Galaxy Zoo: Are Bars Responsible for the Feeding of Supermasssive Black Holes Beyond the Local Universe?
Supermassive black holes are thought to reside in the centers of most galaxies. These massive objects can produce powerful jets of energy that may significantly influence the evolution of their host galaxies. While we believe that black holes have an important role in galaxy evolution, a crucial unknown aspect about black holes is how they are fueled and turn into active galactic nuclei (AGN).
Among the proposed black hole fueling processes, bar-driven secular evolution is among the most popular. Bars are linear structures of stars that stretch across the centers of galaxies. They are theorized to fuel black holes by driving gas from the outskirts of galaxies into the very centers, where supermassive black holes lie.
Previous studies have tested whether bars can fuel black holes by examining whether there is an excess of bars among AGN hosts compared to non-AGN hosts. For the most part, those works found that there was not a significant enhancement of bars among AGN hosts, leading them to conclude that bars do not fuel black holes. But almost all these previous works were limited to the local universe, i.e., in the present, where the number of AGN is the lowest across cosmic time.
In this work, we investigate whether there is an excess of bars among AGN hosts beyond the local universe, i.e., in the distant past, up to 7 billion years ago. In this epoch, the number of AGN hosts is much higher, giving us a better glimpse of the identity of the black hole fueling mechanism. To conduct this experiment, we created two samples: 1) a sample of AGN hosts and 2) a carefully constructed control sample of non-AGN hosts that are matched to the AGN hosts. In order to create the largest samples possible, our experiment utilized three of the most popular extragalactic surveys: AEGIS, COSMOS, and GOODS-S. With these samples, we used the Galaxy Zoo: Hubble bar classifications to identify barred galaxies. Below is a gallery of 6 sets of AGN and their corresponding control galaxies, 2 sets from each survey.
Our main results are shown in the figure below. We have two probes of bar presence—bar fraction (left) and bar likelihood (right)—for the AEGIS, COSMOS, and GOODS-S surveys.
We find no statistically significant enhancement in the bar fraction or the bar likelihood in AGN hosts (green squares) compared to the non-AGN hosts (purple triangles). Our results, combined with previous works at the local universe, indicate that bars are not the primary fueling mechanism for supermassive black hole growth for the last 7 billion years. Moreover, given the growth of supermassive black holes over cosmic time, our results imply that bars are not directly responsible for the buildup of at least half the local supermassive black hole mass density.
Therefore, although among the most popular fueling mechanisms, it seems that bars do not fuel black holes. However, this result does narrow the search for the real black hole fueling mechanism.
At a conceptual level the formation of radio galaxies is pretty simple. According to a basic picture first introduced in the 1970s, a supermassive black hole in the center of a galaxy generates a symmetric pair of oppositely directed, high speed jets or beams of hot, ionized gas as a by-product of energy released or stored from matter falling onto the black hole. Those jets drill holes in the atmosphere of the galaxy and then even far beyond, dumping energy, excavating cavities and possibly entraining gas into the jets and cavities along the way. The jets carry magnetic fields and high energy electrons. Those electrons, spiraling in the magnetic fields light up the jets and the cavities they excavate in the radio band through a process called synchrotron emission.
While calculations based on this cartoon picture can correctly predict a few properties of radio galaxies, anyone who has looked at the images in Radio Galaxy Zoo can see that there must be a whole lot more to the story. Radio galaxies at best have only a rough bilateral symmetry with respect to their host galaxies. Furthermore, no two radio galaxies look alike, and most look pretty complicated; some could only be described as messy. In fact, the physics of radio galaxy formation is really very complex for a whole bunch of reasons that range from inherent instabilities in the dynamics of a fast jet, to the reality that the jets are not steady at the source. Furthermore, the surrounding environments are themselves messy, dynamic and sometimes even violent. All of these influences have impact on the appearances of radio galaxies.
