Some colleagues and I successfully proposed for a symposium on citizen science at the annual meeting of the American Association for the Advancement of Science (AAAS) in San Jose, CA in February 2015. (The AAAS is one of the world’s largest scientific societies and is the publisher of the Science journal.) Our session will be titled “Citizen Science from the Zooniverse: Cutting-Edge Research with 1 Million Scientists.” It refers to the more than one million volunteers participating in a variety of citizen science projects. This milestone was reached in February, and the Guardian and other news outlets reported on it.
The Zooniverse began with Galaxy Zoo, which recently celebrated its seventh anniversary. Of course, Galaxy Zoo has been very successful, and it led to the development of a variety of citizen science projects coordinated by the Zooniverse in diverse fields such as biology, zoology, climate science, medicine, and astronomy. For example, projects include: Snapshot Serengeti, where people classify different animals caught in millions of camera trap images; Cell Slider, where they classify images of cancerous and ordinary cells and contribute to cancer research; Old Weather, where participants transcribe weather data from log books of Arctic exploration and research ships at sea between 1850 and 1950, thus contributing to climate model projections; and Whale FM, where they categorize the recorded sounds made by killer and pilot whales. And of course, in addition to Galaxy Zoo, there are numerous astronomy-related projects, such as Disk Detective, Planet Hunters, the Milky Way Project, and Space Warps.
We haven’t confirmed all of the speakers for our AAAS session yet, but we plan to have six speakers who will introduce and present results from the Zooniverse, Galaxy Zoo, Snapshot Serengeti, Old Weather, Cell Slider, and Space Warps. I’m sure it will be exciting and we’re all looking forward to it!
I’ve used some statistical tools to analyze the spatial distribution of Galaxy Zoo galaxies and to see whether we find galaxies with particular classifications in more dense environments or less dense ones. By “environment” I’m referring to the kinds of regions that these galaxies tend to be found: for example, galaxies in dense environments are usually strongly clustered in groups and clusters of many galaxies. In particular, I’ve used what we call “marked correlation functions,” which I’ve found are very sensitive statistics for identifying and quantifying trends between objects and their environments. This is also important from the perspective of models, since we think that massive clumps of dark matter are in the same regions as massive galaxy groups.
We’ve mainly used them in two papers, where we analyzed the environmental dependence of morphology and color and where we analyzed the environmental dependence of barred galaxies. These papers have been described a bit in this post andthis post. We’ve also had other Galaxy Zoo papers about similar subjects, especially this paper by Steven Bamford and this one by Kevin Casteels.
What I loved about these projects is that we obtained impressive results that nobody else had seen before, and it’s all thanks to the many many classifications that the citizen scientists have contributed. These statistics are useful only when one has large catalogs, and that’s exactly what we had in Galaxy Zoo 1 and 2. We have catalogs with visual classifications and type likelihoods that are ten times as large as ones other astronomers have used.
What are these “marked correlation functions”, you ask? Traditional correlation functions tell us about how objects are clustered relative to random clustering, and we usually write this as 1+ ξ. But we have lots of information about these galaxies, more than just their spatial positions. So we can weight the galaxies by a particular property, such as the elliptical galaxy likelihood, and then measure the clustering signal. We usually write this as 1+W. Then the ratio of (1+W)/(1+ξ), which is the marked correlation function M(r), tells us whether galaxies with high values of the weight are more dense or less dense environments on average. And if 1+W=1+ξ, or in other words M=1, then the weight is not correlated with the environment at all.
First, I’ll show you one of our main results from that paper using Galaxy Zoo 1 data. The upper panel shows the clustering of galaxies in the sample we selected, and it’s a function of projected galaxy separation (rp). This is something other people have measured before, and we already knew that galaxies are clustered more than random clustering. But then we weighted the galaxies by the GZ elliptical likelihood (based on the fraction of classifiers identifying the galaxies as ellipticals) and then took the (1+W)/(1+ξ) ratio, which is M(rp), and that’s shown by the red squares in the lower panel. When we use the spiral likelihoods, the blue squares are the result. This means that elliptical galaxies tend to be found in dense environments, since they have a M(rp) ratio that’s greater than 1, and spiral galaxies are in less dense environments than average. When I first ran these measurements, I expected kind of noisy results, but the measurements are very precise and they far exceeded my expectations. Without many visual classifications of every galaxy, this wouldn’t be possible.
Second, using Galaxy Zoo 2 data, we measured the clustering of disc galaxies, and that’s shown in the upper panel of the plot above. Then we weighted the galaxies by their bar likelihoods (based on the fractions of people who classified them as having a stellar bar) and measured the same statistic as before. The result is shown in the lower panel, and it shows that barred disc galaxies tend to be found in denser environments than average disc galaxies! This is a completely new result and had never been seen before. Astronomers had not detected this signal before mainly because their samples were too small, but we were able to do better with the classifications provided by Zooites. We argued that barred galaxies often reside in galaxy groups and that a minor merger or interaction with a neighboring galaxy can trigger disc instabilities that produce bars.
