How do galaxies grow? One of the most common ways seems to be by merging with other nearby galaxies (a hot research topic that Rita’s talked about in more detail). Seems simple enough, but to really understand how this happens you need to look at a large number of them, at various stages of the merging process.
One way to approach this is by studying active galactic nuclei (AGN) – the extremely bright central regions seen in some galaxies caused by accretion onto their supermassive black holes. Mergers are thought to trigger AGN by funneling cold gas onto the black hole and creating the accretion disc. However, both of the incoming galaxies involved in the merger have their own central black holes, so, if this funelling happens before they themselves merge together, two AGN could be ignited. If you could find systems like this, ideally with a range of separations, what you’ll actually have is a snapshot of many different merging stages. Plus, since AGN only live for a short time, you’ll also be able to get a handle on how long all this takes.
Hundreds of these dual-AGN systems have been detected, but only a small proportion contain AGN that are closer together than 10 kiloparsecs, meaning that we’re missing information on the final stages of the merging process. The situation improved recently with the publication of a paper reporting a pair that are separated by only 4.8 kiloparsecs. One source isn’t enough though to draw conclusions about mergers in general of course, but finding one should make it easier to find more. When it comes to AGN, sometimes two are better than one.
R.C. McGurk, C.E. Max, D.J. Rosario, G.A. Shields, K.L. Smith, S.A. Wright (2011). Spatially-Resolved Spectroscopy of SDSS J0952+2552: a confirmed Dual AGN Submitted to ApJL DOI: arXiv:1107.2651
Within the constellation of Ursa Major, about 134 million light years away, an almighty collision is occurring between two galaxies. As the clouds of gas and dust are swirled together an intense burst of star formation is triggered, but is that all that’s been awoken? Has this galactic merger also provided a hidden central black hole with enough matter to accrete to ignite it into an active nucleus (AGN)? This wasn’t the question that Miguel Perez-Torres and his team set out to answer. They were trying to characterise the compact sources within the inner region of Arp 299 (as the merging pair is known), which they were generally expecting to be supernovae and supernova remnants. This is what they found:
The top panel in this picture from their paper shows the inner region they observed, and all the white blobs are the compact sources they were looking for. What caught their interest though was the line of objects toward the top right of this image. This, they reasoned, wasn’t a chance alignment but could either be a chain of supernovae in a super star cluster approximately 500 years old, or a core and jet of a hidden AGN. Normally, if a galaxy (or pair of interacting galaxies) hosts an AGN its output dwarfs the light coming from all the stars in the galaxy combined. However, sometimes (and possibly more often than we realise) the AGN is weak and can only be seen if you really go looking for it.
Super star clusters are known to exist in Arp 299. However, finding four supernovae in one this size and age is very unlikely they reasoned, as it shouldn’t have housed enough massive stars for this to happen. So they had a closer look – the middle and bottom panels of the picture show their close ups of the top right region (indicated by white dotted lines). See how the blobs are linked in the bottom image? That, they concluded is a clear indication of an AGN core with accompanying radio jet. Interestingly, the object labelled A0 isn’t part of the AGN, but has all the characteristics of a young supernova, making it, as they note, “…one of the closest to a central supermassive black hole ever detected.” This could be the reason why Arp 299’s AGN is so weak – nearby massive stars heating their surroundings and dispelling the material that it would normally accrete.
All three of these are radio images, taken at a frequency of 5 GHz (top and middle) and 1.7 GHz (bottom) with an array of linked radio telescopes stretching from Shanghai to Cambridge . Radio is ideal for this investigation as it is not absorbed by the large amounts of dust enshrouding Arp 299’s nuclear region.
Just goes to show what you can find when you don’t go looking for it.
Perez-Torres, Miguel A.; Alberdi, Antxon; Romero-Canizales, Cristina; Bondi, Marco (2010). Serendipitous discovery of the long-sought AGN in Arp 299-A Accepted for publication in Letters to Astronomy and Astrophysics : 1008.4466
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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