Today’s blog post is written by Radio Galaxy Zoo science team member Anna Kapinska, who works on radio jets and studies how they affect the galaxies which host them at various stages of the Universe’s evolution. This is the first of a two-part series.
As most of you know by now (after classifying hundreds of radio and infrared images in our amazing Radio Galaxy Zoo!) radio galaxies are not the type of object that most of us are used to. There’s no stars and dust; no light from that. It’s all about the jets – outflows of particles ejected from the vicinity of the galaxy’s monstrous supermassive black hole and moving nearly at the speed of light. Some of these particles, electrons, emit light while spiralling in the black hole magnetic field. By no means are the radio galaxies stationary or boring!
Radio galaxies can live for as long as a few hundred thousand or even a few million years. They grow and mature over that time, and so they change. They even don’t disappear straight after they die; that takes time too. So, how does the life of a radio galaxy look like?
Just as for ourselves, humans, we have names for different stages of radio galaxy life. There are newborn and young radio galaxies. There are adults – these are the most often encountered individua. There are also old, large giants. And there are dead radio galaxies – the slowly fading away breaths of magnificent lives – a rare encounter as they don’t stay with us for too long. So, how do these radio galaxies look like and what exactly happens during their lives? Over the next two blogs I will take you through the evolutionary stages of radio galaxies.
Let’s start off with a simple plot astronomers like to use for an overview of a radio galaxy evolution during its lifetime. The plot describes how the radio luminosity changes with the radio galaxy size (Figure 2). The larger the radio galaxy, the older it is (usually), so we can trace the changes in radio galaxy luminosity and structure (size) as it gets older.
The early years
After a radio galaxy is born it will be growing up very quickly – in a blink of an eye its jets will be penetrating dense environments out to the borders of the host galaxy (the one you can see in optical and infrared wavelengths). Astronomers call these sources GPS (Gigahertz peaked sources) and CSS (compact steep spectrum) radio sources. The GPS/CSS sources are very small; they reach only kilo-parsec distances from the central black hole (3.5 thousand light years), which is merely few arc seconds, or even less, in radio images. This is the reason why we usually detect them as compact radio sources; however, one can sometimes see their double radio structure (that is the jets) in very high resolution radio images (Figure 3), hence really they are just mini-doubles!
This stage of radio galaxy life is the only one at which the radio galaxy luminosity rises steeply as the source grows in size. The stage lasts only for tens to hundreds of thousands of years, which is barely 0.1% of the radio galaxy’s lifetime. This means there is a very short window of time when we can spot CSS and GPS sources, but there are many of these sources around and they are also very bright (their radio luminosities are typically the maximum a radio galaxy can reach) so we often detect them.
After the childhood radio galaxies enter the adulthood. Astronomers have dozen of names to describe the adult radio galaxies and this depends on their structure observed in radio waves, but really, what these radio galaxies have in common is their age and size. They are usually tens to hundreds of millions of years old, and between tens and hundreds of kilo-parsecs in size (30 thousands – 3 million light-years). The luminosity of these radio galaxies slowly drops as they penetrate through the intergalactic space. When you inspect the Radio Galaxy Zoo images, these radio sources are the #hourglass, #doublelobe and #plumes. 3C 237 and Centaurus A are fantastic examples of what we usually see! (Figure 4).
You will quickly notice that there are two main types of these radio galaxies; one type that has very strong radio emission at the end of the lobes (#hourglass, #doublelobes) which is the signature of jets pushing through the ambient medium around radio galaxy. These radio galaxies are called FR IIs by radio astronomers, and the bright spots at the ends of the lobes are called hotspots. The second type has their maximum radio luminosity close to the supermassive black hole or half way through the lobes; these are #plumes within the Radio Zoo and are tagged FRIs by radio astronomers. Plumes are less powerful than the hourglass, but they are even more of a challenge to astronomers!
