Radio Galaxies Part 2

A prototypical radio galaxy

Any size: from pc to Mpc First order similar radio morphology (but differences depending on radio power, optical luminosity & orientation) Typical radio power 1023 to 1028 W/Hz

Lobes

Core

JetsHot-spots

The morphology of a radio galaxy may depend ondifferent parameters:

- radio power (related to the power of the AGN?) - orientation of the radio emission - intrinsic differences in the (nuclear regions of) host galaxy - environment

….but radio galaxies are not all the same!

The morphology does not depend on size!

~20 pc

~200 kpc

Effects of the interactionwith the environment

Effects of age

Two main flavors

Fanaroff and Riley type I and II

Edge-brightenedhigh radio power

Edge-darkenedlower radio power

  FRI FRII

Jets Large opening angle low Mach number(but not on small scales)

Very collimatedhigh Mach number(relativistic on small scales)Magnetic Field  Perpendicular

to the jet Parallel to the jet

Hot-spot -- Yes

Lobes Plume-like  Backflow

Spectral index in the Lobes

Steeper away from the nucleus

Steeper toward the nucleus (from hot-spots)

o The reason(s) for these differences is not completely clear; likely related to the nuclear regions.o Differences are seen also in other wavebands.o Possibly also environment: lower-power radio galaxies tend to be in clusters

Two flavors also for the jets: supersonic and highly collimated subsonic with entrainment

This can explain the presence of

hot-spots and the collimation of the

jets.

Radio jets

Simulations of an high Mach number radio jet(velocity of the plasma >> sound speed)

Movie M = 10

Movie hot-spot

What makes the difference?

Intrinsic differences in the nuclear regions?

Accretion occurring at low rate and/or radiative efficiency?

No thick tori?

Well known dichotomy: low vs high power radio galaxies

Differences not only in the radio WHY?

Also important: the resolution of the observations!

Importance of observations at different resolution

M87radio emission at differentfrequencies and resolutions

Often the radio emission is more symmetric on the large scale and asymmetric on the small scale

core

The core is defined based on the spectral index: flat ( ~ 0)

[to find which component is the radio core is not always easy:free-free absorption can complicate the story!]

What are the characteristics of the jets close to the AGN?

Discovered (around 1970-80) in powerful radio galaxies and quasars: apparent change (on the VLBI scale) in the structure of some sources during a period of few months. the velocities appear superluminal the components of the velocities and direction remain constant there are no observed “contractions” a flux outburst seems to be associated with the appearance of new components

Case of 3C273 (quasar) apparent velocity ~10c

Superluminal motions

These projection effects explain: the apparent superluminal motion the asymmetry between the two jets, also the flux of the approaching and receding components are affected by projection

These are among the methods used to find out the orientation of a source

)cos1/(sin vv

Not all the jets are superluminal

VLBI observations of Centaurus A (between 1991 and 1996)

Apparent motion subluminal speed ~ 0.1c

However this does not seem to be characteristics common to all lower power (Fanaroff-Riley I) radio galaxies

Apparent motionof the componentsbetween 4 and 6 cbut very complex.

3C120

The structure can be complicated, perhaps from the interaction with the inter-stellar medium New components can be seen to appear

Going very close to the BHto see how the collimation of the jet works.

rapid broadening of the jet opening angle as the core is approached on scale below 1 mas (0.1 pc).

~ 1 mas = 0.071 pc

43 GHz VLBI

M 87

The jet does not seem to reach a complete collimation until a distance of many tens of Schwarzschild radii (escape velocity = c)

jet emanating from the accretion disk, not yet collimated

Optical Jets

some radio jets have an optical counter-part

few more cases found by HST (high resolution is needed)

the more the jet is “beamed” toward us the more is likely we see the optical part

Origin of the optical emission:likely an extension of the synchrotron spectrum.Lifetime of the electrons very short Acceleration mechanisms need to be present.

Optical jet in M87 (NGC 4486 in Virgo)

M87Radio

Optical

X-ray

X-ray +optical contours

The new X-ray satellite Chandra has shown that many radio jet have also an X-ray counterpart

Jets in optical and X-ray

opticalX-ray

Almost identical morphology from the radio to X-ray band:the optical, UV and X-ray data (and spectral indices) are consistent with synchrotron emission

Other possibility : inverse Compton effect

Relativistic electrons in a radiation field. Because of the interaction with the photons, the electrons loose energy while the outcome are photons with higher energy. This interaction can take place between the relativistic electron producing the radio emission and either the radio photons or the photons from the cosmic micro-wave background.

The electrons must have high (107) and very short life-time (<<yrs)

Radio and X-rayof the jet in Centaurus A

Very detailed image, scale of the arcsec (~18 pc). Small offset between the radio and X-ray emission.The X-ray trace the regions where the electrons with high are accelerated NOW.

pink = radio blue = X-ray

HST, Chandra, Merlin

3C273 (quasar)

An other well studied case.

o Also radio hot-spots are now found to have optical and X-ray counter-parts

A look at the nuclear regions

Many indications that the view of the central regions is “orientation dependent: superluminal motion of the jets broad optical lines free-free absorption

Evidence for a circum-nuclear disk/torus

Seyfert galaxy NGC4258(from water masers)

Evidence of absorption from the X-ray spectra(column density above 1024 cm-2)

Evidence of obscuring “torus”from the optical broad line observed in polarized lightin narrow-line radio galaxies

The inner edge of the “torus” (<0.3 pc) is likely to be completely ionized from the radiation from the AGN.The presence of this ionized medium will cause “free-free” absorption of the radio emissionfrom the nuclear radio components.This effect is prominent at longer wavelengths.

Presence of ionized gas

Component that “appears” only at high frequency(absorbed at lower frequencies) REAL NUCLEUS!?!

Neutral hydrogen in the nuclear regions

From the absorption, the column density of the neutral hydrogen can be derived.Typical numbers are 1020 to 1021 cm-2

VLBI data

HST image

Structure of the circumnuclear torus 1946+708 Mrk 231

Nuclear dust/gas disks very common in radio galaxies (as we will see tomorrow!)

These characteristics (different morphologies, superluminal motions, presence of nuclear tori etc.) are extremely important when we will try to “unify” different type of AGN.

A key parameter is the orientation of the jets with respect to the line-of-sight.