Jet-Environment Interactions in FRI Radio Galaxies

Robert Laing (ESO)

Blandford’s Tasks

2 Map jet velocity fields

4 Understand the changing composition

5 Measure jet pressures

6 Deduce jet confinement mechanisms

7 Infer jet powers, thrusts

Jets in radio galaxies – FR classes

FRI – low power

Morphologicalclassification

FRII – high power

FRI/FRII division

FR classes are clearlydivided in the radio luminosity – stellar isophotal luminosity plane

(Ledlow & Owen 1996)

Varieties of FRI

Sources with well-definedlobe edges are in the majority in complete samples

Cen A motions

Variety of knot speeds on kpc scalesmeasured in radio: 0 – 0.5c

Hardcastle et al. (2003) Also M87

Jet/counter-jet ratio

Core prominence correlatedwith jet/counter-jet ratio

Ij/Icj decreases with distance1 at 10 kpc

Intrinsic symmetry is a goodapproximation for jet bases

For isotropic emission in the rest frame ratio of flux densities per unitlength:Ij/Icj = [(1 + βcosθ)/ (1 - βcosθ)]2+α

Degree of polarization

Asymmetry in Icorrelated with degree of polarization

Sidedness ratio image

3C296Divide I image by a copyof itself rotated by 1800

Explanations for asymmetry

Why does the side-to-side asymmetry decrease with distance from the nucleus?

Why is there an asymmetry in the intrinsic polarization structure which correlates with the intensity asymmetry (and also disappears at large distances)?

Why does polarized emission from the brighter jet suffer less Faraday rotation?

Why is the brighter jet more centrally peaked? The only natural explanation is that the jets are relativistic,

symmetrical, decelerating and faster on-axis than at their edges.

Intrinsic and environmental effects become dominant, but only on larger scales.

Breaking the β – θ degeneracy

For isotropic emission in the rest frame, jet/counter-jet ratio depends on βcosθ – how to separate?

B is not isotropic, so rest-frame emission (IQU) depends on angle to line of sight in that frame θ′

sin θ′ = D sin θ and D = [Γ(1± βcosθ)]-1 is different for the main and counter-jets

So the polarization is different for the two jets

If we knew the field, we could separate β and θ

We don’t, but we can fit the transverse variation of polarization and determine field component ratios

Need good transverse resolution and polarization

Geometry

FR1 jets flare and thenrecollimate

Abrupt brighteningclose to nucleus

Complex fine structure in brightregion

Fits (1)

θ = 8o 38o

Fits (2)

θ = 58o

Fits (3)

θ = 52o 64o

Degree of polarization (1)

θ 8o 38o 52o

Degree of polarization (2)

θ 58o 64o

Velocity β = v/c: deceleration and transverse gradients

3C 31 B2 0326+39

NGC 315 3C296

Velocity, spines and shear layers

β ≈ 0.8-0.9 where the jets first brighten

All of the jets decelerate abruptly in the flaring region, but at different distances from the nucleus.

At larger distances, four have roughly constant velocities in the range β ≈ 0.1 – 0.4 and one (3C 31) decelerates slowly

They have transverse velocity gradients, with edge/on-axis velocity consistent with 0.7 everywhere, except for 3C296, which has a very low fraction edge velocity 0.1 [something to do with the lobe structure?].

No narrow shear layers

Why don’t we see more evolution in the profile, as expected for boundary-layer entrainment?

Acceleration → Deceleration

Open squares: Ij/Icj

Filled squares: VLBIproper motions (Cottonet al. 1999)

Curves: on-axis and edges velocities from jet models (Canvin et al.2005)

M87 similar?

Real acceleration, or arewe seeing a slow outer layeron small scales?

NGC 315

Geometry, velocity and emissivity

NGC 315 (Canvinet al. 2005)

Emissivity profileflattens with distance

Not “adiabatic”until after recollimation

Brightening point is always closer to the nucleus than the start ofrapid deceleration, which in turn is complete before recollimation.

Field component evolution

Toroidal

Radial

Longitudinal

Longitudinal and toroidal components are comparable close to the nucleus

Toroidal dominates at large distances

Both components could bedisordered; ordered toroidal +longitudinal with many reversalsalso consistent

Radial component weak; no obvious regularities

Not simple flux freezing

Conservation law analysis

We now know the velocity and area of the jet.

The external density and pressure come from X-ray observations (Chandra/XMM-Newton).

Solve for conservation of momentum, matter and energy.

Include buoyancy

Well-constrained solutions exist.

Key assumptions:

Energy flux = momentum flux x c

Pressure balance after recollimation

Pressure and density

3C31 0326+39 3C296

Mach number and entrainment rate

Stars

Trends If the internal and external pressures are equal after

recollimation, then the flaring region is overpressured at the brightening point.

p > pmin (an essential consistency check). Assumption of external pressure confinement is self-consistent. The jets are often close to minimum pressure in the outer region.

Densities are low (equivalent to ~1 proton m-3)

Mach numbers are 1 – 3 (transonic)

Entrainment rates are comparable with those expected from stars in the jet volume at the start of the expansion, but not at large distances.

Continuing deceleration in 3C31 due to larger core radius of hot gas?

What are the jets in 3C31 made of?

= 2.3 x 10-27 kg m-3 (equivalent to 1.4 protons m-3) at the flaring point.

For a power-law energy distribution of radiating electrons, n = 60

min-1.1 m-3 (~10-28

min-1.1 kg m-3).

Possibilities include:

Pure e+e- plasma with an excess of particles over a power law at low energies.

e+e- plasma with a small amount of thermal plasma.

Cold protons in equal numbers with radiating electrons and

min = 20 - 50 (not observable).

Jet energy flux

Filled squares: conservation-law analysis

Open squares: X-ray cavities (Birzanet al. 2004)

Lines: Willott et al. scaling relation for FRIIsources

Significantly higher thanprevious estimates.

Questions

What causes jets to brighten and flare in the radio?

Can we tie down the particle acceleration mechanism(s)?

Mass input: stars and gas? Models to test?

Can we detect internal Faraday depolarization now that we understand the foreground better?

Can we get consistent estimates of jet power from cavities and conservation law analyses for the same sources?

Do jets really accelerate on pc scales and then decelerate again?

Can we model FRII, pc-scale and microquasar jets (EVLA, eMERLIN, ALMA, broad-band VLBI)?