The radio galaxy adult stage will be the majority of their lifetime, and that’s why they are the radio galaxies and radio structures one would typically see. Next time we will see what happens when the radio galaxy gets old!
Meet Enno Middelberg, Radio Galaxy Zoo science team member from Ruhr-University Bochum, Germany:
I grew up in northern Germany and developed in interest in astronomy only when I finished school. I went to Bonn to study Physics and Astronomy and, after my PhD, moved to Sydney to work as a postdoc. Here’s where I got involved with radio surveys of large portions of the sky, and also where I got involved in radio interferometric observations of large fields with continent-sized baselines (Very Long Baseline Interferometry). After moving back to Bochum, Germany, I got involved with the Radio Galaxy Zoo project, mainly because I just loved the idea and because I was able to quickly contribute to the project by making the images. Also, for a paper a few years back I had classified more than one thousand radio/IR sources the way RGZ users do it now – and I had to do it three times! So I know what it’s like, and I have seen my fair share of radio/IR images in my life. In general I like to fiddle with computers, software, and technology, so every project which requires crunching through, or trying to filter things out of piles of data is interesting.
Meet Anna Kapinska, Radio Galaxy Zoo science team member from the University of Western Australia:
I was born and grew up in Poland, where I also did my undergrad studies in astronomy. Since then astronomy was the force behind my life paths directing me via the Netherlads and the UK to Western Australia where I currently work as a Research Associate. I was introduced to radio astronomy during my masters studies when I learned all the fundamentals, and the tricky bits too, about radio interferometry. After that I moved towards more theoretical understanding of how radio galaxies grow and how they influence our evolving Universe. For that we still need large samples of radio galaxies of various sizes, shapes and luminosities – and here is where the Radio Galaxy Zoo and all your citizen science work is so invaluable!
Currently, I am also strongly involved in producing sky surveys with two fanstastic new radio telescopes, Lofar and MWA – hopefully we will be able to include these radio sky images in Radio Galaxy Zoo at some point too!
Today’s Radio Galaxy Zoo post is by Ray Norris, our Project Advisor. Ray researches how galaxies formed and evolved after the Big Bang, using radio, infrared, and optical telescopes.
In Radio Galaxy Zoo, some bright radio sources don’t have any infrared sources at all associated with them, and they have been given the hashtag #ifrs, for Infrared-Faint Radio Sources. So what are these IFRS?
In 2006, we discovered about 1000 radio sources in the Australia Telescope Large Area Survey (ATLAS). Conventional wisdom told us that all of these would be visible in the infrared observations taken by the Spitzer Space Telescope, as part of the SWIRE project. So we were astonished to find that about 50 of our sources were not listed in the SWIRE catalog. Could they be bugs in our data? After eliminating iffy sources, we were left with 11 sources that are bright in the radio but invisible in the infrared images. We dubbed these objects Infrared-Faint Radio Source (IFRS). With hindsight, we should have thought of a better name for them. But at the time we didn’t know that they would turn out to be important!
We suggested in 2006 that these might be high-redshift active galaxies – galaxies whose emission is dominated by a super-massive black hole at their centre (the galaxies you are looking at in Radio Galaxy Zoo). This was surprising, because we were finding so many of them that it meant there must be far more supermassive black holes in the early Universe than found by deep optical surveys, such as SDSS (whose images we use in Galaxy Zoo). It’s also far more than can be accounted for by conventional hierarchical models of super-massive black hole formation. Naturally, our colleagues were sceptical, and most of us harboured our private doubts too. But over the next few years we tested this idea and its alternatives. Gradually our confidence grew that these were indeed high-redshift active galaxies.
In 2011 we showed that they were similar to high-redshift radio galaxies (HzRG) but even more extreme. Crucially, we suggested that they follow the same correlations between the radio and infrared emission as the HzRG. If this suggestion turned out to be correct, then that would push them to be amongst the first supermassive black holes in the Universe.