The other side of the coin is that, if they can be understood, these complications may improve opportunities to decipher both the formation processes of the jets as well as the conditions that control their development and dissipation as they penetrate their environments. One part of piecing this puzzle together is expanding our awareness of all the things radio galaxies do, as well as when and where they do what they do. That’s what Radio Galaxy Zoo is about.
On the other hand, to go beyond the cartoon picture of what we see we also have to develop much more sophisticated and realistic models of the phenomena. This is very challenging. Because the detailed physics is so complex (messy!), astronomers have come to depend increasingly on large computer simulations that solve equations for gas dynamics with magnetic fields and high energy electrons. Pioneering gas dynamical simulations of jets in the 1980s already played an important role in confirming the value of the jet paradigm and helped to refine it soon after it was introduced.
Those early simulations were, however, seriously limited by available computer power and computational methods. In important ways the structures they made did not really look much like actual radio galaxies. At best they were too grainy. At worst important physics had to be left out, including the processes that actually produce the radio emission. This made it hard to know exactly how to compare the simulations with real radio galaxies. Thankfully, rapid improvements in both of those areas have led recently to much more realistic and detailed simulations that are starting to look more like the real thing and can be used to better pin down what is actually going on.
Our group at the University of Minnesota has been involved for some years now in pushing forward the boundaries of what can be learned about radio galaxies from simulations. I illustrate below some of the lessons we have learned from these simulations and some of the complex radio galaxy environments that it is now possible to explore through simulations. Each of these simulations was part of the work carried out by a student as part of their PhD training.
The jets responsible for radio galaxy formation propagate at speeds that can be a significant fraction of the speed of light. They are almost certainly supersonic. These properties lead to several related behaviors that are illustrated in Figure 1. It turns out that the flows within such a jet tend periodically to expand and then to contract. As they do so they form a sequence of shocks along the jet. These are visible in the figure. The jet also creates a sonic boom or bow shock in front as it moves forward. A close look at the jet in this figure also reveals that the jet actually does not remain straight as it moves forward. The end of the jet turns out to be unstable, so soon after launch begins to ‘flap’ or wobble. As a result the end of the jet tends to jump around, enlarging the area of impact on the ambient medium.
Many radio galaxies form inside clusters of galaxies, where the ambient medium is highly non-uniform and stirred up as a result of its own, violent formation. This distorts and bends the radio structures. At the same time the energy and momentum deposited by the jets creates cavities in the cluster gas that lead to dark holes in the thermal X-ray emission of the cluster. Figure 2 illustrates some of these behaviors for a simulated radio galaxy formed at the center of a cluster. Even though the source of the radio galaxy is at rest, there are fast gas motions in the cluster gas that obviously deflect the radio galaxy jets. ‘Mock’ radio images representing synchrotron emission by high energy electrons in the magnetic field carried by the jets are shown on the right in the figure at two times. At the same two times mock images of thermal X-rays are shown to the left. The X-ray images have been processed to exaggerate the dark cavities produced by the jets. Note that each image spans about 700 kpc or 2 million light years.
Quite a few radio galaxies in clusters are not made by galaxies anchored in the cluster center, but are hosted by galaxies moving through the cluster. This is especially common in clusters that are in the process of merging with another cluster. In that case the host galaxy can be moving very fast, and even supersonically with respect to its local, ambient medium. Then the radio jets can be very strongly deflected into ‘tails’ by an effective cross wind and eventually disrupted. Figure 3 illustrates the mock synchrotron emission from such a simulated radio galaxy. The abruptness of jet bending depends on the relative speed of the jet with respect to its internal sound speed and the relative speed of the host galaxy through its ambient medium with respect to the sound speed of that medium. So, when strongly bent jets are seen in a radio galaxy it is a strong clue that the motion of the galaxy is supersonic in relation to its environment. When multiple tailed radio galaxies are found in a given cluster it provides potentially valuable information about the dynamical condition of the cluster, since a relaxed cluster ought not to have many galaxies moving at supersonic speeds through the cluster gas.
