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.)

Example galaxies

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.

PG84 morphology density relation

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.

Galaxy Zoo morphology-density relation

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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13 responses to “Do galaxies care where they live?”

  1. Jim says :

    Your paper is very interesting and clear to this novice which is a plus. You must be a fine teacher.

    I am not sure what a lenticular galaxy is, but I think it is a more coherent and also differentiated cluster of stars around a core that also is well defined. To me the ordinary elliptical is usually fuzzier and has a less well-defined perimeter, i.e., lots of stuff at work to make it appear that things are almost flinging off at the “poles” more than the edges.

    I wonder about the conditions in the elliptical clusters. Maybe they are being influenced by almost chaotic gravitational forces. And it does not seem likely that they will somehow evolve into even more dense, fewer galaxies. But they could.

    Maybe spiral galaxies have done their “work” so well that they have cleared out a multitude of ellipticals. I love to speculate and always have. I just don’t have the professional background to say much or coherently.

    I am certainly drawn to the Galaxy Zoo project by more than one thing. First is being able to look at the amazing galaxies and do a little sorting. (I have looked at and analized over 50k so far which by comparison to others is not that much.) I love viewing the postings and comments. And I am eagerly anticipating the future developments in the project.

    Thank you so much.

  2. Steven says :

    Glad you enjoyed the post, Jim. Thanks for all your efforts in the Galaxy Zoo – 50k classifications is amazing!

    I actually haven’t done that much teaching so far, mostly supervising the occasional lab class. Most of my time is spent on research. Hopefully I’ll get a lectureship in the not too distant future, which will allow me to teach more, although as long as I can still do plenty of research!

    Spiral galaxies usually consist of a bulge, like a small elliptical, located at the centre of a disk, which contains the spiral arms. A lenticular, or S0, galaxy is similar, with a bulge and disk, but for some reason doesn’t have spiral arms. Your comment about differences in the ‘perimeter’ of ellipticals and lenticulars is very insightful. Disks, such as those which dominate the outer regions of lenticulars, are often seen to be “truncated”, i.e. they have a well-defined outer edge. Ellipticals, on the other hand, have “fuzzier” edges that just keep on going, getting fainter and fainter. I’m not sure quite what you mean by your comment about the “poles”, though.

    I’ll write about lenticulars, interactions and mergers between galaxies in future posts.

  3. Adam Primus says :

    Thanks for informative & interesting piece ; I can’t remember the last time I read an article that made reference to “the biscuit hoarding characteristics of different galaxies”. Great stuff!

  4. Polly Wilson says :

    I am also a beginner at all this astronomy stuff, but it has truly caught my attention. I do understand ‘biscuits’, the US equivalent to cookies, preferably chocolate chip, and am occasionally guilty of hoarding a few myself. Due to hungry kids and husband. I have seen quite a few lenticular galaxies in photos that I classified as ellipticals, but thought that wasn’t quite the right place to put them. So, I was happy to read that I wasn’t the only one to notice the sort of the same but different type of galaxy, since I would not have been brave enough to write in and try to describe it.

    Question: The photos of the elliptical looks very much the same, fuzzy balls. How are sizes, distances, or number of stars involved in these galaxies be determined, or can they not be?

  5. Steven says :

    Well, Polly, that is quite a big question! To answer it properly would take more space than a blog post, never mind a comment.

    Very, very briefly: all these things can be measured, although it can be difficult!

    You can find lots of information about galaxy astronomy at and about the SDSS survey in particular at

    In fact, those links are so useful, I’ll add them to the list on the blog menu.

  6. saxman says :

    thanx a lot for the really good article!

    i’m a beginner to astronomy too, but might add my thoughts to the topic. please ignore, if it’s too stupid! 🙂 (also i apologize for the bad english)

    i agree with jim – the obvious difference between the two environments (dense / lesser dense regions of space) is the gravitational layout:

    within an “isolated” galaxy the main sources of gravity seem to be the features of this very galaxy:
    – the central bulge, usually with its big black hole in the center, binds the stars to the center
    – the masses of the single stars influence their imediate neighborhood – but “level out” themselfs over the galaxy as a whole
    – due to rotation of the system (=galaxy) we get this forces to form a disc of arms.

    why? well, if i remember correctly from school physics, the gravitational (pulling) force is proportional to 1/r^2, meaning it reduces by the distance squared. (radius 1 ~ gravity 1, r=2 ~ g=.25, r=10 ~ g=.001 etc.) plainly spoken: you feel the gravity of nearby objects overproportionally strong. so
    – local clusterings within a galaxy will attract more nearby stars to “gather together”
    – the massive center will bind those to itself
    (other organizing features, like star formation, gas clouds, novae etc. i will ignore, because this reply is about gravity)
    (i skip the issue of dark matter here, but get to it later)

    what happens, if we add some galaxies around?
    well – distant galaxies have an influence on the galaxy as a whole, but either
    – just “a slight pull” (remember the 1/r^2) to this or that direction, if the galaxies summed gravity is stronger in that direction, or
    – a general “fluff up” of the galaxy, if the summed gravity is evenly distributed
    but the influence will increase if the surrounding galaxies are closer. and especially the seemingly small irregularities in the “gravitational matrix/layout” will get greater importance. (same reason: 1/r^2, but this time r gets smaller -> g gets overproportionally larger)