Fortunately, it was possible to test this hypothesis by measuring the redshifts of less extreme objects, to see if they followed these same correlations. Two new papers confirm that they do indeed follow this correlation. In one, Andreas Herzog and his colleagues use the European Very Large Telescope to measure the redshifts of three of these less extreme objects, and find they lie on the correlation, at redshifts between 2 and 3, just as predicted. In the other paper Jordan Collier and his colleagues take exactly the same data now being shown in Radio Galaxy Zoo, and search for objects which are relatively much brighter in the FIRST (radio) data than in the WISE (infrared) data. 1,317 of these are found, of which 19 have measured redshifts. Again, all but one of these lie in the redshift range 2 to 3. This is strong support for the hypothesis!
Armed with this, we are increasingly confident that the most extreme IFRSs that will turn up in the fainter ATLAS and COSMOS field, to be released in Radio Galaxy Zoo in a few weeks, will include many supermassive black holes formed in the first half-billion years after the Big Bang. According to conventional hierarchical black hole formation models, these shouldn’t exist. So the race will be on to identify them and measure their redshifts using instruments like ALMA.
Not everything in Radio Galaxy Zoo classified as being an IFRS will turn out to be a high-redshift black hole, as the data currently being displayed (FIRST and WISE) are not deep enough to pick out the really high-redshift objects. But when the new ATLAS data are loaded into RGZ in a few weeks, almost every object that appears in the radio but not in the infrared will be one of these enigmatic objects. We can’t wait to see how many you find!
Ivy is an Australian astronomer (currently based at CSIRO) who is interested in the how/why galaxies start and stop forming stars. She is currently obsessed with galaxies that have suddenly stopped forming stars (aka ‘post-starburst’ galaxies), blue spheroidal galaxies and galaxy interactions in general. Having worked with the original Galaxy Zoo project, she became super excited with the prospect of helping to launch Radio Galaxy Zoo. She is very keen to find out how the massive radio jets emitting from central supermassive black holes affect its host galaxy as well as neighbouring galaxies.
When not working for astronomy, she is slave to two feline overlords, Princess Pippa and Master Finian. These two can pass for the internet celebrities: Grumpy Cat and Chemistry Cat (although Finian does not approve of glasses and bowties).
This post was written by Radio Galaxy Zoo team member Stas Shabala, an astronomer at the University of Tasmania.
The supermassive black holes at the hearts of galaxies are supposed to be simple. For someone looking at a black hole from afar, physicists tell us all black holes can be described by just three parameters: their mass, electric charge, and spin. For really big black holes, such as the ones astronomers deal with, things are even simpler: there is no charge (that’s because they are so big that there would always be enough neighbouring positive and negative charges to more or less cancel out). So if you know how heavy a black hole is, and how fast it spins, in theory at least you have enough information to predict a black hole’s behaviour.
Of course the black holes in Radio Galaxy Zoo often have at least one other, quite spectacular, feature – bright jets of radio plasma shooting through their host galaxy and into intergalactic space. Where do these jets come from?, I hear you ask. This is a very good question, and one to which astronomers are yet to find a wholly convincing answer. We have some pretty good hunches though.
The fact that black holes can spin might be quite important. Matter accreted by a black hole will rotate faster and faster as it falls in. Stuff closer to the equator will also rotate faster than stuff at the poles, and that causes the accreting material to flatten out into a pancake, which astronomers call the accretion disk.
The accreting matter near the black hole event horizon (a fancy term for the point of no return – any closer to the black hole, and not even light is fast enough to escape the gravitational pull) is subject to friction, which heats it up so much that individual atoms dissociate into plasma. These plasma (i.e. positively and negatively charged) particles are moving, so they are in fact driving an electrical current. When this current interacts with the rotating magnetic field of the black hole and the accretion disk, the charged particles are flung out at close to the speed of light along the axis of black hole rotation. We can see these fast-moving particles as jets in the radio part of the electromagnetic spectrum. A useful analogy is a car alternator, where electrical currents and magnetic fields are also combined to generate energy.
