Do galaxies care where they live?
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.
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.
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.