Even more complex motions between the host galaxies and the ambient gas are possible. Those can sometimes lead to really exotic-looking radio structures. One beautiful example of this is the radio source 3C75 in the merging cluster Abell 400. Evidently two massive galaxies have become gravitationally bound into a binary system with a separation of about 7 kpc. The orbital period should be around 100 Myr. The pair also appears to be moving together supersonically through the ambient medium. Each of those galaxies has formed radio jets. If it were not for the binary the expected outcome might resemble the situation pictured in Figure 3. However, the binary motions cause each of the two galaxies to oscillate in its motion and this causes the radio jets to develop more complex, twisted shapes before they disrupt into tails. Figure 4 illustrates a preliminary effort to simulate this dynamics. The image on the right shows the real 3C75, where pink is the radio emission (VLA) and blue is thermal X-rays (Chandra). The image on the left traces the distribution of gas expelled by each of the two galaxies in the binary system. This simulation seems to capture the general character of the dynamical situation responsible for 3C75.
From this short set of simulation results it ought to be clear why many different kinds of radio galaxy structures are expected to form. It also ought to be apparent that we need better catalogs of what behaviors do exist in nature in order to see how to focus our simulation efforts and to establish what are the most important dynamical conditions in radio galaxy formation.
One of our scientists Prof. Ray Norris put the call out to the Radio Galaxy Zoo community for a hunt on spiral galaxies hosting powerful radio sources. The first known galaxy of this type is 0313-192, a galaxy much like our Milky Way and has left astronomers baffled.
Figure 1: 0313-192 The wrong galaxy from the Astronomy Picture of the Day. Credit: W. Keel (U. Alabama), M. Ledlow (Gemini Obs), F. Owen (NRAO, AUI, NSF, NASA.
Here is Prof. Norris’ post:
Keep an eye out for any hourglass sources that seem to be hosted by galaxies that look spiral in the infrared. These objects are incredibly rare in the local Universe (only 2 or 3 known) and we may not see any in Radio Galaxy Zoo, but if someone does find one, that would be worth writing a paper about (with the discoverer as co-author, of course). The rarity of radio-loud spirals is thought to be because the radio jets heat up and disrupt the gas in the spiral, switching off star formation, and turning the galaxy into a “red dead” elliptical. But we might find one or two where the jets have only just switched on and haven’t yet destroyed the spiral. See The radio core of the Ultraluminous Infrared Galaxy F00183-7111: watching the birth of a quasar for another example of this process in its very early stage. So keep your eyes peeled and yell out (very loudly) if you find one!
We are pleased to announce that the Radio Galaxy Zoo community has identified over a dozen potential candidates and we are in the process of following these up.
Have you seen any? Head over to Radio Galaxy Zoo to join in on the hunt and let us know what you find.
Meet Julie (aka @42jkb on RadioTalk), a project scientist on Radio Galaxy Zoo!
I’m a postdoctoral fellow at CSIRO Astronomy and Space Science in Australia. This is my first position after obtaining my PhD from the University of Calgary, Canada working on magnetic fields of radio galaxies. My first memories of astronomy and the wonders of the Universe were spending summer nights outside at campfires with my family staring up and counting the number of “shooting stars” we could see. It wasn’t until my second year of undergraduate studies at Western University in Ontario Canada that I considered doing astrophysical research; I was actually going through to be an airline pilot! I haven’t looked back at my decision to change into physics and astronomy and everyday I am amazed at the complexity and beauty of the Universe.
I spend my time researching magnetic fields and how important they are to radio galaxies. You can usually find me at the Australia Telescope Compact Array taking observations of all types of radio galaxies, sitting in front of a computer doing the exact same thing as Radio Galaxy Zoo, learning about life from my daughter, and educating myself on the wonderful country I now live in. I am excited about what Radio Galaxy Zoo has to offer the astronomical community and what the Universe will unfold for us through this project. Thank you for taking part!
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!