There are many things we don’t know. For example, we don’t know for sure where most of the jet energy comes from. It could be from the accreted matter, or the spin of the black hole, or a combination of both. We are also not sure exactly what sort of charged particles these jets are made up of. Understanding black hole jets is one of the great unsolved mysteries in astronomy. By studying a huge number of these jets at different points in their lifetimes, Radio Galaxy Zoo — with your help — will help us solve this puzzle.
To introduce you to the Radio Galaxy Zoo team, we’re doing a series of blog posts written by each team member — in no particular order. Meet Stas Shabala, our team Project Manager from the University of Tasmania, Australia:
I grew up in Tasmania, a gorgeous part of the world which also happens to be the place Grote Reber, the world’s first radio astronomer, called home for 50 years. After finishing university, I made a pilgrimage that these days is more or less standard for young Australians – I moved to the UK. I ended up staying for six years, and it was during my time in Oxford that I became involved with Galaxy Zoo. Normal galaxies are interesting but – given our history- a Tasmanian’s true heart will always be with radio astronomy. That’s why I have such a soft spot for Radio Galaxy Zoo.
Recently, I’ve been trying to figure out why radio galaxies come in so many different shapes, sizes and luminosities. Data from Radio Galaxy Zoo will go a long way to answering these questions. I’ve also had lots of fun using active black holes as beacons to accurately measure positions on Earth. It’s just like navigation by stars, but much more precise because stars move around in the sky a fair bit, whereas black holes don’t. The neat thing is, these measurements make it possible to study all sorts of geophysical processes here on Earth. It’s such a cool concept- using black holes to measure the movement of tectonic plates!
This is the second half of a detailed description of tailed radio galaxies from RGZ science team member Heinz Andernach. If you haven’t yet read the first part, it’s here: please feel free to leave any questions in the comments section.
Apart from distance or angular resolution, another reason for causing a NAT may be projection of the inner jets along the line of sight, as seems to be the case for NGC 7385 (PKS 2247+11). In the low-resolution image above, the inner jets are not resolved, but the corresponding far outer tails start to separate widely about half-way down their length. At much higher resolution:
the two opposite jets can be clearly seen to emanate from the very core, the central point-like source in this contour plot. However, the jet heading north-east (upper left) has been bent and diluted by almost 180° such that it runs behind the other jet for about 200 kpc (about 650,000 light-years) before the two jets separate and can be distinguished again in the lower-resolution image. The combination of these images is also a good example showing that different interferometers (large and small) are needed to show all features of a complex radio source.
However, not all apparently tailed radio sources would show their double jets near the core at high resolution. One curious example is IC 310, among the first “head-tails” to be discovered, has stubbornly resisted to show double jets, and is now accepted as a genuinely one-sided jet of the type that BL Lac objects, implying that its jet points fairly close to our line of sight.
An atlas of the radio morphologies in general of the strongest sources in the sky can be found here. Several NATs and WATs can be distinguished on the collection of icons. However, this atlas only comprises the 85 strongest sources in the sky. The enormous variety of bent radio galaxies present in the FIRST radio survey was explored with automated algorithms by Proctor in 2011, who tabulated almost 94,000 groups of FIRST sources attaching to them a probability of being genuine radio galaxies of various morphological types. The large variety of bent sources can be seen e.g. in her Figures 4, 7 and 8. However, this author made no attempt to find the host galaxies of these sources. Users of Radio Galaxy Zoo will eventually come across all these sources and tell us what the most likely host galaxy is.