    what you could do is creating a small mathematical simulation of this gravity field/matrix/layout/how-ever-we-call-it:
    – a galaxy consists of ~ 10^7..10^13 solar masses, radius ~ 10^2..10^6ly (both roughly values)
    – lets take a medium one, say 10^10 s.mass by 10^4ly radius
    – form a matrix/grid of mesuring points within 3D, doesn’t have to be that fine – 100x100x100 points should be enough for a start
    – implement a rotation algorithm in one plane as a simple shifting of the values to the next grid in the direction of rotation (speed ~ 225km/s), stepping 10^1..10^3 y
    – fill grid with the distribution of stars (populate the x/y plane and lets say planes 49..51 of the z axis), pre-form central bulge (of course in the z-axis, too) and some spiral arms
    – calculate the gravity influences in all 3 directions for each rotation step and each of the grid points (skipping empty cells)
    – transform the matrix by the resulting movement vector (circulation and gravitational pull)
    – for otical study you can color code the points regarding the z part (like green: local gravity dominates, no z pull / red: z pull upwards / blue: z pull downwards) but thats just gimmicks – you only need the numbers
    – then add a wider grid for the surrounding galaxies (lets say also 100x100x100 points over a larger space), where each galaxy is a single grid point
    – implement a random fill alorithm for the second grid (within rational parameters of course)
    – let the galaxy rotate for a while (some hundred million years)
    – determine, if the z space populates (“elliptical”) or not (and if the general structure remains intact) – “spiral”
    – save the outcome together with the neighborhood scenario
    – repeat for different random neighborhood scenarios
    you should come up with something like “a lot of near galaxies -> more likely elliptical”

    to finetune the system you then can add some “self made dark matter” 😉
    – as a means to preserve the general structure (like an offset for the central galaxy’s gravity field)
    – to match the systems outcome with reality:

    if the simulation suggests, that the influence of the neighboring galaxies “kicks in” too late (=close), extend the offset field. this should “feel” the other galaxies gravitational pull sooner (as dark matter is influenced by gravity) and by deforming should transmit the effects to the central galaxy.

    or may be you (or someone else) has already done that…

  7. Daniel Lawton says :

    Thanks for summing some results! That’s one reason I classify. To see the results. There’s nothing like an organized slope, that makes me feel like something was accomplished.

  8. starry nite says :

    Hello, Steven. Thanks for the post; this blog is great! I’m an amateur, but my guess is that lenticulars have a more defined edge because they are still spinning, which keeps the stars (or most of them) within a rough boundary. My thought is that when the rotation of a spiral slows, the arms lose definition and “fuzz” into a lenticular or ring. That’s assuming that there are no interactions with other objects or inherent instability that causes the spiral to fly apart first. On that subject, I think also that there may be more loose stars and clouds in inter-galactic space than is assumed currently. I’ve seen a lot of irregulars and spirals losing bits and pieces, and that stuff isn’t going to get incorporated into other galaxies until their paths cross.

  9. Mark McC says :

    Hi Steven.
    Thanks for taking the time to write such great descriptions. Its fascinating reading even if I haven’t taken it all in yet and I agree with all the positive comments above.
    Make sure you save them to a file and you will be a published author on galaxy descriptions in no time.

  10. Steven says :

    saxman: Given you’re a beginner in astronomy, but presumably at least some science background, your ideas are very good. The simulation you suggest would give some insight, and indeed a number of similar simulations have been performed. The approach of simulating the gravity with higher resolution than its surroundings is common. Unfortunately the real situation is rather more complicated than you suggest and even a simple model would have to take into account more effects.

    The gravitational interactions of neighbours is indeed thought to have a significant influence on a galaxy’s morphology. However, galaxies move relative to one another, and it is the short-lived close-approaches which have the largest effect, as I’m sure you can appreciate. Motions of neighbours must therefore be included, although this is fairly simple in your framework, although to reliably simulate the effect the positions and motions of the neighbours must be realistic – not just random.

    There are various other environmental effects, including mergers, gas-stripping and shocking, the gravitational interaction with the cluster’s own dark matter halo, and the changing distribution of galaxy masses with environment, which could all be important in explaining the observed trends. I’ll give more of a description of these mechanisms in a future post.

    Finally, accurately simulating the evolution of just one galaxy, whether isolated or in a group, is currently well beyond our capabilities. The complex interplay between environment, galaxies, gas, star formation and active galactic nuclei, spans such a wide range of scales, easily a factor of a million, that the processes can not be followed consistently. Simplified models can, however, give us an understanding of some of the processes involved, and we can constrain the roles of some mechanisms by comparing with observations.

  11. Steven says :

    Mark McC: Just to note that, along with all the Zookeepers, I am a ‘published author’ of a number of articles in scientific journals. I haven’t produced much for the public yet, though, which I’m sure is what you meant. Chris, on the other hand and for the few of you who don’t know, is also a public communication of science guru!

  12. Hanny says :

    Hi Steven, just read your piece. It took me a while because I thought it would be a bit over my head. However, your explanations are very clear (even for a Dutch amateur!) Thanks!

  13. Debby Delvalle says :

    Hi, i just thought i’d submit and allow you to know your blogs layout is actually messed up around the K-Melonbrowser. Anyhow maintain up the beneficial function.

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