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Right, it’s about time this blog went extragalactic again. As Douglas Adams wrote, “Space…is big. Really big. You just won’t believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space”. With all that Universe available we shouldn’t spend all our time talking about things in our own back garden! Today, therefore, I want to talk about some new results concerning the behaviour of quasars and radio galaxies – two types of radio loud, active galactic nuclei or AGN.
Before I start though I think I need a paragraph or two to explain what exactly an AGN is, to save those of you who don’t know from having to follow my links to Wikipedia above! Everybody else, I’ll try and be brief but please skip ahead if it gets boring…
So, the first thing to note is that it looks like most, if not all, galaxies have a massive black hole at their centre. Mostly they sit there, minding their own business, not drawing attention to themselves (the one in the Milky Way, the galaxy we live in, is like this – we know it’s there because people have tracked the stars that orbit it). Occasionally though the black hole is surrounded by a rapidly rotating disk of gas and dust, in the process of accreting onto it (I always think of this as being analogous to water swirling round a plughole). Collisions in the disk heat the material and result in the emission of massive amounts of radiation from this small region – so much that the galaxy’s nucleus outshines the combined light of all its stars! Hence the term ‘active nucleus’ or AGN. A further twist to this picture is that about a tenth of these objects also produce twin giant, radio emitting, jets, which emerge perpendicular to the accretion disk, and extend far beyond the extent of the host galaxy, before depositing their energy in huge puffed up lobes. It’s not clear what starts these jets as there’s no telescope good enough to see into this region.
Imagine now viewing one of these AGN from lots of different angles – it would look completely different depending on how it was oriented towards you. When these things were first discovered they were classed as many different types of object because of this, and it took a long time before people realised they could all be linked together. The picture below (ref. here) is a good illustration of this for a radio loud AGN (though bear in mind that not all AGN have all of these features). Looking edge on, the bright central nucleus is obscured by a large, dust torus so the light from the host galaxy isn’t drowned out, and only the jets are seen – a radio galaxy. Increase the angle and the nucleus is no longer obscured so it overshadows everything (including, sometimes, the jets) – a quasar. See here for a more detailed explanation of this!
Ok, now everybody hopefully has some idea what a radio galaxy and a quasar are, and how they’re related we can get back to the point, assuming anyone’s still interested (please still be interested). AGN lifetimes are pretty short compared to the age of the galaxy hosting them – they only last for as long as they have fuel. However, they could presumably reignite if they were refueled, maybe through a merger. When the AGN switches off, the jets would also disappear, but the lobes would linger, slowly depleting the reservoir of energy that’s been deposited in them. This means that the remnants of a previous cycle of activity could still be present at the beginning of the next one. This is exactly what was seen in four radio galaxies by Schoenmakers et al. in a paper published in 2000. They called them Double Double Radio Galaxies as they have two pairs of lobes – one new and one old. Since then, about ten more of these have been identified (including one with three lobe pairs), but none in other radio loud AGN as their orientation makes their jet/lobe structure harder to disentangle.
This all changed last month when Jamrozy et al. presented the first clear detection of a double double structure in a quasar. This is good news for the unification model – different types of radio loud AGN should behave in the same way if the only difference between them is orientation. It’s also more evidence for episodic activity. All that’s left to do now is figure out exactly why this happens… Finding and investigating more of these Double Double sources should hopefully help.
Schoenmakers, A., de Bruyn, A., Rottgering, H., van der Laan, H., & Kaiser, C. (2000). Radio galaxies with a ‘double-double morphology’ – I. Analysis of the radio properties and evidence for interrupted activity in active galactic nuclei Monthly Notices of the Royal Astronomical Society, 315 (2), 371-380 DOI: 10.1046/j.1365-8711.2000.03430.x
M. Jamrozy, D. J. Saikia, & C. Konar (2009). 4C02.27: a quasar with episodic activity? Accepted for publication in MNRAS arXiv: 0908.1508v1