March’s talk: The environmental impact of radio galaxies by Prof. M. Hardcastle

Martin Hardcastle (University of Hertfordshire) wanted to call this a follow up talk from the talk he gave us in April 2017!  ‘The environmental impact of radio galaxies’

Prof Hardcastle originally used information from the Spitzer infrared nearby galaxies survey which gave statistics for different kinds of galaxies.  Of interest to him are the largest galaxies and clusters of galaxies, as these have the strongest radio emissions.  There is so much more information out there now, and the idea of dark matter holding galaxies and galaxy clusters together makes so much more sense.

The galaxy cluster Abell 1689 is a case in point; it has a high density of matter and when viewed in x-rays by the Chandra satellite the centre of the cluster is full of hot gas.  A Hubble view shows rather beautiful gravitational lensing.  The density of matter is so high in the centre that as galaxies form they appear to be falling towards the centre, where the temperature appears to reach 10 million kelvin.  How?  Yet the outer reaches cool quite slowly, over tens to 100 millions of years.  Why are the central regions staying so warm?  You’d expect the central galaxies to get more and more massive as everything gravitates inward, and outer layers of the cluster to get cooler.  

This is what he calls the ‘cooling flow problem’.  

(Here come black holes again….)

Evidence indicates that the central galaxy has a massive black hole.

The Milky Way is a larger than average spiral galaxy with a black hole in the centre, of a mass about 3 million times the Sun’s mass.  The central black hole of some galaxies is much bigger.  The one in NGC 4649 (a spherical galaxy) has a mass of about 2 billion solar masses.  Larger ones are available, and if you’re wondering about their size, even the largest would be (only?) the size of our solar system.  The M87 black hole (the famous doughnut image compiled by the Event Horizon radio telescope collaboration two years ago) is about one billion solar masses.

Radio galaxies  can have a radio profile much much bigger than their optical counterpart.  The largest so far identified is 15 million light years across.

The reason for these huge sizes appears to be that when the central black hole attracts matter onto it, conservation of momentum causes the resulting accretion disc to be highly wound up.  That orbiting matter becomes a superheated plasma as it concentrates inward and the magnetic field lines become more and more tangled.  These field lines are what pushes out the charged particles in the form of jets coming out of the poles.  The jets have fascinated us because they get accelerated to relativistic speeds in order to escape the pull of the black hole.  We have known for years that they can even appear to be faster than light if they are coming towards us.  We know of a quasar with a jet apparently at 15.4 times the speed of light.   

You may have seen the pictures such as Cygnus A with its polar jets and radio envelope lobes.  X-ray views also show an envelope of hot blue gas, which continues to stay heated.  Centaurus A is also shown to have an envelope of hot gas pushing out from the poles and at a great speed in a bow shock.

When you get a galaxy cluster like the Perseus cluster, you can actually see the hot gas has some little irregularities in it.  This appears to be caused by what was spat out by the various galaxies and indicates that their central black holes went through quiescent phases where there was no stuff reaching them.  (Much like our Milky Way monster at the moment, although there are large ‘Fermi Bubbles’ of hot gas above both poles, indicating that it has been active in the last few million years.)

A lot of radio data is now available from the LOFAR northern 2 metre sky survey (called LoTSS = LOFAR Two metre Sky Survey).  There was a data release in February 2019, but it is too large for the scientists to wade through.  That means an appeal has gone out for citizen scientists to jump in (https://lofargalaxyzoo.nl) to look for optical counterparts to the radio data.  It seems the largest mass galaxies have these polar lobes.  The survey is hopefully to be completed this year, but the existing data release has produced lots of pretty pictures of tiny spots with huge radio lobes.  (That’s just for the northern hemisphere, remember.)

23,000 radio galaxies have so far been identified and these tend to be the ones with the highest masses and the most hot gas.  How much energy is coming out from their central black holes and how much is there in the jets and lobes?  It seems the hot gas is ten times the mass of the stars.  Will all this energy slow down the cooling of the universe?

The LOFAR data release number 2 has help coming from the William Herschel Telescope (La Palma) which is measuring the radio sources’ redshifts.  (WEAVE = WHT Enhanced Area Velocity Explorer, which is a spectrograph analysing LOFAR galaxies with redshifts less than z= 0.4)

Further data and support is being provided by the SKA (Square Kilometre Array) and the X-ray survey satellite e-ROSITA.  But human eyeballs are needed to pore over the data in order to find the tiny smudges that are these huge radio galaxies’ visual counterparts.

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