A couple weeks ago, I talked about the Voorwerp (”object”), the strange blue object that Hanny posted to the Galaxy Zoo forum. She asked if anyone knew what it was, and we sure didn’t. Part of the problem was that we didn’t have a spectrum for it, so it could have been literally anywhere from right next door in our galaxy to the edge of the universe. Our colleague Bill Keel took a spectrum, which he posted about here in the blog, and found that the Voorwerp is associated with the galaxy above it. We’ve since been looking around for other colleagues that can help us figure out what the Voorwerp is.

Thanks to Matt Jarvis, who was observing at the 4.2m William Herschel Telescope in La Palma, we’ve been able to get some better images. The William Herschel Telescope is bigger than the telescope that gathers images for the Sloan Digital Sky Survey (SDSS is 2.4 m; WHT is 4.2 m), and the images that Matt took are longer exposures, so we can see fainter features in them. The conditions were also quite good (good “seeing” in astronomer’s lingo) and so the image has very good resolution (it’s “sharper”) as the atmosphere didn’t blur things too badly.

So what kind of data did we get? We got three images in filters very similar to the SDSS ones. We got a g, r and i-band image. Those correspond roughly to green, red and infra-red for human eyes. Just to make things confusing though, we colour g in blue, r in green and i in red to stay consistent with the SDSS/GZ images. Without further ado, here are the original SDSS and new WHT images:

Original SDSS image

New WHT image

The WHT image is rotated with respect to the SDSS image; look at the orientation of the galaxy and the Voorwerp to see how they compare. Once you mentally rotate the images so they match, you can see clearly that the Voorwerp is quite a lot bigger than we initially thought, because so much of it was too faint to be visible in the SDSS image. This immediately makes us want to get an even deeper g-band (blue colour) image to see just how much bigger it is! For that, we will probably go to the world’s largest telescopes such as ESO’s Very Large Telescopes, Gemini or Keck.

To give you an idea just how big the Voorwerp is by now, look at the spiral galaxy next to it. This galaxy is a very massive spiral galaxy, likely as big or bigger than our own Milky Way! That’s really, really big!

If you look at the new WHT image of the Voorwerp, you can also see a huge, gaping hole. From the SDSS images, it wasn’t really clear whether the fuzzy structure there was anything real, but the WHT image makes it clear that this is a genuine hole. Again, just to put it into proportion, that hole has a diameter of something like 10 000 light-years. We have no good idea of what could punch such a large hole. One possibility is that a massive burst of star formation occurred there, causing a string of powerful supernova explosions, causing an expanding bubble. Such holes presumably caused by supernovae have been seen in other galaxies, but as far as we know, nothing anywhere near this size.

In his last post, Bill mentioned that the spectrum of the Voorwerp showed some very odd emission lines, in particular Helium II (HeII) and Neon V. HeII only really appears in spectra when there is something really hot around to excite the gas - something hotter than the hottest star. This could be an active galactic nucleus(i.e. gas falling into a supermassive black hole, and heating up as it falls), or perhaps some high velocity shocks. We’re busy analysing the spectrum to understand better what’s going on here.

By a luck coincidence, the Voorwerp turned out to be at a redshift where the HeII line “redshifted” into a common narrow-band filter. Such a filter blocks all light except in a very narrow wavelength range, and so lets us take an image focusing only on those areas which are emitting light in that wavelength range. Below is the image of the Voorwerp in the wavelength range of the HeII line:

The Voorwerp in HeII

The HeII emission clearly comes from a good chunk of the whole Voorwerp (again,a deeper image might show even more), so whatever is exciting the gas in the Voorwerp seems to do it over quite a large volume.

What’s next? We really still have no idea of what the Voorwerp really is. The more data we take on it, the stranger it gets. Many of us are busy trying to convince friends of ours on observing runs to take observations of the Voorwerp so we can figure out what it is.

That’s how an observational science like astrophysics works: you find something new, you don’t know what it is, so you take more data to try and understand it better and form some hypothesis about what’s actually going on and then you confirm it with more data. But we’re still at the very start of this process. The mystery deepens… *cue scary music.