Hi, I’m Meg Schwamb (normally from Planet Hunters and Planet Four), but not to fear, I’m not here to talk about planets. With the Oxford Galaxy Zoo Team, I’ve been helping to observe on the Caltech Submillimeter Observatory on Mauna Kea. Chris blogged about our first night. I thought I’d give a quick update, before final preparation for the start of tonight’s observing.
It’s been quite a world-wide effort. I’m currently based in Taipei, Taiwan. So I’m remotely logging into the telescope and instrument controls from home while Chris, Brooke, and Becky have been logging in remotely from Oxford Zooniverse HQ. Then we’re in a Skype call, so we can communicate and know who’s commanding the telescope and helping support the person running the observation.
Chris and the Zooniverse’s Rob Simpson talked more about the details of why we’re observing with the CSO these past few nights and what the experience has been like on their latest episode of Recycled Electrons, which you can find here.
The weather the past few nights hasn’t been great, we were closed Sunday night in Hawaii, we opened part of the night last night and closed due to high humidity in the middle of the night and never reopened. A few hours ago, the primary observers whose time this is, made the call that the conditions are not good enough for their project, but they are good enough for us to observe. Since I’m 7 hours ahead of the UK, one of my tasks is to be checking the Mauna Kea weather reports and waiting for the decision from the lead observer of the primary program. So about an hour ago, I phoned to start waking up the Oxford team.
The conditions are looking pretty good on the mountain. So I think we’ll have a smooth night in turns of humidity and wind. The optical depth is looking as good as our first night. I’m off to start my final checks and preparations, as I’m the lead observer of tonight’s observers (which includes Becky and Brooke in Oxford), so I make the calls of when to open, close, when we move to the next target.
If things are moving smoothly, we’ll try and update the blog occasionally. In the meantime, enjoy the view of sunset from of the CFHT webcams on the submit of Mauna Kea.
9:42pm Hawaii time – We’re all pointed on source and taking data. Been on there observing for about an hour now. We”ll move off soon to do a pointing check on a carbon star and then back integrating on our target galaxy. (Meg)
1:03am Hawaii time- We’re still on the same target. We were thinking of maybe moving off, but decided to stay on to see what some other features in the data look like with more time. We’ll be moving to our end of night source in about 40 minutes, and sit on that for the next several hours. Becky and Brooke are driving the telescope (Meg).
2:49am Hawaii time – Weather continues to about the same. We’re on to another source. Below here’s an image of the spectrograph data GUI windows that we see. The telescope has two spectrographs that simultaneous take data on the source. The bottom one FFTS2 covers a broader range and is higher resolution than the top spectrograph (FFTS1)
4:05 am – We’ve decided to stay on the same source for the rest of the night. So we’re just going to be sitting and taking observations on source then a system temperature calibration and then back to observing on source for the rest of the night. We shut an hour before sunrise so around 4:42am Hawaii local time (Meg).
Hello from the summit of Mauna Kea, Hawai’i! We’re here to follow up on a host of Galaxy Zoo blue ellipticals, trying to use the Caltech Submillimeter Observatory to catch the signature of Carbon Monoxide – gas which might provides the fuel for star formation.
Sadly, we’re not in Hawai’i – I’m in the office in Oxford (my sunrise is below), Becky is in Bristol and we’re joined by Meg Schwamb from Planet Hunters on her first extragalactic observing run. Conditions look good, if a bit windy, and I’ll try and keep you informed as the night wears on.
EDIT: We have an open dome and the weather is looking good. Here’s a dark webcam image you can squint at to pick out a telescope and sky.
And first observations for calibrations are on Mars! Here’s an excitingly noisy spectrum with a nice broad absorption line in the middle – you’re looking at CO (carbon monoxide) in the atmosphere of Mars. The width can even tell you about the current wind speed on Mars. From Oxford to Hawaii to Mars to back to you at home.
EDIT : Well, that was interesting. It turns out it helps if you know a telescope – none of us have used the CSO before and it’s been quite hard work to get our heads round the right software. Still, we successfully observed our first target – an unprepossessing, rather distant blue elliptical by Sloan standards (see below) and on first glance didn’t quite see anything. It set before we could quite confirm that there was nothing there to see, and we’ve moved on to a second galaxy, stopping off on the way by a cool star in order to calibrate the system.
