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 our eighth birthday! The team have done a great job exploring the various ways the number eight connects to the Galaxy Zoo Universe and that collection of blogs does a brilliant job of illustrating the dramatic variety of places we’ve explored together. Some of them were familiar, but others we didn’t even dream of before the start of the project.
Once you start thinking about it, thinking of Galaxy Zoo as an exploration, as a journey undertaken as a group makes a lot of sense. Lots of you have joined us for the whole journey, as we’ve travelled further and further from familiar ground, while others – just as welcome – have walked only a little way. The science team, too, has grown as it has become apparent quite how much can be done with your classifications, and the whole grand parade has attracted a following of computer scientists, web developers and other assorted camp followers.
I’m writing this on my way to report on the arrival of New Horizons at Pluto for the Sky at Night. For the first time, we’ll see close up images of a world that until now has been little more than a point of light. The missions is part of the glorious tradition of Solar System exploration, but our journey through the datasets provided by the Sloan Digital Sky Survey and by Hubble are voyages of exploration too. We need not travel to distant galaxies to understand them; encountering something new and never-before-seen in your web browser is thrill enough. Thanks for all the classifications of the last eight years – here’s to many more.
As we approach the 8th anniversary of the Galaxy Zoo project, it is a great opportunity to look back at one of the most fascinating discoveries of citizen science in Galaxy Zoo – the “Green Pea” galaxies. Volunteers on the forum first noted these galaxies due to their peculiar bright green color and small size. Their discovery was published in our 8th paper: ‘Galaxy Zoo Green Peas: discovery of a class of compact extremely star-forming galaxies’ and is noted on the blog here. But the story doesn’t end with their discovery.
In the years since the publication of their discovery paper by the Galaxy Zoo Science Team, the Green Peas are beginning to fulfill their promise as a living fossil of galaxy evolution. Because they aren’t too far away, they provide a unique local laboratory in which we can investigate processes key to the formation and evolution of galaxies in the early universe. They are living ‘fossils,’ undergoing extraordinary, intense starbursts unlike any other galaxies known in the local universe. Their color is due to a large amount of emission in an oxygen line [OIII]/5007A that made their appearance green in the images.
Follow-up studies of the Green Peas have looked in great detail at their abundances of various elements, something that cannot be done in their high redshift analogs. The results of these studies show that they have energetic outflows of gas and lower oxygen abundances than other typical local galaxies with similar masses. They also suggest what might be responsible for ionizing the gas in the galaxies and producing those bright emission lines (e.g., Wolf-Rayet stars). Their clumpy morphologies (or shapes) have been confirmed and suggest that star formation in the peas occurs in several separate knots throughout the galaxy. Their radio emission implies they have strong magnetic fields, larger than that of the Milky Way. All of these results paint a picture of galaxies very similar to those that formed in the early Universe.
Results from studies of these galaxies can provide challenges to commonly accepted models. For example, the strong magnetic fields challenge models that suggest magnetic fields grow slowly over time and observations of the variation in Lyman alpha emission line profiles and strengths challenge models of the dependence of the emission line shape on gas properties in the galaxy. The Green Peas have held up their promise of lending new insights into galaxy evolution by characterizing an active mode of star formation, which contrasts with the typical more passive evolution dominating the local galaxy population. Studies of the Peas have suggested that a galaxy’s evolutionary pathway may depend on stochastic initial conditions, leading insights into our understandings of how galaxies throughout the Universe form.
Dear Radio Galaxy Zoo volunteers,
Thanks again for all your help so far in classifying radio galaxies through RGZ. We’re rapidly approaching our 1 millionth classification, probably by the end of this week (Jan 15-17) at the current rate. Don’t forget that we’ll be awarding prizes!
In the meantime, we’re excited to announce that we’ve just finished processing a new set of images for RGZ. There are 2,461 new images in total: the radio images are from a survey named ATLAS, carried out by the ATCA telescope in Australia. The corresponding infrared images come from the Spitzer Space Telescope as part of a survey named SWIRE.
Due to the differences in telescopes (ATCA has fewer dishes and a different arrangement of them than the VLA, while Spitzer has a much bigger mirror than WISE) and the depths of the two surveys, the data will look a little bit different. If you’ve done lots of classifications on Radio Galaxy Zoo already, you may notice more elongated radio beams in the ATLAS data, as well as a slightly larger size of the smaller unresolved noise spots. ATLAS can also detect fainter objects than the FIRST survey.