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A couple weeks ago, Chris and I attended the meeting of the American Astronomical Society in Austin, Texas. At the end of the meeting, I uploaded the poster I had presented there:

Galaxy Zoo public outreach poster

In a comment to that post, mushroom made a fascinating suggestion:

Could be a fun visual communication exercise to try to make another poster which conveys the same information without using any text.

This is a fascinating idea. I talked it over with the team, and we’re not sure how to go about telling the Galaxy Zoo story without text. But we’re sure that with your creativity, someone can figure it out! And, to phrase mushroom’s suggestion in a different, more general way:

How would you tell the story of Galaxy Zoo?

Here is your chance. We invite you to “remix” the Galaxy Zoo poster, telling the story of Galaxy Zoo in your own words and images. You can start from the original work, or strike out on your own. Here is the original poster, as a JPEG image and as a Word file:

Galaxy Zoo public outerach poster (JPEG)

Galaxy Zoo public outreach poster (Word)

I have created a topic in the Galaxy Zoo Latest News forum called “Remix the Galaxy Zoo poster!” Post your ideas in that thread, or in comments here on the blog. Post your creations there, or if they’re too big to be uploaded, E-mail them to me at raddick “at” jhu.edu.

If any of you are wise in the ways of Photoshop, E-mail me, and I can send you the original Photoshop CS2 file.

We’re really looking forward to seeing what you come up with, and we hope this is fun for you too!

Have fun, and keep telling the story!

Details

You may change anything you want about the poster, as long as you leave the following elements. You may move them around, but you should maintain them visibly on the poster:

1) The author names and institutions (”TeamMembers” in the Word file). This is in the standard format for scientific posters and papers.

2) The names of the volunteers, which now appear at the end of the poster. But, of course, feel free to add your own name to our randomly-chosen list!

3) The logos of the institutions involved with Galaxy Zoo.

4) The copyright statement, including the Creative Commons logo.

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A user on the forum asked me what I meant by the word ‘random’ in my previous blog post. A statistician could explain this more precisely, but I’ll give my understanding of it in an astronomy context.

A random process is one in which a variety of alternatives could occur, but beforehand we cannot know which alternative actually will occur. The alternatives need not be equally likely, and the probabilities of each alternative happening (the probability density function) may be known very well. If we repeat the process many times, the number of times each alternative occurs will be very close to the probabilities we might already know. But we don’t know which alternative will actually happen each time the process occurs.

A random system is one which arises as a result of random processes. Everything has some element of randomness in it, nothing is perfectly ordered, but some things have a lot more order to them than others. The chair you are probably sitting on while reading this is a highly ordered system. Even though the atoms in it are jiggling around randomly to some extent, their overall motions are generally very ordered. The random motions of the atoms are not very large compared with the order that was instilled in the chair when it’s materials were constructed, whether that was by metal cooling in a mould or wood slowly forming in a tree.

The motions of stars in a spiral galaxy disk have a lot of order to them, as they are mostly formed by relatively gentle physical processes that maintain information about the history of the system. The galaxy started rotating long ago, and the stars which formed in it are still rotating today. In an elliptical galaxy some process has happened, for example a merger of two galaxies, which disturbs that order. The random process at work is the multiple close interactions of pairs of stars. The motion of two stars after they have passed close by one another is very dependent on the exact values of their motion before the interaction. If each of those stars then interacts with other stars we quickly get to a state where we could never know the stars’ initial motions well enough to predict their motions at a later time. This is an example of a chaotic system, and it can even occur for three objects interacting through gravity, never mind the billions of stars in a typical galaxy. There are many alternative paths the stars could travel along, and beforehand we don’t know which paths the individual stars will take. Despite this, we can go part way to determining what the result will be overall, a big fuzzy ball. The stars in elliptical galaxies are not moving totally randomly, there are certain ranges of orbits that are more populated than they should be for a truly random system. This order is a ‘memory’ of the initial motions of the stars in the merging galaxies. However, most of the stars’ movements are dominated by random motions.

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Galaxy Zoo features in this week’s Carnival of Space, a collection of all the best space blog writing from the past seven days. This week it’s in a 50s detective story format…and it’s definitely worth a look.

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Does where we live make a difference to the kind of person we are? This is a question that has been addressed many times by social scientists, and certainly with more refined thought than the following example, but it will serve our purposes.

