Jet Physics: implications for feedback. Robert Laing (ESO)

Transcription

1 Jet Physics: implications for feedback Robert Laing (ESO)

2 Aims Quantify the physics of jets in low-power radio galaxies by fitting 3D models to very deep, high-resolution radio images. Geometry Velocity field Composition (electron, positron, proton, Poynting flux, thermal plasma,...) Particle energy spectrum Magnetic field structure and strength inside and outside Interactions with the external medium Study kpc scales initially, where we have adequate spatial resolution now. Then work towards the nucleus. This talk: overview + new results on jet propagation in lobed FRI sources + new energy flux estimates feedback

3 Deep, multiconfiguration VLA observations Spectra High-resolution IQU, Faraday rotation corrected for Faraday images rotation Optical/IR (stellar mass loss) Kinematic models of jets Particle acceleration CO line, dust B in IGM (cool gas) Entrainment mechanism Dynamical models Jet images Models of hot gas distribution Chandra/XMM Robert Laing (ESO) Alan Bridle, Bill Cotton (NRAO) James Canvin (ex-oxford) Daria Guidetti (Bologna/ESO) Paola Parma (Bologna) Mark Birkinshaw, Diana Worrall (Bristol) Martin Hardcastle, Judith Croston (Herts)

4 The sources (1) - tails

5 The sources (2) - lobes

6 Spectral variations - 3C270 Jets shielded from contact with the IGM (except close to the nucleus?)

7 Relativistic flows in FRI jets Proper motions in Cen A (0.5c) and M87 (5c) on kpc scales Superluminal motion on pc scales (e.g. NGC 315) Evidence for relativistic flows in BL Lac objects (fast variability; TeV gamma rays; high brightness temperatures). There must be side-on counterparts: FRI radio galaxies are the only candidates. TeV detection of M87 and Cen A (maybe 3C66B) Jet/counter-jet asymmetry decreasing away from the nucleus, correlated with intrinsic polarization, fractional core power. Asymmetry in external Faraday rotation/depolarization correlated with jet sidedness.

8 Relativistic effects in jets Energy spectrum Doppler boosting Doppler factor Jet/counter-jet ratio (isotropic emission) Aberration Superluminal motion vapp = 30c

9 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

10 Making a model Model assumes intrinsic (side-to-side) symmetry, axisymmetry and stationary flow. Symmetry assumption must fail badly at large distances, but seems to be accurate close in. Choose parameterised functional forms for the geometry (angle to line of sight, flow streamlines; velocity field ; ratios of toroidal:longitudinal:radial field; emissivity. Calculate Stokes parameters, taking proper account of relativistic aberration (beaming); convolve and evaluate χ2. Optimise to determine best parameters of the model. Looked at tailed (plumed) sources first: jets propagate in contact with the external medium close to the nucleus, lobed sources are very similar, but trickier to separate jets.

11 Geometry FR1 jets flare and then recollimate Abrupt brightening close to nucleus Complex fine structure in bright region

12 Fits θ = 52o 38o

13 Degree of polarization θ 52o 38o

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

15 Velocity, spines, shear layers and all that β 0.8 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, most have roughly constant velocities in the range β and one or two (3C 31 certainly, NGC 315 maybe) decelerate slowly Evidence for evolution in the velocity profiles from ~top-hat to centrally peaked ( boundary-layer entrainment?) No narrow shear layers

16 Velocity parameters

17 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.

18 Field component evolution Longitudinal and toroidal components are comparable close to the nucleus Toroidal Toroidal dominates at large distances Ordered toroidal + longitudinal with many reversals or both components disordered? Radial component weak; no obvious regularities Not simple flux freezing Radial Longitudinal

19 Backflow? th Is one jet intrinsically wider? No: we see evidence for narrow structures on both sides of the nucleus but sometimes as minima on the counter-jet side. What if there is mildly relativistic backflow in the material immediately surrounding the jets?

20 Velocity fields in hydrodynamic simulations Fast backflow is expected in lobed FRI sources and is seen in hydrodynamical simulations with appropriate parametersfor FRI, but: - Can be an artefact of axisymmetry or low resolution - real backflows probably become turbulent and slow down - beware of open boundary conditions Simulation by Perucho & Marti (2007)

21 Data-model comparisons ( ) θ = 31o

22 Velocity fields (v/c)

23 Magnetic fields in backflows Dominant field component in backflow is toroidal: reminiscent of: (Begelman, Blandford & Rees 1984)

24 Backflow: conclusions Symmetrical backflow model gives a surprisingly good fit to the brightness and polarization of and Backflow is expected in lobed FRI sources: the surprise is that it can be described by an axisymmetric, fully symmetrical model. The backflow we model must also have enhanced emissivity (with α 0.6) iimmediately around the jet outflow. Typical velocities c, increasing away from axis Why so fast? [Not that well constrained, in fact]. Why does backflow disappear close to the nucleus? Does the backflow magnetic field contribute to collimation? Currents? No counter-examples yet. Clear predictions, so easy to test with more sources.

25 Conservation law analysis 1D version 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

26 Mass, energy and momentum flux conservation (Bicknell 1994)

27 Pressure and entrainment rate

28 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?

29 Comparison with other power estimates Open squares: Birzan et al. (2004) cavities Filled squares: conservation law Lines: Willott et al. (1999) FRII's

30 Direct comparison with cavity estimate XMM (Croston et al. 2008) get 6 x 1035 W Conservation law analysis gives 8 x 1036 W

31 What are the jets in 3C31 made of? ρ = 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, = 60 γmin-1.1 m-3 (~10-28 γmin-1.1 kg m-3). n 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 γmin = (not observable).

32 Mixing-layer model Yang, Kaiser, RL et al. (2009)

33 Mixing-layer model Flaring region Outer region

34 Quasi-1D and Mixing-layer models Mixing-layer model starts with a denser jet. Energy flux and eventual mass flux are similar for the two models.

35 The IGM is magnetised implications for feedback Rotation measure images (Guidetti et al., in prep.)

36 Summary Kinematic models of deceleration from relativistic speeds in FRI jets now 10 examples Model velocity, emissivity, field ordering Backflow? Two flavours of conservation-law model: No variations across the jet (stellar mass input) Mixing layer (boundary-layer entrainment) Entrainment required only in the flaring region (except for 3C31) Jet powers similar to those deduced from X-ray cavities; detailed comparisons needed next On to FRII jets...