionized lines, which are closer in, should have larger
FWHM and shorter time-lags.

59. AGN Unification Schemes
Luminous AGN are classified as:
• Seyfert galaxies (Type I and II)
• Quasars, QSOs
• BL Lacs
• Radio galaxies (in `Broad line’ and `Narrow line’ variants)
LINERs
All powered by accretion onto supermassive black holes.
But why so many classes - are these all physically distinct
objects?

60. AGN: Basic Ingredients
An AGN consists of the following basic ingredients:
Black Hole (power source)
Accretion Disk (UV/x-rays)
Jet (radio)
-Core (compact, flat-spectrum, radio-to-gamma emission)
-Jet
-Lobes & Hotspots (extended, steep spectrum)
Broad-Line Region (BLR)
Narrow-Line Region (NLR)
molecular (dusty) “torus" (feeding and obscuration)
host galaxy (feeding)

61. Seyferts 1 and 2: Unification Scheme
In the broad-line region (BLR)
The Keplerian orbital speeds of the clouds around the central massive body will be large => lines are Doppler broadened.
Density is high => no forbidden lines are emitted
In the narrow-line region (NLR)
The Keplerian orbital speeds of the clouds will be much smaller => lines are narrow
Density is low => forbidden lines are emitted

62. Seyferts 1 and 2: Unification Scheme
So, if the above Seyfert were viewed from direction (1), you would see:
Broad permitted lines
Narrow Forbidden Lines
Bright continuum from the central engine
i.e. a Seyfert 1
If, on the other hand, it were viewed from direction (2), you would see:
No broad permitted lines (obscured by dust torus)
No bright continuum from the obscured central engine
except in the infrared and X-ray region, which gets through the dust
i.e. a Seyfert 2

63. Seyferts 1 and 2: Unification Scheme: Evidence for Torus