Consider one person, Victor, living in a small countryside village, and another, Claire, who lives in the centre of a city. The nearest shops to Victor are many miles away. When he has a sudden biscuit craving and opens the cupboard to find, to his horror, that his wife finished off the last packet the previous evening, it is a great effort for him to travel to the shops to get another. Claire, on the other hand, has merely to stroll to the corner of her road to satisfy her craving for something crunchy. However, while Claire often finds herself nipping out for a packet of biscuits, Victor rarely has the need. He always makes sure he buys plenty of biscuits on his regular weekly shopping trip, and there is always the packet hidden at the back of the other cupboard that his wife hasn’t noticed. Victor is very organised, while Claire clearly isn’t, at least when it comes to biscuits. Does this have anything to do with where they live?

Of course, biscuit buying habits, although important, aren’t the only thing one can say about an individual. Each person is complex and unique, imperfectly describable even by a very large number of personality traits. However, there are simple and obvious ways of crudely dividing up the population. Although we have so far confined ourselves to biscuits, the chances are that Victor is generally more organised than Claire. Perhaps there is a way of dividing people into groups by how organised they are. I’ve no idea, but there are small number of general personality traits, like introvert and extrovert, that describe how many specific personality traits tend to group together, such that you can give reasonably good description of someone by just a few words.

By now you are sure to be wondering what the hell this has got to do with galaxies. Well, to date there has been very little research into the biscuit hoarding characteristics of different galaxies, but like people, galaxies are extremely complex objects. There are so many processes simultaneously going on inside them that we just can’t fully describe each one, never mind understand how those processes go towards forming the properties of the individual as a whole. However, one thing about galaxies, that you can’t help noticing when you’ve looked at a enough of them, is how cleanly they can be split into two different types: spirals and ellipticals. Spirals are, at least in some respects, very organised. Most of their stars are travelling in circles around the galaxy centre in an ordered manner. Ellipticals, on the other hand, are in disarray. Their stars move around on many different, random orbits. (It is interesting how the appearance of order, a nice smooth elliptical galaxy, appears when many unorganised things happen at once, but that is a whole other topic.)

We’ve made the distinction between spirals and ellipticals completely obvious in Galaxy Zoo by only giving you those two options, along with “star/don’t know”. Even so, if we’d just sat each of you at a table with a pile of galaxy pictures to sort, without giving you any instructions about how to do it, most of you would probably have arrived at the same way of dividing them up. Those of you who value simplicity would have formed two or three piles. The pickier ones amongst you would probably be surrounded by lots of neat little stacks, containing galaxies with two sprawling spiral arms, with many tightly wound arms, big blobs, small blobs, red, blue, and so on. Nevertheless, the main distinction, the difference between all the galaxies on your left and those on your right, would probably be whether they possess a disk, often containing spiral arms, or whether they are just a big, smooth elliptical.

Of course, as many of you will have noticed, not all galaxies do fit into a nice category. So, as well as your stacks of spirals and ellipticals, you would be likely to have a collection of weird objects. However, these only form a small fraction of the whole population of galaxies. Whether you choose to hide your pile of odd galaxies away to one side, or display it smack right in front of you, again depends on your character. The projects examining blue ellipticals or Hanny’s Voorwerp belong to the latter class - confronting the occasional odd object to see what secrets it can tell us. The analysis I have been working on has more of the former character: as most objects are elliptical or spiral, let’s ignore the few weird ones and study how the majority behaves. One problem with working with the majority is that this is very many objects, hundreds of thousands of galaxies. To analyse a dataset this large we have to use statistics, for example we consider the fraction of objects that are elliptical, and how that changes when we only look at galaxies with certain properties.

If you did the galaxy sorting exercise described above you would be reproducing work performed by many astronomers over the past ninety years, including Hubble, de Vaucouleurs and Sandange. This subject is called morphology, literally the study of the ‘forms’ that galaxies take. Strictly morphology doesn’t include a description of the colours of galaxies, but rather their shape or appearance in greyscale.

The distinction between spirals and ellipticals was noted even before it was fully accepted that these objects reside outside our own galaxy. It was also noticed, almost immediately, that spirals and ellipticals are distributed differently on the sky. They all tend to cluster together in groups, rather than being evenly or randomly arranged, but ellipticals cluster much more strongly than spirals. Ellipticals live in galaxy cities, alongside many others, whereas spirals prefer the villages and isolation of the cosmos’ countryside.