The new SWIRE infrared images have about twice the angular resolution of WISE (it can separate objects down to 3 arcseconds apart) and are more than 20 times as sensitive. That means you’ll likely see more infrared objects in the new images, and might have more choices for likely host galaxies for radio emission.
Since the images are mostly similar, the task for RGZ hasn’t changed (in fact, the original tutorial image was from ATLAS data). We’re still asking you to pick out individual radio components (or groups of components) and match them to their IR host galaxies. The new images will be randomly mixed in with the older images; you should see an ATLAS image every 6th or 7th classification, on average. If you’re curious whether a galaxy you’ve just classified is in ATLAS, the easiest way is to look at it in Talk: the new galaxy names will begin with a “C” (eg, “CI3180”) and will have declinations that are negative (eg, -27.782) showing that they’re in the Southern Hemisphere.
We’ll post a longer blog post very shortly with more information on ATLAS, SWIRE, and what we’re hoping to learn from these new images. In the meantime, please post here or on Talk if you have any questions!
And keep up the classifications in the next few days — hopefully you can be our 1 millionth image!
Last October, Galaxy Zoo began including new images from the UKIDSS survey on the main site. These are many of the same galaxies that were classified in GZ2, but the images come from a completely different telescope and a different wavelength — the infrared. While there’s a lot of science we’ll be able to do comparing galaxy morphologies at different wavelengths, many volunteers have noticed artifacts (features that aren’t real astronomical objects) in the UKIDSS images that can look very different from what you’re used to seeing in the SDSS or Hubble images:
- green squares
- rings and ghosts
- grid patterns and speckles
These are only a small percentage of the images we’re looking at, but it’s important to identify them and try to separate them cleanly from the galaxies we’re classifying. So here’s our “spotter’s guide” to UKIDSS image artifacts.
All of the UKIDSS images you see in Galaxy Zoo are what we call “artificial-color” — we use images captured by the telescope’s infrared detector, and then combine the different infrared wavelengths into a single color image. For our images, we use data from the Y-band filter (1.03 microns) for the red channel, J-band filter (1.25 microns) for green, and K-band (2.20 microns) for the blue channel.
The images in Y, J, and K were taken at separate times and with different detectors and filters. So for changes in either the camera or the sky, these will often only show up in one color in the GZ images.
Some users have identified a persistent pattern in the images that looks like four little green pixels arranged in a square (looks a little like the UKIDSS logo!). This is from the J-band images.
The origin of the squares comes from the way that UKIRT processes data. Each patch of the sky is imaged in multiple exposures, and then these exposures are combined to get the final, deeper image. So each pixel in the image comes from four different locations on the detector. In the case of J-band images, the telescope actually took 8 different exposures during the dither pattern. For a few of the observing runs, the telescope lost the guidestar which keeps it positioned at the correct location; that means that the expected number of counts at the position of a bright star is lower due to the bad frame in the interlaced data. Normally, the software algorithms in UKIDSS drop the bad frames and correct for this effect; as GZ volunteers have identified, though, there are some cases where it didn’t work perfectly. (Many thanks to UKIDSS Survey Scientist Steve Warren at Imperial College London for his help in explaining this phenomenon.)
Since the exposure pattern is in a square, the bad pixels will show up where there’s a bright star and one of the four frames is bad (meaning counts are lower than they should be). That’s the origin of the pattern showing up in some images.
As mentioned above, the telescope takes multiple exposures for each part of the sky that it images. To improve this, for some of the bands, it images the same part of the sky for a second round, but offsets the location of these by either an integer or half-integer pixel. The reason for this is so we can improve the angular resolution of the telescope – that is, distinguishing small features in the galaxy that are normally blurred out by either the Earth’s atmosphere or the limiting power of the telescope itself.
In the final data products, images from these offset frames are combined onto a fixed pixel scale in a process called interleaving. In some sources (bright ones especially), the gridding isn’t perfect and you can see some of the scale for this in the images.
Another feature people have spotted are what have been called “ghosts”: these can be either regular or irregularly shaped objects appearing in a couple specific colors. There might be multiple causes for these, but one of the most common is the presence of an actual contaminant (a speck of dust, for example) that got into the optics of the telescope. Since the telescope isn’t designed to focus on nearby objects, the point source is distorted, usually into a ring-like shape. The color of these images, like the green squares, depends on what band they were imaged in; red for Y-band, green for J-band, or blue for K-band.
