Galaxy evolution 101Posted: October 3, 2009
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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