To use more scientific language, ellipticals are concentrated in high density regions, where many galaxies are located in a small volume of space. Spirals, on the other hand, are usually found in low density environments, where galaxies are separated from others by large distances. As mentioned earlier, the dependence of galaxy morphology on the density of surrounding galaxies was noticed early in the 20th century. However, it wasn’t until the 1980’s that it was well quantified in two landmark papers by Dressler (1980), looking specifically at large galaxy clusters, and Postman and Geller (1984), who extended the relationships to lower density environments around clusters and smaller groups. These studies tried to classify galaxies as ellipticals, spirals, or lenticulars. This last type is a galaxy morphology somewhere between a spiral and an elliptical: with a disk, but with no spiral arms. Lenticulars are tricky to classify, and so in Galaxy Zoo so far we haven’t asked the classifiers to try and identify them. Galaxy Zoo “ellipticals” will contain normal ellipticals, and most of the lenticulars. This issue will be discussed more in future posts.

This figure shows the morphology-density relation from Postman and Geller (1984) and Dressler (1980), based on around 9000 galaxies. The lines show the fraction of ellipticals (red), lenticulars (orange), and ellipticals + lenticulars (purple) versus a measurement of local density. The different lines of the same colour just indicate three different sources. You can see that as local density increases, going from left to right in the figure, the fraction of ellipticals and lenticulars increases.

With the latest Galaxy Zoo data provided to me by Anze, I set to work analysing how a galaxy’s morphology depends on the environment it lives in. The initial thing I had to do was carefully measure and correct for any biases in the morphological classifications. This in itself is interesting, although it tells us more about people and the telescope than about galaxies, so I won’t discuss it further here. The next thing to do was to find out about the environments of the galaxies - specifically the local galaxy density. These were kindly provided by Ivan Baldry, an astronomer at Liverpool John Moores University who has done lots of work on the variation of galaxy colours with environment.

When I had my corrected dataset, with measurements of environment added in, the first thing I looked at was the relationship between the fraction of galaxies that are elliptical and local galaxy density.

This figure shows the morphology-density relation for nearby galaxies from Galaxy Zoo, based on 100733 objects. The light shading indicates the very small uncertainties on the relation.

It is difficult to directly compare the Galaxy Zoo morphology-density relation with that by Postman & Geller (1984) shown further above. This is because the local density was measured in a different way, and they include lenticular galaxies separately. However, it is easy to see that the overall behaviour is the same. In regions of high density the fraction of elliptical galaxies increases. The Galaxy Zoo relation is much more accurate, as it is based on more than ten times the number of galaxies, and very clearly defined, which will enable future studies and models to easily compare with it. It shows clearly that morphology depends smoothly on local galaxy density over all environments. Even in the lowest density regions there is some dependence.

Now is a good time to think back to Victor and Claire. Like Victor, organised spiral galaxies tend to live in areas of low density. Disorganised ellipticals are found where many galaxies cluster together, somewhat comparable to the city Claire lives in. But is Victor organised because he lives in such an isolated place, and is forced to be; or is he just an intrinsically organised person, and so living in the countryside didn’t seem such a problem? Likewise, is Claire disorganised because of where she lives? Do the plethora of nearby shops make biscuit hoarding unnecessary? Or is she simply a disorganised person, and so chose to live in the city to avoid having to be organised? If Victor moved to the city, would be become more disorganised? Would the place he lives change his personality?

Obviously galaxies don’t choose where they live, in the sense that Victor and Claire can, but the analogy is still strong. Are there more ellipticals in clusters because that’s where ellipticals happen to be, or because something about where they live has turned them into ellipticals? If otherwise identical galaxies form in areas of different densities, would they be the same, or is there something happening in dense regions that changes galaxies into ellipticals? Maybe something about dense regions turns organised galaxies into disorganised ones.

One of the powers of Galaxy Zoo is the staggering number of galaxies we have data for. It is possible to divide up our sample by a variety of galaxy properties, such as their mass and colour, and still have enough galaxies in each slice to see how environment affects that particular subsample. Each of these different properties tells us something different about a galaxy, and enables us to go someway to disentangling their intrinsic properties from recent changes. I’ll discuss the things we’ve learned by doing this in future posts.

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