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Radio Galaxies part 4. Apart from the radio the thin accretion disk around the AGN produces optical, UV, X-ray radiation. The optical spectrum emitted by the gas depends upon the abundances of different elements, local ionization, density and temperature .

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Apart from the radio

the thin accretion disk around the AGN produces

optical, UV, X-ray radiation

The optical spectrum emitted by the gas depends upon the abundances of different elements, local ionization, density and temperature.

  • Photons with energy > 13.6 eV are absorbed by hydrogen atoms.

In the process of recombining, line photons are emitted and this is

the origin e.g. of Balmer-line spectra.

  • Collision between thermal electrons and ions excites the low-energy level

of the ions, downward transition leads to the emission of so-called

“forbidden-line” spectrum (possible in low density conditions).


Optical spectrum, what can we derive:

  • which lines
  • flux/luminosity
  • width (kinematics)
  • ionization mechanism (line ratios)
  • density/temperature of the emitting gas
  • morphology of the ionized gas
  • (any relation with the radio?)
  • continuum and stellar population

using spectra and narrow band images


Ionization parameter:

  • ratio between ionizing photon flux/gas density
  • Temperature of the emitting gas
  • Mass of the emitting gas


models for different

ionization parameters

Examples of

diagnostic diagrams


Broad line regions (BLR):

  • typical size (from variability)
  • of 10-100 light-days (Seyferts) up to
  • few light-years (few x 0.3 pc, quasars).
  • electron density is at least 108 cm-3

(from the absence of broad forbidden lines)

  • typical velocities 3000-10000 km/s
  • Narrow line regions (NLR):
  • typical density 103 to 106 cm-3
  • gas velocity 300 – 1000 km/s
  • large range in size: from 100-300 pc to tens of kpc
powerful radio galaxies energetics
Powerful radio galaxies: energetics

Quasar luminosity:1044 — 1047 erg s-1

Luminosity integrated over lifetime:1057—1062 erg

  • Radiation
  • Jets
  • Winds

Jet power:1043 —1047 erg s-1

Jet power integrated over lifetime:1057 — 1062 erg

Total wind power:1043 — 1046 erg s-1

Wind power integrated over lifetime:1056 — 1061 erg

+ Starburst-induced superwinds….

emission line haloes 1kpc scale
Emission line haloes: <1kpc scale
  • Kinematics.The emission line kinematics comprise a combination of gravitational motions, AGN-induced outflows, and AGN-induced turbulence
  • Black hole masses.Now possible to determine direct dynamical masses for nearby PRG using near-nuclear emission line kinematics
  • Feedback.The outflow component provides direct evidence for the AGN-induced feedback in the near-nuclear regions

the presence of the nuclear activity could influence the evolution

of the galaxy (e.g. clear gas away from the nuclear regions)

cygnus a viewed by hst
Cygnus A viewed by HST

NICMOS 2.0mm

Optical images


2.0 micron image


Evidence for a super-massive black hole in Cygnus A


broad permitted line seen in polarized line: only the scattered component can be seen

Broad- and narrow line radio galaxies

become undistinguishable

emission line nebulae 1 5kpc scale
Emission line nebulae: 1-5kpc scale
  • Kinematics.Emission line kinematics a combination of AGN-induced and gravitational motions
  • Ionization.Gas predominantly photoionized by the AGN

Outflows.Clear evidence for emission line outflows in Cygnus A and some compact radio sources, but outflow driving mechanism uncertain


Example of complex kinematics


700 km/s

Complex kinematics

of the ionized gas in

coincidence with the

radio emission:

this suggests interaction

between radio plasma and ISM

emission lines in powerful radio galaxies


[O III]λλ4959,5007

z = 0.1501 ± 0.0002

FWHM ~ 1350 km s-1

Δz ~ 600 km s-1


Relative flux


[O II] λλ3727

z = 0.1526 ± 0.0002

FWHM ~ 650 km s-1



[O II]

[Ne III]

[Ne V]

(Tadhunter et al 2001)

Emission lines in (powerful) radio galaxies

Wavelength (Å)

emission line nebulae 5 100kpc scale
Emission line nebulae: 5-100kpc scale
  • Kinematics.Activity-induced gas motions are important along the full spatial extent of the radio structures, regardless of the ionization mechanism
  • Jet-induced shocks.The shocks that boost the surface brightness of the structures along the radio axes also induce extreme kinematics disturbance
  • Gravitational motions.Require full spatial mapping of the emission line kinematics in order to disentangle gravitational from AGN-induced gas motions
  • Starbursts.Starburst-induced superwinds may also affect the gas kinematics out to 10’s of kpc

Gas with very high ionization at

8 kpc from the nucleus

Even if the nucleus is obscured by the torus, the extended emission line regions can tell us about the UV radiation from the nucleus.


Contours: radio

Colors: ionized gas

In some cases the radio

galaxy seems to have a

strong effect on the

medium around.

Diagnostic diagrams important to understand which mechanism is dominant


Radio galaxies at high redshift

  • Morphology of the extended emission line regions
  • depends on the size of the radio source
  • Alignment between the emission lines and the radio axis
  • Interaction between radio and medium: does this also trigger star formation?

Any difference (in the optical lines) between

low and high power radio galaxies?



radio galaxy


radio galaxy

What makes the difference?

Well known dichotomy:

low vs high power radio galaxies

Differences not only in the radio


Intrinsic differences in the nuclear regions?

Accretion occurring at low rate and/or radiative efficiency?

No thick tori?


No optical core

Optical core

The central regions of low-power radio galaxies

No strong obscuration: optical core very often detected


From HST and X-ray

The HST observations:

  • High rate of optical cores detected
  • Correlation between fluxes of

optical and radio cores

But so far we haven’t seen broad permitted lines


The optical continuum of Radio Galaxies

Usually the old stellar

population is the dominant - as usual in elliptical galaxies - but in

some cases a young stellar population component is observed

(typical ages between 0.5 and 2 Gyr).


  • consistent with the merger
  • hypothesis for the triggering
  • of the radio activity.
  • but not a single type of merger
  • AGN appears late after the


old stellar pop.

young stellar pop.

power law





Results from

UV imaging

Allen et al. 2002


The young stellar component may come from

  • a recent merger
  • We can use the age of the stars to date when this merger
  • occurred
  • To be compared with the age of the radio source