What kinds of science shall we use these great datasets and statistics for next? My next priority with Galaxy Zoo is to develop dark matter halo models of the environmental dependence of galaxy morphology. Our measurements are definitely good enough to tell us how spiral and elliptical morphologies are related to the masses of the dark matter haloes that host the galaxies, and these relations would be an excellent and new way to test models and simulations of galaxy formation. And I’m sure there are many other exciting things we can do too.
You know those odd features in some SDSS images that look like intergalactic traffic lights?
They aren’t intergalactic at all: they’re asteroids on the move in our own solar system. They move slowly compared to satellite trails (which look more like #spacelasers), but they often move quickly enough that they’ve shifted noticeably between the red, green, and blue exposures that make up the images in SDSS/Galaxy Zoo. When the images from each filter are aligned and combined, the moving asteroid dots its way colorfully across part of the image.
These objects are a source of intense study for some astronomers and planetary scientists, and the SDSS Moving Object Catalog gives the properties of over 100,000 of them. Planetary astronomer Alex Parker, who studies asteroids, has made a video showing their orbits.
I find their orbits mesmerizing, and there’s quite a lot of science in there too, with the relative sizes illustrated by the point sizes, and colors representing different asteroid compositions and families. There’s more information at the Vimeo page (and thanks to Amanda Bauer for posting the video on her awesome blog).
One of the most common questions we receive about asteroids from Galaxy Zoo volunteers is whether there will ever be a citizen science project to find them. So far, as the catalog linked above shows, the answer has been that computers are pretty good at finding asteroids, so there hasn’t been quite the need for your clicks… yet. There are some asteroids that are a little more difficult to spot, and those we’d really like to spot are quite rare, so stay tuned for a different answer to the question in the future. And in the meantime, enjoy the very cool show provided by all those little traffic lights traversing their way around our solar system.
Great news everybody! The latest Galaxy Zoo 1 paper has been accepted by MNRAS and has appeared on astro-ph: http://arxiv.org/abs/1402.4814
In this paper, we take a look at the most crucial event in the life of a galaxy: the end of star formation. We often call this process “quenching” and many astrophysicists have slightly different definitions of quenching. Galaxies are the place where cosmic gas condenses and, if it gets cold and dense enough, turns into stars. The resulting stars are what we really see as traditional optical astronomers.
Not all stars shine the same way though: stars much more massive than our sun are very bright and shine in a blue light as they are very hot. They’re also very short-lived. Lower mass stars take a more leisurely pace and don’t shine as bright (they’re not as hot). This is why star-forming galaxies are blue, and quiescent galaxies (or “quenched” galaxies) are red: once star formation stops, the bluest stars die first and aren’t replaced with new ones, so they leave behind only the longer-lived red stars for us to observe as the galaxy passively evolves.
Blue Ellipticals & Red Spirals
The received wisdom in galaxy evolution had been that spirals are blue, and ellipticals are red, meaning that spirals form new stars (or rather: convert gas into stars) and ellipticals do not form new stars (they have no gas to convert to stars). Since you’re taking part in Galaxy Zoo, you know that this isn’t entirely true: there are blue (star-forming) ellipticals and red (passive) spirals. It’s those unusual objects that we started Galaxy Zoo for, and in this paper they help us piece together how, why and when galaxies shut down their star formation. You can already conclude from the fact that blue ellipticals and red spirals exist that there is no one-to-one correlation between a galaxy’s morphology and whether or not it’s forming stars.
Blue, Red and…. Green?
A few years back, astronomers noticed that not all galaxies are either blue and star forming or red and dead. There was a smaller population of galaxies in between those two, which they termed the “green valley” (the origin of the term is rather interesting and we talk about it in this Google+ hangout). So how do these “green” galaxies fit in? The natural conclusion was that these “in between” galaxies are the ones who are in the process of shutting down their star formation. They’re the galaxies which are in the process of quenching. Their star formation rate is dropping, which is why they have fewer and fewer young blue stars. With time, star formation should cease entirely and galaxies would become red and dead.
The Green Valley is a Red Herring
Ok, why is this green valley a red herring you ask? Simple: the green valley galaxies aren’t a single population of similar galaxies, but rather two completely different populations doing completely different things! And what’s the biggest evidence that this is the case? Some of them are “green spirals” and others are “green ellipticals”! (Ok, you probably saw that coming from a mile away).