Distant clusters of galaxies, important for cosmological studies, tend to get “drowned” in a large number of foreground galaxies present in their directions, so they are difficult to be distinguished on optical images. One way to find such clusters is by means of their X-ray emission, but since X-rays can only be detected from Earth-orbiting telescopes like ROSAT, ASCA, XMM-Newton, Chandra and Suzaku (to name only a few), this is a rather expensive way of detecting them. A “cheaper” way of looking for distant clusters is to use NATS and WATs as “beacons”. In fact, authors like Blanton et al. (2001) have followed up the regions of tailed radio sources and see the variety of morphologies:
Note that several of these WATs appear like twin NATs, but actually they are a single WAT, being “radio quiet” at the location of their host galaxies (not shown in the image above), but their jets “flare up” in two “hot spots” on opposite sides of the galaxy where they suddenly bend. The authors confirmed the existence of distant clusters around many of them. So, whenever RGZ users identify such tailed radio galaxies, we know that with a high probability we are looking in the direction of a cluster of galaxies.
An excellent introduction (even though 34 years old!) to radio galaxy morphologies and physics is the article by Miley 1980 (or from this alternate site). This author already put together a few well-known radio sources into what he called a “bending sequence”:
Today’s post is written by Heinz Andernach (Univ. of Guanajuato, Mexico), a member of the Radio Galaxy Zoo science team and an expert on radio galaxies. This is the first half of a detailed science post explaining what we know — and what we don’t know — about tailed radio galaxies, along with how Radio Galaxy Zoo volunteers are helping us understand them. There’s a lot of information here, so if you have questions please ask them in the comments.
In 1968, Ryle and Windram found the first examples of a type of radio galaxies, whose radio emission extends from the optical galaxy in one direction in the form of a radio “tail” or “trail”. These examples were NGC 1265 and IC 310 in the Perseus cluster. Soon other examples were found in the Coma cluster of galaxies (NGC 4869, alias 5C4.81), as well as the radio galaxy 3C 129. The latter lies right in the plane of our Galaxy, and its membership in a cluster of galaxies was only confirmed much later, hampered by the dust obscuration of our own Milky Way. All three of these tailed radio galaxy were located close to another radio source, with their tails pointing more or less away from their radio neighbor, leading these authors to suspect that the tails were blown by winds of relativistic particles ejected from the radio neighbor. Soon thereafter higher-resolution observations of these sources revealed that the host galaxies showed the same two opposite radio jets as known from other, less bent, radio galaxies like Cygnus A, but close to the outskirts of the optical galaxies the jets would both bend in some direction, thought to be the direction opposite to the motion of the host galaxy through the so-called “intergalactic” or “intracluster” medium, which had been discovered from X-ray observations at about the same time. Later on, doubts were cast on this scenario, as it was found that the optical hosts of tailed radio galaxies in clusters did not move about within their clusters with high enough velocities to explain the bends in the jet.
A detailed study of the prototypical WAT source 3C 465 (image above) at the center of the rich Abell cluster of galaxies A2634 by these authors did not result in any plausible explanation for the bending of their radio trails. Later on, with more detailed X-ray images of clusters of galaxies, it was found that tailed radio galaxies occur preferentially in high-density regions of the intracluster medium (i.e. where the X-ray intensity is high; for more information, see this 1994 study) and even cluster mergers were made responsible for the formation of radio tails in this 1998 study. More recently authors seem to converge on the compromise idea that the combination of high ambient density and modest speeds of the host galaxy with respect to the ambient medium are able to produce the bends, but in this blog I would rather like to concentrate on the variety of morphologies shown by these objects in order to help RGZ users to classify them.
Above all, the term “tailed radio galaxies” should never be separated from the word “radio”. Some authors talk about “tailed” or “head-tail (HT) galaxies”, but the tail always occurs in their radio emission, usually far beyond the optical extent of their host galaxies. So, let us reserve the term “head-tail galaxies” for a future when optical tails may be detected in certain galaxies. Also, please note that while galaxies sometimes show optical tails due to tidal interactions, these are of totally different origin than the radio tails we discuss here.