EDIT : End of a long night. One disadvantage of observing remotely is that we have to be very cautious, so we’re commanded to shut the telescope an hour before sunrise. We got data, certainly, but it’s not one of those nights in which wonderous things are apparent immediately. We have more chances for the rest of the week if the weather cooperates, so watch this space.
In the UK on a seventh anniversary the traditional gift is one made of wool. So considering it’s the SEVENTH anniversary of Galaxy Zoo TODAY (July 11th) our very own, super talented Karen Masters has knitted us the Galaxy Zoo logo!
If you feel like getting your astronomical knit on for this momentous occasion, here’s a few inspirational photo’s.
Karen also knitted our favourite Penguin Galaxy as a birthday present for the lovely Alice :
And check out the skills of Jen Greaves and the NAM Knitters who knitted a WHOLE GALAXY CLUSTER at this year’s National Astronomy Meeting in Portsmouth:
For those of you itching to stretch your creative muscles but haven’t been struck by that bolt of inspiration yet, here’s a KNITTING PATTERN (a first I believe for the Galaxy Zoo blog) for the Galaxy Zoo logo to keep you busy…
To honour the SEVENTH anniversary of Galaxy Zoo (July 11th) we’ve put together a gallery of the Science Team’s favourite images from the site (and why) for your visual pleasure…
Chris’s favourite: I love the flocculent spiral galaxies. The ones you can stare at and still have no idea how many spiral arms they have.
Karen’s favourite: I’m a sucker for merging galaxies, despite the fact that I work on barred galaxies mainly! This one which looks like the yin-yang symbol (or maybe a heart) is a particular favourite. It’s amazing that the Universe can be so vast that we can find galaxies in so many different shapes.
Kevin’s favourite: The penguin galaxy shows the power of human pattern recognition – and a crucial stage in galaxy evolution!
Brooke’s favourite: when the latest Galaxy Zoo launched, the volunteers made a find almost right away that turned out to be a very rare kind of object called a gravitational lens. I love this image because it shows not just the variety of things that are out there in the Universe — in this case the very distant universe — but also the rare place that Galaxy Zoo itself occupies. It’s a diverse community and diverse images like this are part of the reason why.
Kyle’s favourite: did you know that we can spell Galaxy Zoo out of galaxies? The users originally started collecting a list of galaxies that look like letters and now we have writing.galaxyzoo.org thanks to Steven. Since it’s the anniversary, here’s my favourite letter G.
Bill’s favourite: Hanny’s Voørwerp really started something – the blue stuff in the image – other teams are now finding similar objects at smaller and larger distances too.
Becky’s favourite: this amazing image has SO much going on in it – mergers, interactions, spirals, bars, ellipticals, grand designs, foreground stars etc. It feels like a visual representation of thoughts in my head at times, which is clearly why I love it.
The Hubble Ultra Deep Field!!!! Just to remind us all why we’re all here. Every single thing you can see in this image is a galaxy – even the most minuscule of dots. And the size of the image on the sky is about 1/20 the size of the Full Moon… Let’s just all take a minute to let that sink in as we stare and wonder...
Friday 11th July 2014 is the SEVENTH anniversary of Galaxy Zoo! So to celebrate this momentous achievement, we’ve put together a list of seven of the greatest Galaxy Zoo discoveries (so far!); all thanks to YOU, the classifiers…
1. Chirality of Spiral Galaxies
One of the first major results from Galaxy Zoo wasn’t even Astronomical. It was Psychological. One of the questions in the original Galaxy Zoo asked whether spiral galaxy arms rotated clockwise or anti-clockwise; we wanted to check whether they were evenly distributed or whether there was some intrinsic property of the Universe that caused galaxies to rotate one way or the other. When the Science team came to analyse the results they found an excess of anti-clockwise spinning spiral galaxies. But when the team double checked this bias by asking people to classify the same image that had been flipped there was still an excess of anti-clockwise classifications; so it’s not an astronomical phenomenon. Turns out that the human brain has real difficultly discerning between something rotating clockwise or anti clockwise; check out this video if you don’t believe me – you can watch the dancer rotate both ways! Once we’d measured this effect we could adjust for it, and we went on to establish that spirals which were near each other tending to rotate in the same direction.