So, we have both green spirals and green ellipticals. First: how do we know they must be doing very different things? If you look at the colour-mass diagram of only spirals and only ellipticals, we start to get some hints. Most ellipticals are red. A small number are blue, and a small number are green. If the blue ellipticals turn green and then red, they must do so quickly, or there would be far more green ellipticals. There would be a traffic jam in the green valley. So we suspect that quenching – the end of star formation – in ellipticals happens quickly.
In the case of spirals, we see lots of blue ones, quite a few green one and then red ones (Karen Masters has written several important Galaxy Zoo papers about these red spirals). If spirals slowly turn red, you’d expect them to start bunching up in the middle: the green “valley” which is revealed to be no such thing amongst spirals.
Galaxy Quenching time scales
We can confirm this difference in quenching time scales by looking at the ultraviolet and optical colours of spirals and ellipticals in the green valley. What we see is that spirals start becoming redder in optical colours as their star formation rate goes down, but they are still blue in the ultraviolet. Why? Because they are still forming at least some baby stars and they are extremely bright and so blue that they emit a LOT of ultraviolet light. So even as the overall population of young stars declines, the galaxy is still blue in the UV.
Ellipticals, on the other hand, are much redder in the UV. This is because their star formation rate isn’t dropping slowly over time like the spirals, but rather goes to zero in a very short time. So, as the stellar populations age and become redder, NO new baby stars are added and the UV colour goes red.
It’s all about gas
Galaxies form stars because they have gas. This gas comes in from their cosmological surroundings, cools down into a disk and then turns into stars. Galaxies thus have a cosmological supply and a reservoir of gas (the disk). We also know observationally that gas turns into stars according to a specific recipe, the Schmidt-Kennicutt law. Basically that law says that in any dynamical time (the characteristic time scale of the gas disk), a small fraction (around 2%) of that gas turns into stars. Star formation is a rather inefficient process. With this in mind, we can explain the behaviour of ellipticals and spirals in terms of what happens to their gas.
Spirals are like Zombies
Spirals quench their star formation slowly over maybe a billion years or more. This can be explained by simply shutting off the cosmological supply of gas. The spiral is still left with its gas reservoir in the disk to form stars with. As time goes on, more and more of the gas is used up, and the star formation rate drops. Eventually, almost no gas is left and the originally blue spiral bursting with blue young stars has fewer and fewer young stars and so turns green and eventually red. That means spirals are a bit like zombies. Something shuts off their supply of gas. They’re already dead. But they have their gas reservoir, so they keep moving, moving not knowing that they’re already doomed.
Ellipticals life fast, die young
The ellipticals on the other hand quench their star formation really fast. That means it’s not enough to just shut off the gas supply, you also have to remove the gas reservoir in the galaxy. How do you do that? We’re not really sure, but it’s suspicious that most blue ellipticals look like they recently experienced a major galaxy merger. There are also hints that their black holes are feeding, so it’s possible an energetic outburst from their central black holes heated and ejected their gas reservoir in a short episode. But we don’t know for sure…
So that’s the general summary for the paper. Got questions? Ping me on twitter at @kevinschawinski
Last time we discussed the early and mid stages of radio galaxy life that take up the majority of the radio galaxy lifetime. Today we will go much further following paths of aging radio galaxies.
‘Only few of us get here’
As we discussed last time, radio galaxies are typically between tens and hundreds of kilo- parsecs in size (30 thousands – 3 million light-years). However, some of our buddies will grow to enormous sizes. Once a radio galaxy reaches one Mega-parsec in size (3.3 million light-years across) it’s called a giant – that is, a giant radio galaxy. Not every radio galaxy will reach such enormous sizes; only the most powerful ones whose environments are not extremely dense do. We don’t see too many giant radio galaxies. There are two main problems. One is that they are of low radio luminosity, and so our telescopes are not always sensitive enough to detect more than a subset of radio galaxies reaching this stage of their lives. The other problem is that giants are often composed of numerous bright knots spread over a large area and it’s difficult for us to tell which of these knots are associated with the giant and which are from unrelated sources.
Giant radio galaxies are usually hundred of thousands, or more, years old and they are very large and extended. They can tell us a lot about what is going on within the space in between galaxies in groups and clusters, and that’s why radio astronomers cherish these giants! The largest giant radio galaxy known is 4.5 Mega-parsecs across (named J1420-0545), which is almost 15 million light-years! In radio images these radio galaxies extended over 20 or 30 arc-minutes, which means you will normally see only one of their lobes at a time in any of the Radio Galaxy Zoo images we classify. This is also the reason why we would tag these Radio Galaxy Zoo images as #overedge or #giants.
