Counting galaxy mergers you can’t see

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I’m going to pick up where I left off a while ago, when we talked about galaxy evolution. I have a staggering backlog of papers to read on my desk, most of which have the words “merger history”, “mass assembly” or “galaxy pairs”. All of these expressions are more or less equivalent, and they relate to the one of the processes we believe regulates galaxy growth – merging. The other one is star formation – by which I mean the process of turning cold gas into stars – but we’ll talk about that some other time.

We know that galaxies merge. We do not only see it  (go to Galaxy Zoo’s Mergers site to revel in pretty mergers images, and also to help astronomers get some real science out of them), but it is also a prediction of our current model of structure formation. I’ll cover that model and prediction in another post, but for now let me just say that we are in a position in which measuring the rate at which galaxies of different mass or luminosity merge in the Universe is becoming very important as a way to constrain our models of galaxy and structure formation. In other words, it’s time to get quantitative.

So what we want to know is, on average, how many galaxies merge per unit volume, per unit time, as the Universe evolves. If you sit down and think about this for a moment or two, you’ll quickly come to the conclusion that simply counting galaxies that are merging (which you can identify by looking at the images) is one way to go. But this is only possible relatively near by – as we go to higher redshift it becomes increasingly hard to get good enough images. Still, a number of people have been working hard at measuring this, and pushing this sort of analysis forward.

Another way to go, is to simply count galaxy pairs that are closer than a given physical distance. You can assume that if galaxies are too close then gravity will win at some point, and the galaxies will merge. The upshot is that you don’t need really good images to actually see interacting galaxies and you can take this to higher redshift. The downside is that you need to make assumptions about what this physical separation should be and, perhaps more importantly, how long it will take them to merge – the dynamical timescale. Another disadvantage comes from the fact that you miss pairs of galaxies in which one of the two is very faint – so you are limited to counting pairs of bright galaxies. The jargon for the merging of two galaxies of similar mass or luminosity is “major merging”. Some neat pieces of work have come out of this, and have measured the major-merger rate of luminous galaxies to a respectable redshift. The last one I read (but by no means the only, nor the last!) was by  Roberto de Propris et al. (2010) who did this up to a redshift of 0.55, but there are measurements of this quantity which span the last 8 Gyrs of the lifetime of the Universe or so (equivalent to a redshift of 1).

A few weeks back however, I read another paper which took a different and rather interesting approach to the subject. This is the work of Sugata Kaviraj et al. (2010), and their idea is as follows. The types of measurement like the ones I described in the above paragraph give you a number of how many major mergers there are at some point in the Universe. These mergers, however, leave a signature in the shapes of the galaxy for a certain time – they look disturbed (i.e., not smooth), until they final relax into one larger, smoother, and stable galaxy. However, this means that you should be able to predict how many galaxies of a given mass, on average, should look disturbed at any point in time by assuming a measured rate of mergers in the past.

And so they did. They took a whole load of high-resolution images from the Hubble Space Telescope and looked for signs of disturbed elliptical galaxies. What they found (amongst other neat things that I don’t have time to go into), is that there are too many of these disturbed galaxies if we assume that the other rates are correct. But hang on in there for a minute – the other rates are limited to major mergers because we can’t see the minor mergers when looking at pairs. So Sugata Kaviraj and collaborators postulate that the excess is due to these minor mergers – we can’t see them happening at high redshift, but we can see their effect at lower redshift. Moreover, they also observe these minor mergers to be significantly more dominant than major mergers since redshift of one, suggesting that galaxies have been growing from accreting smaller (fainter) galaxies in the recent Universe, but this was potentially very different at high redshift.

Other people have found this sort of behaviour in some way or another (including me!), but I was happy to see a rather neat way to.. well.. see (and measure) the unseen. R. De Propris, S. P. Driver, M. M. Colless, M. J. Drinkwater, N. P. Ross, J. Bland-Hawthorn, D. G. York, & K. Pimbblet (2010). An upper limit to the dry merger rate at ~ 0.55 ApJ arXiv: 1001.0566v1 Sugata Kaviraj, Kok-Meng Tan, Richard S. Ellis, & Joseph Silk (2010). The principal driver of star formation in early-type galaxies at late epochs: the case for minor mergers MNRAS (submitted) arXiv: 1001.2141v1

Galaxy evolution 101

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I think it’s about time that we start covering some ground in galaxy evolution, here in weareallinthegutter. We won’t do it in one post. We won’t do it in 100 either, simply because galaxy evolution is not yet solved. But of course, that only makes it more exciting.

Let us start with some of the basics then, and lay down our aims. The goal of galaxy evolution, in its broader terms, is to explain how galaxies are born and how they evolve throughout cosmic history. A successful theory will give a framework which, given an ensemble of galaxies at one point, can predict how these same galaxies (or another ensemble just like it) will end up in the future.

We have an advantage here, as Astronomers, in that we can look at the Universe during different stages of its past evolution. The trick is in the finite speed of light – for example we see a star which is 10 light years away as it was 10 years ago. In other words, light takes 10 years to travel from this star to us, and an observer sitting on a planet around this star would see me not sitting at my computer right now, but 10 years younger and sitting someplace else (probably a lot warmer).

So the further we look, the further back in time we’re travelling. If you’re studying galaxy evolution, then, this is incredibly advantageous: by looking at galaxies which are at different distances from us, we are looking at how galaxies looked at different stages of the cosmic evolution. Our job is to draw a coherent story line through these stages.

What, then, should our observations be? Let us start simply, with two sets of galaxies – one set near us, and one set far away from us. Each galaxy has a set of characteristics which we may want to study – for example its shape, known in the business as morphology; its colour; its brightness; its mass; its chemical composition; its dynamics (the way it moves); or even its neighbourhood, or environment. The truth is, there are many ways in which one could describe a galaxy, much in the same way as I could choose a variety of characteristics to describe a person. I could go for height, arm length, hair colour, eye colour, number of eye lashes, gender, etc. Some, you will agree, are more useful than others, depending on what I’m studying about a person or group of people. It’s the same with galaxies.

It turns out that one of the most defining characteristics of a galaxy is its colour. And not just any colour – galaxies tend to either be blue, or red. The colour is related to the age of the dominant stellar component – old stars are red, young stars are blue – so the colours themselves are easily explained. But what is surprising is that galaxies tend to sit very much in either the red or in the blue side of the fence. There are very, very few galaxies which sit on the fence and are, for example, green. This on itself is very revealing – it means that whatever process makes galaxies go from red to blue (or the other way around) must happen quickly. If this transition is fast, it means we are less likely to observe a galaxy in this period which explains why we see so few galaxies perching on the fence.

Good. Now, remember that we have two sets of galaxies – one near, and one far from us. If we have some theory of how galaxies go from blue to red and vice-versa, we should be able to predict the fraction of red and blue galaxies in the present (those near us) by measuring it in the past (in those far from us). Our observations of the near Universe should therefore help prove or disprove our theory for galaxy evolution.

This is the mantra of many a paper in galaxy evolution. Observables get more or less complicated – for example, instead of just looking at how the number of red and blue galaxies evolves, we can look at how bright they are, how fast they make stars, how they’re distributed in space, their environment, etc. But essentially, this is what galaxy evolution is all about – and it’s hard!

Two papers recently have caught my eye on this particular matter, so let me very briefly tell you about them. Last month, Tinker et al. looked at these sets of clouds of red and blue galaxies at different distances from us, and tried to make sense of the time-scale of the process which drives the blue-to-red transition. The process itself is still unconstrained, but what they did find is that whatever dominates this evolution today is different from what dominated it in the early Universe. And a bit later in the month, Zucca et al. studied how this transition depends not only on the epoch, but also on the environment of the galaxies. Interestingly, they found that in very dense regions (i.e., more packed regions of the Universe, where there are more galaxies per unit volume) most of the blue-to-red transition happened over 7 Gyr ago. However, in more sparse regions of the Universe, this transition is still happening today.

So the picture is complex – galaxies appear to evolve via different processes according to the age of the Universe, and according to their environment. This is not a surprise, but it is exactly this sort of observational constraints which help test, prove and most often disprove several ideas for galaxy evolution – they are as important as they are technically and instrumentally hard.

I’ll leave you now with this very brief and basic first introduction to galaxy evolution, but I promise to come back with more observational constraints, and with some explanation of what the theorists have to offer.

Jeremy L. Tinker, & Andrew R. Wetzel (2009). What Does Clustering Tell Us About the Buildup of the Red Sequence? ApJ arXiv: 0909.1325v1

E. Zucca, S. Bardelli, M. Bolzonella, G. Zamorani, O. Ilbert, L. Pozzetti, M. Mignoli, K. Kovac, S. Lilly, L. Tresse, L. Tasca, P. Cassata, C. Halliday, D. Vergani, K. Caputi, C. M. Carollo, T. Contini, J. P. Kneib, O. LeFevre, V. Mainieri, A. Renzini, M. Scodeggio, A. Bongiorno, G. Coppa, O. Cucciati, S. delaTorre, L. deRavel, P. Franzetti, B. Garilli, A. Iovino, P. Kampczyk, C. Knobel, F. Lamareille, J. F. LeBorgne, V. LeBrun, C. Maier, R. Pello`, Y. Peng, E. Perez-Montero, E. Ricciardelli, J. D. Silverman, M. Tanaka, U. Abbas, D. Bottini, A. Cappi, A. Cimatti, L. Guzzo, A. M. Koekemoer, A. Leauthaud, D. Maccagni, C. Marinoni, H. J. McCracken, P. Memeo, B. Meneux, M. Moresco, P. Oesch, C. Porciani, R. Scaramella, S. Arnouts, H. Aussel, P. Capak, J. Kartaltepe, M. Salvato, D. Sanders, N. Scoville, Y. Taniguchi, & D. Thompson (2009). The zCOSMOS survey: the role of the environment in the evolution of the luminosity function of different galaxy types A&A arXiv: 0909.4674v1