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 Morphological classification FRII – high power

FRI/FRII division FR classes are clearly divided in the radio luminosity – stellar isophotal luminosity plane (Ledlow & Owen 1996)

Varieties of FRI Sources with well-defined lobe edges are in the majority in complete samples

Cen A motions Variety of knot speeds on kpc scales measured in radio: 0 – 0.5c Hardcastle et al. (2003) Also M87

Jet/counter-jet ratio Core prominence correlated with jet/counter-jet ratio I j /I cj decreases with distance  1 at 10 kpc Intrinsic symmetry is a good approximation for jet bases For isotropic emission in the rest frame ratio of flux densities per unit length: I j /I cj = [(1 + βcosθ)/ (1 - βcosθ)] 2+α

Degree of polarization Asymmetry in I correlated with degree of polarization

Sidedness ratio image 3C296 Divide I image by a copy of itself rotated by 180 0

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 then recollimate Abrupt brightening close to nucleus Complex fine structure in bright region

Fits (1) θ = 8 o 38 o

Fits (2) θ = 58 o

Fits (3) θ = 52 o 64 o

Degree of polarization (1) θ 8 o 38 o 52 o

Degree of polarization (2) θ 58 o 64 o

Velocity β = v/c: deceleration and transverse gradients 3C 31 B NGC 315 3C296

Velocity, spines and shear layers β ≈ 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: I j /I cj Filled squares: VLBI proper motions (Cotton et al. 1999) Curves: on-axis and edges velocities from jet models (Canvin et al. 2005) M87 similar? Real acceleration, or are we seeing a slow outer layer on small scales? NGC 315

Geometry, velocity and emissivity NGC 315 (Canvin et al. 2005) Emissivity profile flattens with distance Not “adiabatic” until after recollimation Brightening point is always closer to the nucleus than the start of rapid 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 be disordered; ordered toroidal + longitudinal with many reversals also 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 3C C296

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 > p min (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 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 (~  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 = (not observable).

Jet energy flux Filled squares: conservation-law analysis Open squares: X-ray cavities (Birzan et al. 2004) Lines: Willott et al. scaling relation for FRII sources Significantly higher than previous 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)?