Astrocladistics and galaxies: a milestone

Astrocladistics has the power to renew our classification of galaxies. I have explained why this is necessary. But after more than 10 years of developments, where are we standing in this ambitious and long term goal?

A first step has been made forward in our publication: A six-parameter space to describe galaxy diversification. Fraix-Burnet, D., Chattopadhyay, T., Chattopadhyay, A.K., Davoust, E., Thuillard M. 2012, A&A 545, A80 Astronomy & Astrophysics  (

Groups have been defined on the cladogram, and relative average properties have been derived thanks to boxplots like these ones (dispersion velocity, effective radius and absolute magnitude):

boxplotsThe important point is not the absolute differences between the 8 groups for one parameter, but the relative differences of all available parameters between the groups. These physical properties of the galaxies provide hints on their formation history. I do not enter the details here, it is explained in the paper, but for instance if a group has a low velocity dispersion and a low radius, it is probably a group gathering galaxies that were not gravitationally perturbed, as opposed to galaxies with high velocity dispersions and a high radius. Also, between two groups with similar velocity dispersions, the gravitational perturbations were probably different when they show different average radii.

All the available parameters should be used to describe the average properties of each group. We are here far from the Hubble classification that considers only one parameter. Note that we now know, thanks to the heavy but numerous numerical simulations, that there are many ways of forming galaxies with elliptical shapes. In other word, morphology is an homoplasy. It is thus impossible to reconstruct an history from this parameter since several paths are possible.

In the paper, we do not try to do any taxonomy. We simply show that from all the properties, we can derive a history for the galaxies in each of the groups. Further, we then deduce the transformation events at the internal nodes. These events are the equivalent of innovations in biological diversification.

The cladogram with the historical information looks like this:


For me, this is a great achievement. This is the first concrete realization of the initial goal of astrocladistics. I knew from the start that it would not be possible to obtain a general new classification of galaxies at once. Since we cannot analyze a large sample, it will take time to put the millions of observed galaxies into few classes. Here, we have only a thousand objects…

This diagram is also a clear demonstration that cladistic analyses can provide valid results. The average properties of the groups are derived without any physical model or assumptions, without any a priori belief. The grouping cannot be disputed, except with other multivariate and objective tools. On the contrary, the histories for the groups have been derived by using knowledge of the physics of galaxies and their evolution. We also used results from numerical simulations. This is an interpretation, but it comes after the grouping, not before, and can be debated. The transformation events at the internal nodes come out from these histories. And strangely or not, a progression from simple and gentle processes to complicated and very violent ones appear naturally. Magic, isn’t it?

So what?

First, instead of taking one or two parameters, classifying galaxies accordingly, making some physical models followed by some computation, redoing the same for other parameters, and discussing at length how the different classifications and constraints of the models can match, we show that one can first classify galaxies with all parameters, and then do the physics. In this conditions, we do not care about the classification, but concentrate on the physics and discuss about it. In addition, the classification integrates all the available information, so that the physics is necessarily more correctly constrained.

Second, I have not said that we have performed an objective selection of the parameters that provides the most robust clustering. This set of parameters, six indeed, may be now used to perform cladistic analyses on more samples to increase the number of galaxies. Ideally, one should select again the more discriminant parameters for other samples, at least to check that we do not miss something, or that it does not depend too much on the sample.

Third and last, we do not have found a new physics, or new kinds of objects. Not yet. But this is a first step, a crucial one for me: we can now do physics with astrocladistics. So, let’s go on, complete the cladogram, and … do physics.

Stay tuned!

  1. Why? | Astrocladistics

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