But radio galaxies will not grow to infinity, they will eventually die. What happens then is that the radio galaxy starts fading away. Physically, at this point, the supermassive black hole stops providing jets with fresh particles, which means the jets and lobes or radio galaxy are not fed with new material. The electrons in the radio galaxy lobes have a finite amount of energy they can release as light, and so the lobes simply fade away until they are no longer visible with our telescopes. Dying radio galaxies become progressively less powerful, and less pronounced: no bright jet knots nor hotspots are present within the lobes anymore (see Figure 6). Eventually we can’t see these radio galaxies any more with our telescopes. You would typically mark these radio sources as #relics or #extended in Radio Galaxy Zoo images. It takes only ten thousand years for the brightest features of radio galaxy lobes to disappear, which is barely 0.01% of the total lifespan of radio galaxy. Again, just as the birth, it’s a blink of an eye!
So is that the end?
Well… not really! Astronomers have seen evidence that radio galaxies re-start. What does that mean? That means radio galaxies sort of resurrect. After switching off, the supermassive black hole is radio silent for a while, but it can become active again; that is the whole cycle of radio galaxy life can start all over again. A single host galaxy can have multiple radio galaxy events. We still don’t know the ratio of how long the galaxy is in quiet, silent stage, to how long is in its active, violent radio galaxy forming stage. We also do not know if all galaxies go through the active, radio galaxy forming stage, or whether it’s just some of them. And we don’t know what exactly is the process that makes the galaxies switch on and off. But details on that… that’s yet another story!
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!
We’re in the middle of an observing run at the Lick 3m Shane telescope, with the first part devoted to polarization measurements of the Voorwerpje clouds (which is to say, giant clouds of ionized gas around active galactic nuclei found in the Galaxy Zoo serendipitous and targeted searches), and just now switching to measure spectra to examine a few new candidate Voorwerpjes, and further AGN/companion systems that may shed light on similar issues of how long AGN episodes last.
Polarization measurements can be pretty abstruse, but can also provide unique information. In particular, when light is scattered, its spectra lines are preserved with high fidelity, but light whose direction of polarization (direction of oscillation of its electric field when considered as a wave) is perpendicular to the angle it makes during this operation is more likely to reach us instead of being absorbed. This is why polarized sunglasses are so useful – glare from such scattering light can be reduced by appropriate orientation of the polarizing filter.
In our context, polarization measurements tell us something about how much of the light we see is secondhand emission from the AGN rather than produced on the spot in the clouds (admittedly as a side effect of the intense UV radiation from the nucleus), and will show us whether we’re fortunate enough that there might be a dust cloud reflecting so much light that we could look there to measure the spectrum of the nucleus when it was a full-fledged quasar. (This trick has worked for supernovae in our galaxy, which is how we know just what kind of supernova was seen in 1572 despite not having spectrographs yet).
Polarization wizard Sebastian Hoenig (now at the Dark Cosmology Center in Copenhagen) has already produced preliminary calibrations and maps from these new data. Here are some visualizations. In each case, the lines show the direction of polarization. Their length and color show the fraction of light which is polarized at points where there is enough to measure. This fractional polarization tells about the mix of light arising on the spot (even if secondhand due to UV radiation ionizing the gas) and that reflected from dust particles. There is a telltale annular or bull’s-eye pattern when the scattered light originates in a central source, which we see over and over (as if we hadn’t figured out to blame the galaxy nuclei anyway).
First up is a personal favorite, UGC 7342 (the last one to have its Hubble images obtained, and among the largest and brightest of the Galaxy Zoo sample).
The next one, Markarian 78, is less familiar, oddly because it makes perfect sense (so it has not figured much in the followup observations). In this case, we see a bright and obvious active nucleus, one which is powerful enough to light up the giant gas clouds without having changed over the past 60,000 years or so.
For comparison, here is a polarization map of IC 2497 and Hanny’s Voorwerp itself, from data obtained last year (the first time the weather let us get useful results). Sometimes we can hear the Universe laughing – a quick simulation shows that the reflected light from the nucleus, when it was a quasar, is just a bit too faint for us to have seen its signature broad emission lines in any of the Voorwerp spectra.
As we switch into measuring spectra for the next few nights, the aim changes to a combination of looking at a few new Voorwerpje candidates from the Galaxy Zoo forom, and a set of newly-identified AGN/galaxy pairs which may let us study the same issues of AGN lifetimes. We can sort of settle into a routine – Anna Pancoast does calibrations and setup during the day and hands over to Vardha Bennert to finish observations during the night. I typically get to work before Vardha finishes the last galaxy observation (thanks to the time-zone difference) and transfer data to start analysis, so we can change the next night’s priorities if something interesting shows up. It takes a (global) vllage, but then if there’s been any single meta-lesson from Galaxy Zoo, that would be it.
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!
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.
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”: