The Universe is pretty huge, and to understand it we need to collect vast amounts of data. The Hubble Telescope is just one of many telescopes collecting data from the Universe. Hubble alone produces 17.5 GB of raw science data each week. That means since its launch to low earth orbit in April 1990, it’s collected roughly a block of data equivalent in size to 6 million mp3 songs! With the launch of NASA’s James Webb Telescope just around the corner – (a tennis court sized space telescope!), the amount of raw data we can collect from the Universe is going to escalate dramatically. In order to decipher what this data is telling us about the Universe we need to use sophisticated statistical techniques. In this post I want to talk a bit about a particular technique I’ve been using called a Markov-Chain-Monte-Carlo (MCMC) simulation to learn about galaxy evolution.
Before we dive in into the statistics let me try and explain what I’m trying to figure out. We can model galaxy evolution by looking at a galaxy’s star formation rate (SFR) over time. Basically we want know to how fast a particular galaxy is making stars at any given time. Typically, a galaxy has an initial constant high SFR then at a time called t quench (tq) it’s SFR decreases exponentially which is characterised by a number called tau. Small tau means the galaxy stops forming stars, or is quenched, more rapidly. So overall for each galaxy we need to determine two numbers tq and tau to figure out how it evolved. Figure 1 shows what this model looks like.
Figure 1: Model of a single galaxy’s SFR over time. Showing an initial high constant SFR, follow by a exponential quench at tq.
To calculate these two numbers, tq and tau, we look at the colour of the galaxy, specifically the UVJ colour I mentioned in my last post. We then compare this to a predicted colour of a galaxy for a specific value of tq and tau. The problem is that there are many different combinations of tq and tau, how to we find the best match for a galaxy? We use a MCMC simulation to do this.
The first MC – Markov-Chain – just means an efficient random walk. We send “walkers” to have a look around for a good tq and tau, but the direction we send them to walk at each step depends on how good the tq and tau they are currently at is. The upshot of this is we quickly home in on a good value of tq and tau. The second MC – Monte Carlo – just picks out random values of tq and tau and tests how good they are by comparing the UVJ colours and our SFR model. Figure 2 shows a gif of a MCMC simulation of a single galaxy. The histograms shows the positions of the walkers searching the tq and tau space, and the blue crosshair shows the best fit value of tq and tau at every step. You can see the walkers homing in and settling down on the best value of tq and tau. I ran this simulation by running a modified version of the starpy code.
Figure 2: MCMC simulation for a single galaxy, pictured in the top right corner. Main plot shows density of walkers. Marginal histograms show 1D projections of walker densities. Blue crosshair shows best fit values of tq and tau at each step.
The maths that underpins this simulation is called Bayesian Statistics, and it’s quite a novel way of thinking about parameters and data. The main difference is that instead of treating unknown parameters as fixed quantities with associated error, they are treated as random variables described by probability distributions. It’s quite a powerful way of looking at the Universe! I’ve left all of the gory maths detail about MCMC out but if you’re interested an article by a DPhil student here at Oxford does are really good job of explaining it here.
So how does this all relate to galaxy morphology, and Galaxy Zoo classifications? I’m currently running the MCMC simulation showing in Figure 2 over the all the galaxies in the COSMOS survey. This is really cool because apart from getting to play with the University of Oxford’s super computer (544 cores!), I can use galaxy zoo morphology to see if the SFR of a galaxy over time is dependent on the galaxy’s shape, and overall learn what the vast amount of data I have says about galaxy evolution.
It’s been a good amount of time since the Galaxy Zoo: Hubble and Galaxy Zoo: CANDELS projects were finished, tackling more than 200,000 combined galaxies thanks to the efforts of our volunteers. While we’ve had a couple of science papers based on the early results (Melvin et al. 2014, Simmons et al. 2014, Cheung et al. 2015), a full release of the data and catalog has taken slightly longer. However, we’ve been working hard, testing the data, and developing some new analysis methods on both image sets. This month has been really exciting, and we now have drafts for both papers that are just about finished. Once they’ve been accepted to the journals (and revised, if necessary), we’ll have some much longer posts discussing the results, and of course attaching the papers themselves. Hopefully that’ll be quite soon.
As a small teaser, here’s a little movie I just made of the Galaxy Zoo: Hubble paper as it went through the various drafts by different members of the science team. If only all paper writing were this easy …😉
Once upon a time, there was an experimental project called Galaxy Zoo: Mergers. It used ancient, mystical technology to allow volunteers to run simulations of merging galaxies on their computers, and to compare the results of many such simulations. Their mission: to find matches to more than fifty nearby mergers selected from Galaxy Zoo data.
Amongst the chosen galaxies were not just run-of-the-mill, everyday mergers, but also the various oddities that the volunteers found, such as the Penguin galaxy. The team led volunteers through a series of tournaments designed to pit potential solutions for a particular galaxy against each other. In total, more than 3 million simulations were reviewed producing the results described in the paper, now accepted by the journal MNRAS, and in the dataset visible at the main Galaxy Zoo data repository. This represents a huge amount of effort, and a speeding up of the process – in the paper, we note that previous fits to mergers have taken months of effort to complete.
Which is not to say the analysis, led by Anthony Holincheck and John Wallin, has been easy. In a recent email to the Galaxy Zoo team, John commented:
This is by far the most complex project I have ever worked on. Most papers that model interacting galaxies contain one or two systems where the author uses a few dozen simulations. We just published a paper that modeled 62 different systems using a brand new modeling technique where the 3 million simulation results were reviewed by citizen scientists. Best of all, the 62 models were done using the same code and the same coordinate system so others can reproduce them. Doing this with other published simulations is nearly impossible.
I know an immense amount of effort went into making sure that the results weren’t wasted, and the paper thus represents a happy ending to a tale that’s been running a long time. But it is not really an end; we are already planning to observe some of these galaxies as part of surveys like MaNGA that can measure the way that the galaxies’ components are moving today, allowing us to test these models. We also hope a library of models might be useful for other astronomers, and will be looking to try and revive this kind of project.
Read more about Galaxy Zoo: Mergers in this old blog post blog.galaxyzoo.org/2012/03/27/the-finale-of-merger-zoo.
This post was written as a contribution by Timothy Friel, an undergraduate Australian National University student studying Theoretical Physics and Science Communication. Tim is conducting research into citizen science projects and their social media communication strategies.
Meet two of our fantastic Zooniverse members who have been recognised as co-authors for a RGZ submitted paper.
In March 2016, the Radio Galaxy Zoo (RGZ) team submitted a paper which is co-authored by two of our SuperRGZooites. Thanks to the help of citizens around the world, over 1.6 million classifications have been made. However, a very special thanks must go to two citizens who have been greatly involved in our most recent submitted paper.
Meet Ivan Terentev and Tim Matorny, our Citizen Science co-authors.
How did you discover Radio Galaxy Zoo and become involved?
Tim: I had a passion for research and to be involved with generating new knowledge. So I began to look and met [the world of] citizen science and tried many different projects. I was already familiar with the Zooniverse, when I got email about new project – RGZ.
Ivan: I became involved in RGZ from its beginning, more or less, in December 2013, and at that time I was part of the Zooniverse for two years. I was mostly contributing to the Planet Hunters project back then, but occasionally I switched to different projects just to look for what they have to offer. And it was during one of these “Let’s try something different” moments that I discovered RGZ through the announcement post in the Galaxy Zoo blog.
What parts kept you interested and motivated to stay a part of this project?
Tim: The team of scientists and their active participation is an important part. Their blog posts, comments and links have helped me to learn about the project and my involvement with the goals.
Looking for host radio lobes which are separated by a 10′ [minutes] or looking at the behaviour of jets in galaxies clusters is really exciting for me. I like that RGZ covers a wide range of data: radio, optics, IR, X-ray.
Ivan: If we are talking specifically about RGZ, it would be the RGZ Talk community and the fact that RGZ Science team is eager to communicate with simple volunteers and involve them in the research process. But a large portion of my motivation [for RGZ] is the same as for the rest of the Zooniverse projects. You see, I am sci-fi fan and it made me interested in space exploration. I like to watch documentaries about the astronomers, their work and all the amazing stuff in the universe around us and through the Zooniverse I can actually be involved in the process of science and help to shape the future, even if it just by a very tiny fraction. I never thought that something like this would be possible before I discovered Zooniverse.
How do you feel about being a co-author of a scientific research paper?
Tim: I am still amazed and feel more motivated to look for stunning new radio galaxies.
Ivan: This isn’t the first time actually, I am also a co-author for three papers from the Planet Hunters, BUT it is always awesome, like every single time! Although, I keep my head cool over that since most of the work was done by the professional scientists. A huge thanks to them for the acknowledgment of my small contribution in the form of inviting me to be a co-author in their paper. With this RGZ paper, I got a chance to see the whole process of science starting from the simple question “What is that?” and then people trying to figure out what is going on, schedule observations, discussing things and I have been a part of it! All the way through the process, ending with the actual published science article. It was an amazing experience!
Without the contributions made by our volunteers all over the world, we would not have been so successful in our endeavours.
However, we have only reached 57% of our classification target. Head to www.bit.ly/RadioGalaxyZoo1 to become involved and you could be co-authoring another great discovery with us!
I’m happy to report that in the last several days, we’ve simultaneously finished the initial sets of galaxies from both the DECaLS survey and the second subset of simulated galaxies from Illustris. This has meant the completion (since last September) of more than 50,000 galaxies seen 40 times apiece, for more than 2 MILLION classifications.
So far, your work is helping reveal new insights based on this deeper data. One very preliminary result: as we’d predicted, the better conditions in DECaLS (bigger camera, better night sky seeing, larger telescope mirror) are revealing galaxies that were classified in SDSS as smooth, but in fact with faint or extended disks and features that are now visible. This is really exciting, and is helping to modify our ideas of the assembly histories of these galaxies.
The Galaxy Zoo site is still active – we’ve reactivated a few of the DECaLS DR1 galaxies to slightly improve our statistics, but shortly we’re going to add new sets of (real images) to continue the next phase. I’ll post more as soon as we’ve finalized our plans.
As always, our sincere thanks! Time to start our analysis and continue the science…
Following on from the excellent summary of the hi-lights in 2015 for the Radio Galaxy Zoo project, here’s a similar post about results from Galaxy Zoo.
This year we collected 4,755,448 classifications on 209,291 different images of galaxies. You continue to amaze us with your collective efforts. Thank you so much for each and everyone of of these classifications.
The year started with Galaxy Zoo scientists at Mauna Kea observing galaxies, and reported in this wonderful series of blog posts by (former) Zooniverse developer Ed Paget.
We celebrated 8 years of Galaxy Zoo back in July, with this blog series of all things 8-like about Galaxy Zoo.
Back in May we finished collecting classifications on the last of our Hubble Space Telescope images. At the AAS in Florida this week, Kyle Willett and Brooke Simmons presented posters on the planned data releases for the classifications.
We both launched and finished classifying the first set of images of simulated galaxies from the Illustris Simulation (read more here: New Images for Galaxy Zoo: Illustris and here: Finished with First Set of Illustris Images). We also launched our first set of images from the DECaLS survey, which is using the Dark Energy Camera (New Images for Galaxy Zoo: DECaLS)
We also launched a new Galaxy Zoo side project – Galaxy Zoo Bars (one of the first projects built on the new Zooniverse Project Builder software), measuring bar lengths of galaxies in the distant Universe. The entire set were measured in less than a year, so thank you to any of you who contributed to that, and if you missed it don’t worry, we have plans for more special projects this year.
We launched a new web interface to explore the Galaxy Zoo classifications.
Our contributions to the peer reviewed astronomical literature continue. Papers number 45-48 from the team were officially published in 2015. They were:
– Galaxy Zoo: the effect of bar-driven fueling on the presence of an active galactic nucleus in disc galaxies, Galloway+ 2015.
– Galaxy Zoo: Evidence for Diverse Star Formation Histories through the Green Valley, Smethurst+ 2015.
– Galaxy Zoo: the dependence of the star formation-stellar mass relation on spiral disc morphology, Willett+ 2015.
You can access all 48 team papers using your classifications at the Zooniverse Publication Page. Remember that all Zooniverse papers published in the Monthly Notices of the Royal Astronomical Society – which includes most of the Galaxy Zoo papers – are available open access to any reader, and if we happen to publish elsewhere we always make the post-acceptance version available on the arxiv.org.
All of our papers include a version of this acknowledgement to our classifiers: “The data in this paper are the result of the efforts of the Galaxy Zoo volunteers, without whom none of this work would be possible. Their efforts are individually acknowledged at authors.galaxyzoo.org.” We all hope you all know how grateful we are for each and every one of your classifications.
This year saw publication of the first paper on Hubble observations of Voorwerpje systems accompanied by an HST press release.
One of those papers from (mostly) outside the GZ team discussed a rare examples of double radio sources from spiral hosts, something Radio Galaxy Zoo will find many more of: “J1649+2635: a grand-design spiral with a large double-lobed radio source”, Mao et al. 2015.
Another exciting thing about this year has been the number of papers from non team members using the classifications which are now public (see data.galaxyzoo.org). To date almost 300 astronomical papers have been written which cite the original description of Galaxy Zoo (Lintott et al. 2008) and the two data release papers so far (Lintott et al. 2011 for GZ1 and Willett et al. 2013 for GZ2) have 164 and 34 citations respectively. The number of papers in the Astrophysics Data System which contain the words “Galaxy Zoo” (which you can search in ADS Labs) is an astonishing 700 (409 for refereed publications).
These are just some of the high-lights I’ve pulled together. If I’ve missed your favourite feel free to add it in the comments below. All in all it’s been a great year. Here’s to an equally good 2016!
Our first paper “Radio Galaxy Zoo: host galaxies and radio morphologies derived from visual inspection” was published in Monthly Notices of the Royal Astronomical Society (MNRAS) in September;
upon the recommendation of our referee, our paper on hybrid morphology radio sources will be split into two papers; and
the giant wide angle tail (WAT) discovery paper will be available soon.
progress on the giant WAT is continuing to bring up more interesting information including our JVLA data – potentially 3 additional papers;
we obtained 4 hours to obtain a spectrum for four of our green DRAGN with the observations scheduled for March 2016; and
- with all your work, RGZ has discovered over 100 new giant radio galaxies!
matching of RGZ classifications to SDSS;
merging Galaxy Zoo data with Radio Galaxy Zoo data;
our observations with the JVLA on the hybrid radio sample is complete with 60 hours of observing time; and
- we are working with the International Astronomical Union (IAU) to get the RGZ name official.
Martin Hardcastle (Hertfordshire)
Sarah White (ICRAR/Curtin)
Francesco de Gasperin (Leiden)
Many bargains must be made in pursuit of an academic career, and chief among them is an openness to a nomadic early-career life in exchange for a better chance at staying permanently put somewhere later. Grad students and postdocs move around. Not only do we travel all over the world sharing and discussing our research, but the relatively short duration of postdocs, and the fact that in astronomy doing at least 2 of them is now the norm, means we regularly pull up roots and dash off to live somewhere else. My friends have collectively done postdocs on all continents, including Antarctica. Including places thousands of miles from friends and family; including places where they can neither read nor speak any of the native languages.
In this context, I am so, so lucky. My first postdoc moved me only a medium distance (across just one ocean), and to a place where I could at least understand the words, even if I didn’t always get every nuance of meaning. At Oxford I made lifelong friends and built great collaborations, and I thought the research itself was pretty good, too.
Turns out NASA agrees with me. Last year I applied for and was awarded an Einstein Fellowship, which is an early-career award lasting 3 years, an independent postdoc that can be taken to any institution in the US. They’re very competitive (I had applied the previous year without success), and I was thrilled to be awarded one at my top-choice host institution. My first day was last week.
Here’s what the 2015 Fellows page has to say about my research plans:
Brooke uses a variety of multi-wavelength data, including highly accurate galaxy morphologies from the Galaxy Zoo project, to research the connection between supermassive black holes and the galaxies that host them. This connection appears to exist over many orders of magnitude in black hole and galaxy mass, but its fundamental origin is still a puzzle. As an Einstein Fellow at the University of California, San Diego, Brooke will investigate supermassive black hole growth in the absence of galaxy mergers, using a rare sample of galaxies which have never had a significant merger yet host growing black holes. These active nuclei, selected because their host galaxies lack the bulges which inevitably result from a galaxy merger, provide powerful leverage to disentangle the complex drivers of black hole growth and determine the origin of observed black hole-galaxy correlations.
During my fellowship I’m planning on moving forward with the research we first published in 2013 investigating bulgeless galaxies with growing black holes. That is: it’s Galaxy Zoo research.
Galaxy Zoo research brought me to Oxford, and now it has brought me to California. UCSD is a great place, and I’ve already made some really excellent scientists. UCSD is also part of the Southern California Center for Galaxy Evolution and has access to some of the world’s best telescopes, so the future is full of potential.
For now, though: I wouldn’t be here, watching sunsets from my office, without your contributions to Galaxy Zoo over the years. Thank you.
This post was written with the help of Sugata Kaviraj, senior lecturer at the University of Hertfordshire in the UK. Sugata has been a member of the GZ science team for several years and is leading our analysis of tidal debris in the DECaLS images.
One of the biggest changes to the questions we’re asking for the new DECaLS images asks users to help us classify galaxies that are either merging and/or exhibiting “tidal debris”. While mergers have been part of Galaxy Zoo since our very first classifications, tidal debris is looking for something more specific, which the new DECaLS data is particularly suited for.
In astronomy, “tides” are a type of force exerted on an object by the effect of gravity. Specifically, it refers to the gravitational force exerted by one body on another – since the force exerted by gravity depends on the distance to the object (specifically, it’s proportional to the inverse square of the distance), the nearer side of the object will feel a stronger force than the farther side of the object. On Earth, the differential force caused (mostly) by the gravity of our Moon acts on the liquid in the oceans, causing the rise and fall in sea levels each day.
When the object is a solid body (like a planet or moon), tidal forces can strain and compress the body, resulting in internal heat and sometimes driving geological activity. A galaxy isn’t a solid body, but composed of individual stars/gas clouds/dark matter particles bound within its own gravitational field. When a galaxy is subjected to tidal forces, it pulls the galaxy apart, causing irregularities in shape that can take many forms depending on the magnitude and direction of the forces involved.
One of the main reasons we want to identify tidal features specifically is to make better measurements of the merger history of galaxies. A complication is that the event of merging with a galaxy isn’t an instantaneous event – depending on the relative masses, velocities, and orientations of the merging galaxies, tidal forces strip out long tails of stars and gas from the galactic centers.
The Mice (NGC 4676): colliding galaxies with tails of stars and gas distorted by tidal forces. Source: Hubblesite.org
Ultimately, the nuclei of the two galaxies will fully merge; if that happened sufficiently far in the past for the orbits of stars to relax, then it’s difficult for observers to determine if an elliptical galaxy today was the result of a merger.
Tidal debris features, however, are longer-lived signals of a merger sometime in a galaxy’s past. If we only asked about mergers, we’d be restricting the sample to galaxies that we’re lucky enough to observe “right in the act” of merging. By identifying the tidal debris as well, we can make a more complete census of galaxies that had a merger at some point in their past. One project that this is critical for is looking at the history of galaxy populations, and trying to figure out whether star formation and/or active black holes might be powered by merger events.
Since tidal debris features can be very faint (ie, having low surface brightnesses), the deeper DECaLS images that we’re currently classifying are much better at picking out these features than SDSS. That’s the main reason we’re focusing on trying to detect them in the current set of images.
Tidal debris can come in many different forms, including extended light, faint shells, dust lanes, or satellites in the process of being assimilated to clearly distorted galaxies which have presumably have had a recent interaction. The features can definitely be faint, but we’d like to ask that when you reach this question, please take a careful second look at the area around the main galaxy and see if you can spot anything. Examples are also available in the help text, and we hope that you’ll discuss features you’re not certain about with the moderators, scientists, and other volunteers in Talk.
Please let us know, here or on Talk, what questions you have. Thanks for your help in tackling a new and interesting scientific problem.
One of the best things about being a scientist is the opportunity to attend conferences – you get to visit a new place, meet your colleagues in person, learn about what they’ve been doing, and get a chance to share your exciting research with them. I’m lucky (through the assistance of the American Astronomical Society, the Italian National Institute for Astrophysics, and the University of Minnesota) to participate in a conference this week on the future of extragalactic radio surveys in Bologna, Italy. I’m getting my first chance to share results from Radio Galaxy Zoo and to learn about other, new results in the area of extragalactic radio science!
The conference is four days, from Tuesday – Friday; I’m going to try to make a blog post each day. I’m going to try give a quick overview of all talks/posters on the day, as well as more details on talks which I thought were particularly interesting. I know I won’t do justice to many of the interesting research topics being presented, but I won’t have time to give every topic the breadth they deserve.
The first day of the workshop started with several talks covering current and upcoming surveys in radio astronomy. These include radio telescopes in the Northern Hemisphere. The two main telescopes discussed were the Very Large Array (VLA) in New Mexico, USA, which will run surveys like VLITE (a low frequency survey which will run constantly on the telescope in parallel with other observations), and VLASS, a new all-sky survey with many similarities to the current FIRST data in Radio Galaxy Zoo. LOFAR is a low-wavelength radio telescope with stations centered around Europe; it will open up similar resources, but at significantly lower frequencies than the VLA and thus probing different physical phenomena. In the Southern Hemisphere, the EMU survey in Australia and the MIGHTEE survey in South Africa will carry out similar responsibilities.
I gave a talk at the end of this session on Radio Galaxy Zoo, covering our first accepted paper and some of our early science results. If you’re interested, I’ve put my talk online here.
The afternoon had two sessions on science: one on radio continuum and star formation, and one on radio observations of the transient universe.
I think after the first day that I’m filled with a great sense of optimism about radio astronomy. We’ve got a fantastic new telescope being built in the next several years: the Square Kilometer Array. It’ll be the largest telescope ever built, addressing a huge number of scientific questions. We’re currently in the stage of building prototype telescopes, but those telescopes are already producing useful science – some of which I learned of today. We have a reasonable understanding about how things like magnetic fields affect both the formation and evolution of galaxies. Radio observations have a unique way of detecting and leveraging these detections; through polarization of the radio signal, we can measure the magnetic field and directly probe (through its signal) the interactions with matter between its source and our telescopes. New phenomena like fast radio bursts are, I think, a really neat way of measuring both the amount and distribution of matter in the Universe – this has implications for everything from star formation to cosmology.
Really excited for the rest of the week (including more Radio Galaxy Zoo results) – will post again tomorrow!