Active Galaxy Jets – An exhausting business

Diana WorrallUniversity of Bristol

ThreadA massive black hole (BH) lives at the centre of every

galaxy. ~10% of the BHs are ‘active’ – luminous centre

~10% of these eject radio-emitting jets of relativistic particles and magnetic field, of high mechanical power

Jets heat the surroundings, effect the evolution of stars in the galaxy and the growth of the BHs

Feedback between the very large (>100,000 light years) and very small ( < 1 light day) scales

Argument for massive black hole in centre of our Galaxy is robust

Ghez et al.

Infrared light sees through to centre of Galaxy. Stellar orbits. Kepler’s laws.

4.5 106 MO

All galaxies seem to contain central black holes of MBH ~ 106-109 Mo

Wisdom of the last decade:• Some nearby galaxies have good

measurements of MBH using

motions of central stars.• These masses correlate with the

random speeds of stars in their large-scale stellar bulges

(M- relation)

then used to infer MBH

Seems that the black hole and the galaxy grow together

In ~10% of galaxies, the radiative luminosity from near the black hole outshines starlight.

• Black holes are so massive that infalling material gains huge speeds.

• If material fell into the black hole there would be no radiation.

• Material comes in at glancing angles and orbits the black hole. Builds up. Friction -> heat -> radiation

~1% of galaxies have radio-emitting jets

• Pictures made with arrays of radio telescopes show jets extending far beyond the stars of the galaxy

• Bright centres may be partially obscured by dust rings

Images from VLANew Mexico

Synchrotron Radiation

Why do jets light up in the radio?

• Outflowing plasma of magnetic field and particles

• Light particles (electron and positrons) moving at speeds close to that of light emit synchrotron radiation. Mechanism is known because the radiation is polarized.

radio

What particles are jets made of?

• There must be electrons to give synchrotron radiation

• Charge must be neutral• Other particles could be positrons or

protons (~2000 times more massive)

How are jets produced?

Difficult theoretical issues, Maybe related to a magnetic dynamo.

Jet asymmetry can be attributed to the brighter jet coming towards us and special relativity if outflow is at speed close to that of light

Relatively fast outflows, but how fast?

X-ray synchrotron emission (with Chandra)

In the nearest galaxy jets, the X-rays show knotty structures, probing sites where electrons are boosted to the highest energies.

Core

~3000 light years

More distant & luminous (quasar) jets are of similar length in X-ray and radio.

Plausible explanation is X-rays are from low-energy electrons scattering the cosmic microwave background radiation. Then jets must travel near c on vast scales for special relativity to assist.

Much kinetic power (~1039 W)

PKS 0637

Radio contoursChandra image

Another indicator of the huge power in jets is that theybore holes in the diffuse gas through which they travel.

This ‘gas’ comprises free electrons and protons of density about 1000 particles per cubic metre moving less fast than particles in jets (electrons at ~0.1c, protons at ~0.002c).

Emission from this gas is in the X-ray

McNamara & Nulsen

200 kpcHydra A

McNamara et al

30 kpc

200 kpc

MS0735.6+7421

Measure how much gas has been pushed aside.

‘Guesstimate’ the age of the source to get the power in the expansion (usually ~10 million years)

Results confirm substantial jet power, >30 times the total radiative power.

Power results in heating and helps explain why BHs and galaxies grow together

more jet

lessgas infall

less blackhole growthand star formation

lessjet

moregasinfall

more blackhole growthand star formation

Feedback

Summary• All galaxies have central massive BHs.

• ~1% produce jets with outflow powers >~30 times the total radiative power of the galaxy.

• Something ‘black’ can dictate a galaxy’s energy!

• Something the size of a pea can regulate something the size of the earth!

Where is the jet power dissipated? Details uncertain but….

Paradigm is that distributed heating over cosmic time occurs principally through multiple, small, buoyant cavities (motions in Swiss Cheese, gas mixing).Need: 1) inflated lobes 2) jets changing direction 3) intermittencyPower dissipation can lead to less gas infall to fuel the black hole. Also, heating stops the gas cooling to make stars.This may help explain why black holes and galaxies grow together.