2. Blue Ellipticals
The enigmatic blue ellipticals in many ways started the Galaxy Zoo. Galaxies largely divide into two: spiral galaxies like our Milky Way shining with the blue light of young stars being constantly born, and the “rugby ball-shaped” elliptical galaxies who no longer make new stars and thus glow in the warm, red light of old stars. Clearly, when galaxies stop making new stars, they also change their shape from spiral to elliptical. But how exactly does this happen? And what happens first? Do galaxies stop forming stars, and then change their shape, or the other way round? Answering that question is the first step in understanding the physics of transforming galaxies. With the Galaxy Zoo, we found a whole population of blue ellipticals: galaxies which have changed their shape, but still have young stars in them. With their help, we’ve been making a lot of progress in galaxy evolution. It looks like a galaxy merger, a giant cosmic collision, changes the shape of galaxies from spiral to elliptical and then somehow – and very rapidly! – star formation stops. We don’t know quite why yet, but we think active black holes are involved. This is hugely relevant for us as in a few short billion years, the Milky Way will crash into our neighbour, the spiral Andromeda galaxy. And for a short time, the Milky Way and Andromeda will be a blue elliptical before star formation in the newly-formed Milky-Dromeda ceases. For ever.
3. Red Spirals
Ellipticals are red, Spirals are blue, Or so at least we thought, until Galaxy Zoo…. Think of your typical spiral galaxy and you’ll probably picture it looking rather blueish. Thats’s what astronomers used to think as well – suggest a red spiral to Edwin Hubble and he probably would’ve told you not to be so ridiculous. Before Galaxy Zoo if astronomers saw something looking red they generally tended to think it was elliptical; however to the untrained eye, the colour does not bias any classifications, which means that you all found lots of red spirals and discs which were hiding in plain sight. This put the cat amongst the pigeons for our galaxy evolution theories because, as said earlier, we thought that when galaxies stop making new stars, they also change their shape from spiral to elliptical. The red spirals mean that we now have a different evolutionary path for a spiral galaxy where it can stop making new stars and yet not change its shape. We now think that those spiral galaxies which are isolated in space and don’t interact with any neighbours are the ones that make it to the red spiral stage.
4. Green Peas
The Green Peas, discovered by Citizen Scientists due to their peculiar bright green colour and small size, are a local window into processes at work in the early Universe. Although, they were in the data for many years, it took humans looking at them to recognise them as a class of objects worth investigating. First noticed in some of the earliest posts of the Galaxy Zoo Forum in 2007, a group of dedicated citizen scientists organised a focused hunt for these objects finding hundreds of them by the summer of 2008, when the Galaxy Zoo science team began a closer look at the sample. The Peas are very compact galaxies, without much mass, who turn out new stars at incredible rates (up to several times more than our entire Milky Way Galaxy!). These extreme episodes of star formation are more common to galaxies in the early Universe, which can only be directly observed very far away at high redshifts. In contrast to the distant galaxies, the Peas provide accessible laboratories that can be observed in much greater detail, allowing for new studies of star formation processes. Since their initial discovery, the Peas have been studied at many wavelengths, including Radio, Infrared, Optical, UV and X-ray observations and detailed spectroscopic studies of their stellar content. These galaxies provide a unique probe of a short and extreme phase of evolution that is fundamental to our understanding of the formation of the galaxies that exist today.