Here’s one example: you can see the green and blue ring to the right of the galaxy in the color GZ image. The raw data (in black and white) shows the same ring in multiple locations, which tells us that it remained in the same position on the detector, but appears several times as the telescope moves over the sky.
We hope this has been useful, but please continue to discuss these in Talk and on the forums; particularly if there are any artifacts that impede your ability to make a good galaxy classification. Happy hunting, and thanks for continuing to participate with us on Galaxy Zoo.
Galaxy Zoo Volunteers Share Pain and Glory of Research
The Sloan Digital Sky Survey (SDSS) has compiled a list of more than 1 million galaxies. To glean information about galaxy evolution, astronomers need to know what type of galaxy each one is: spiral, barred spiral, elliptical, or something else. The only reliable way to classify galaxies is to look at each one, but all the world’s astronomers working together couldn’t muster enough eyeballs for the task. A volunteer online effort called Galaxy Zoo, launched in 2007, has classified the entire catalog years ahead of schedule, bringing real statistical rigor to a field used to samples too small to support firm conclusions. The Galaxy Zoo team went on to ask more-complicated classification questions that led to studies they hadn’t thought possible. And in a discussion forum on the Galaxy Zoo Web site, volunteers have pointed to anomalies that on closer inspection have turned out to be genuinely new astronomical objects.
Unfortunately accessing the full article requires a subscription.
Here in the Oxford University astrophysics group we’ve been thinking about astronomy games to play with the school and public groups that come to our telescope evenings — do you remember the “Top Trumps” card game? Well how about “Galaxy Top Trumps”?!
If you grew up in the UK then you may have spent your school lunch hour playing this game. It’s a really simple card game: each pack of Top Trumps has a theme – cars, footballers, fighter planes, now they cover pretty much anything you can imagine (except galaxies). For example, each card in a cars pack would have a picture of a car and a handful of numbers for that car, such as its top speed or fuel consumption. The player on the dealer’s left chooses one of these numbers, and players compare the number on the top card in each of their hands. The card with the best number wins the trick, and its owner collects the other cards. Play continues until one player holds all the cards, or lunch hour ends and double maths begins, whichever comes first!
At school we became experts on cars as a result of playing this game (did you know that a diesel Ford Fiesta uses three times less fuel per kilometre than a Land Rover Discovery?). So, we had a first go at making astronomical Top Trumps – we made a pack for stars, and one for planets, and are finding that playing the game is a great way of introducing these objects to people, showing them the wide range of things out there, and giving them an idea of their relative properties. (Did you know that Saturn’s moon Titan is bigger than Mercury? And Betelgeuse is 140,000 times brighter than the Sun?)
Now we’d like to make a Galaxy pack, and we’d like your help! We think galaxies are a perfect theme, and we should be able to design a pack that is fun to play and through which people can learn a little bit about the amazing objects in the Zoo. Designing a mathematically perfect pack can get complicated, as James Grime explains in this great video, so for now we just want to focus on cool galaxies and good numbers to pick!
So, which galaxies and which attributes? There are some starting thoughts below, but what we really want are your ideas! What numbers would you like to see on the cards? Are we missing an interesting class of object? Did we forget your favourite galaxy?! Let us know!
You can head on over to this new forum thread or leave a comment here!
We probably won’t be able to get telescope time to measure new numbers to put on the cards (apparently the Time Allocation Committees aren’t so keen on proposals that are motivated by Top Trumps!), so we need to select attributes that we can either look up or get pretty easily from public data like the SDSS. We think things like radius, stellar mass, colour, star formation rate, supermassive black hole mass and redshift might be good. But what do you think?
As for choosing the galaxies, the obvious place to start is with the best-known, prettiest nearby galaxies that no pack would be complete without: Centaurus A, Andromeda, the Sombrero galaxy and maybe the Omega Centauri globular cluster (Are globular clusters galaxies? It’s a good conversation starter!). Any others? Then we might add one each of the basic Hubble types: elliptical, S0, Sa, Sb, Sc and Irregular (let us know if you have a favourite!). Then with all the recent Galaxy Zoo work on bars, we could include both barred and unbarred examples of these — which would make the best cards? We might also want examples of some of the unusual samples defined by Zooites: green peas and red spirals. And what about particular Zoo favourites like Hanny’s Voorwerp? Does that belong in the galaxies pack?