Tailed radio galaxies are often subdivided into wide-angle (WAT) and narrow-angle tailed (NAT) radio galaxies, referring to the opening angle between the two opposite jets emanating from the nucleus of the optical galaxy, where we expect the supermassive black hole doing its job of spewing out the jets. However, the distinction between WATs and NATs depends strongly on the angular resolution and/or the distance to the radio source. E.g., the first HT radio galaxy to be discovered (NGC 1265; images below) may be called a WAT at high resolution, but appears as a (much larger) NAT at lower resolution, shown by this sequence of high, medium and low resolution radio images:
Today’s post is from Enno Middelberg, RGZ science team member and astronomer at Ruhr-University Bochum, Germany and expert in interferometry. Enno has kindly agreed to share some details of this complex and highly useful technique for improving the resolution of images.
Radio waves from cosmic objects have been observed since the 1930s, starting with Karl Jansky and Grote Reber. In the beginning, astronomers used single telescopes, some of which looked more or less like TV antennas (and some looked just weird, for example Karl Jansky’s self-made telescope). Whatever the telescope looked like, astronomers understood very well that the resolution of their instruments would never be quite as good as at optical wavelengths. The fundamental reason for this has to do with diffraction theory and Fourier transforms, but the outcome is rather simple: the smallest separation on the sky a telescope can “resolve”, which means, that it can actually tell that there are two things and not one slightly extended thing, is given by the fraction λ/D. Here, λ represents the length of the waves observed (some centimetres in radio astronomy), and D represents the diameter of the telescope (some tens of metres). One can easily calculate that this fraction is of order 0.001-0.004 for a radio telescope, but for an optical telescope the number is much smaller, of order 0.00000005 or so. This means that optical telescopes could separate things on the sky which were much smaller together than the first radio telescopes.
Astronomers had tried to improve on this early on, using something called interferometry. The wavelengths could not be changed (otherwise they wouldn’t be radio telescopes any more, right?), and telescopes could only be made as big as 100m (otherwise they would be too heavy and too expensive). So astronomers took two of the telescopes they had and combined their signals into one. Such a contraption with two telescopes is called an interferometer, and its resolving power is no longer given by the diameter of the dishes, but by their separation. So simply moving the two telescopes further away from one another would increase the resolution – what a fantastic idea! In the 1960s, this technique was much advanced by British astronomer Sir Martin Ryle in Cambridge, and he was awarded the Nobel Prize in Physics for his work in 1974.
In the following decades, Martin Ryle’s innovation was improved upon by astronomers all over the world, creating radio interferometers of various sizes and forms. Radio telescopes sprouted like mushrooms. Ever more powerful telescopes were build: the Very Large Array, theAustralia Telescope Compact Array, the Effelsberg and Greenbank giant single dishes, and many more. Most recently, technical advances have made it possible to build completely digital radio telescopes, such as Lofar. Even though these instruments consist of many more than two radio telescopes, the measurements are always made between any two of them: the Very Large Array, for example, has 27 telescopes, which yields 351 two-telescope inteferometers. Using many more such interferometers improves the image quality and, of course, the sensitivity of the final images.
Radio images are most commonly reproduced as contour images. This makes them easier to analyse and interpret when printed, and contours are better when very bright and very faint portions of an image have to be shown at the same time. If such information was represented in a grey-scale image, the differences in brightness would not be decipherable. Radio astronomers love contour plots. My wife calls them “fried eggs” and always asks me if the kids can colour them in…
The radio images you’re seeing here are the results of the Australia Telescope Compact Array Large Area Survey (astronomers love acronyms!), or ATLAS for short. Between 2006 and 2009 we have collected data on two small regions in the southern sky to create the basis for an investigation of the way that galaxies evolve. We have used these data to create the radio images you’re seeing when you classify sources. The infrared images were made with the Spitzer telescope, to compare the radio to infrared emission. Radio and infrared waves are not necessarily emitted by the same material and can therefore be displaced from one another in a galaxy. That’s why we need your help to determine what radio blobs belong to which infrared blob!