5. The Voørwerp
Probably the most famous new discovery of Galaxy Zoo has been Hanny’s Voørwerp. Hanny van Arkel called attention to it within the first few weeks of the Galaxy Zoo forum with the innocent question ”What’s the blue stuff?”, pointing to the SDSS image of the spiral galaxy IC 2497. The Sloan data alone could indicate a gas cloud in our own Galaxy, a distant star-forming region, or even a young galaxy in the early Universe seen 10 billion light-years away. After a chase to obtain new data, above all measurements of the cloud’s spectrum, with telescopes worldwide, an unexpected answer emerged – this galaxy-sized cloud was something unprecedented – an ionization echo. The core of the galaxy hosted a brilliant quasar recently on cosmic scales, one which essentially turned off right before our view of it (so we see the gas, up to 100,000 light-years away from it, shining due to ultraviolet light form the quasar before it faded). This had never before been observed, and provides a new way to study the history of mass surrounding giant black holes. Further observations involved the Hubble Space Telescope and Chandra X-ray Observatory (among others), filling in this historic view. The cloud itself is part of an enormous stream of hydrogen, stretching nearly 300,000 light-years, probably the remnant of a merging collision with another galaxy. As it began to fade, the quasar started to blow out streams of energetic particles, triggering formation of stars in one region and blowing a gaseous bubble within the galaxy. In keeping with the nature of Galaxy Zoo, the science team deliberately had much of this unveiling play out in full view, with blog entries detailing how ideas were being confronted with new data and finding themselves supported, discarded, or revised. The name Hanny’s Voørwerp (which has now entered the astronomical lexicon) originated when an English-speaking Zoo participant looked up “object” in a Dutch dictionary and used the result “Voørwerp” in a message back to Hanny van Arkel. Following this discovery, many Zoo volunteers participated in a focused search for more (the “Voørwerpjes”, a diminutive form of the word) – as a result we now know 20 such clouds, eight of which indicate fading nuclei. Other teams have found similar objects at smaller and larger distances; Hanny’s Voørwerp really started something!
6. Bars make galaxies redder
A galactic bar is a straight feature across a spiral galaxy. It’s the orbital motions of many millions of stars in the galaxy which line up to make these bars, and in computer simulations almost all galaxies will form bars really quickly. In the real world it’s been known for a long time that really strong (obvious) bars are found in about 30% of galaxies, while about 30% more have subtle (weak) bars. One of the big surprises about the Galaxy Zoo red spirals was just how many of them had bars. In fact we found that almost all of them had bars and this got the science team really curious. We followed this up with a full study of which kinds of spiral galaxies host bars using the first classifications from Galaxy Zoo 2. In this work we discovered a strong link between the colour of disc galaxies and how likely they are to have a bar – with redder discs much more likely to host bars. We now have half a dozen papers which study galactic bars found using Galaxy Zoo classifications. Put together these works are revealing the impact galactic bars have on the galaxy they live in. We have found evidence that bars may accelerate the processes which turn disc galaxies red, by driving material into the central regions to build up bulges, and clearing the disc of the fuel for future star formation.
7. Bulgeless galaxies with black holes
Supermassive black holes are the elusive anchors in the centres of nearly every galaxy. Though they may be supermassive, they are quite difficult to spot, except when they are actively growing — in which case they can be some of the most luminous objects in the entire Universe. But how exactly they grow, and why there seems to be a fixed mass ratio between galaxies and their central black holes, are puzzles we haven’t solved yet. We used to think that violent collisions between galaxies were The Way you needed to grow both a black hole and a galaxy so that you’d end up with the mass ratio that we observe. And violent collisions leave their signatures on galaxy shapes too. Namely: they destroy big, beautiful, ordered disks, re-arranging their stars into bulges or forming elliptical galaxies. So when we went looking for pure disk galaxies with no bulges and yet with growing central black holes we weren’t sure we would find any. But thanks to the volunteers’ classifications, we did. These galaxies with no history of violent interactions yet with large central supermassive black holes are helping us test fundamental theories of how galaxies form and evolve. And we are still looking for more of them!
So here’s to SEVEN more years – keep classifying!