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Towards a Phylogenetic Analysis of Galaxy Evolution: a Case Study with the Dwarf Galaxies of the Local Group

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Towards a Phylogenetic Analysis of Galaxy Evolution: a Case Study with the Dwarf Galaxies of the Local Group A&A 455, 845–851 (2006) Astronomy DOI: 10.1051/0004-6361:20065098 & c ESO 2006 Astrophysics Towards a phylogenetic analysis of galaxy evolution: a case study with the dwarf galaxies of the Local Group D. Fraix-Burnet1,P.Choler2, and E. J. P. Douzery3 1 Laboratoire d’Astrophysique de Grenoble, BP 53, 38041 Grenoble Cedex 9, France e-mail: [email protected] 2 Laboratoire d’Écologie Alpine, BP 53, 38041 Grenoble cedex 9, France 3 Laboratoire de Paléontologie, Phylogénie et Paléobiologie, Institut des Sciences de l’Évolution de Montpellier, 34095 Montpellier Cedex 5, France Received 27 February 2006 / Accepted 25 April 2006 ABSTRACT Context. The Hubble tuning-fork diagram has always been the preferred scheme for classifying galaxies. It is based only on morphol- ogy. In contrast, biologists have long taken the genealogical relatedness of living entities into account for classification purposes. Aims. Assuming branching evolution of galaxies as a “descent with modification”, we show here that the concepts and tools of phy- logenetic systematics that are widely used in biology can be heuristically transposed to the case of galaxies. Methods. This approach, which we call “astrocladistics”, is applied to dwarf galaxies of the Local Group and provides the first evo- lutionary tree for real galaxies. Results. The trees that we present here are solid enough to support the existence of a hierarchical organisation in the diversity of dwarf galaxies of the Local Group. They also show that these galaxies all stem from a common ancestral kind of object. We find that some kinds of dIrrs are progenitors of both dSphs and other kinds of dIrrs. We also identify three evolutionary groups, each one with its own characteristics and own evolution. Conclusions. The present work opens a new way to analysing galaxy evolution and a path towards a new systematics of galaxies. Work on other galaxies in the Universe is in progress. Key words. galaxies: fundamental parameters – Galaxy: evolution – galaxies: formation 1. Introduction primer hypotheses of the evolution of galaxies leading to the fa- mous Hubble or tuning-fork diagram (Hubble 1936), there has Since Hubble (1922, 1926), classification of galaxies relies been no further attempt to base classification of galaxies on his- largely on morphology: spirals (flattened galaxies with spi- torical/evolutionary relationships. Nowadays, the physical and ral arms), barred spirals, ellipticals (galaxies with no obvious chemical processes are often roughly understood individually pattern), and irregulars (Kormendy & Bender 1996; Roberts and sometimes even modelled (e.g. Sauvage et al. 2002); but & Haynes 1994). Subdivisions have been created since then they are not collectively integrated, so that grasping the causes (de Vaucouleurs 1959; Sandage 1961) in an attempt to trans- of galaxy diversity in their entire complexity remains a difficult form morphology into a more quantitative parameter. The use of task. a limited number of additional properties – radio and X-ray prop- erties, environment, nucleus activity, gas content, star formation, Similar concerns have already been addressed in the study colours, and others – has led to the recognition of numerous of biological diversity. Nearly 150 years ago, Darwin (1859) classes that are essentially catalogues of objects. This traditional suggested that the hierarchical classification of living organisms approach to classification might look inadequate for describ- should reflect their genealogical relationships. This was an in- ing the now acknowledged complexity of galaxies. Multivariate credibly successful idea that drove biological systematics into a analysis of these properties has been once proposed to tackle the new era. There is now a common agreement that a natural classi- problem of synthesizing the classificatory information included fication should be derived from phylogenetic trees, i.e. a branch- in independent characteristics (Whitmore 1984; Watanabe et al. ing structure describing the evolutionary relationships of a set 1985). However, these approaches are still based on estimations of biological entities or taxa (Wiley et al. 1991). Accordingly, of the general similarities. the concepts and methods of phylogenetic systematics have been In fact, galaxies are complex systems in which several phys- successfully applied to other sets of entities for which histori- ical and chemical processes govern the evolution of their ba- cal or evolutionary relationships could be documented; examples sic constituents: stars, gas, dust, molecules, and probably black are found in linguistics (Wells 1987) and stemmatics (Robinson holes (e.g. Vilchez et al. 2001). Surprisingly, besides Hubble’s & Robert 1996). We therefore hypothesize that galaxies can be classified Figure 3 is only available in electronic form at in a natural hierarchy of nested groups reflecting evolution. http://www.edpsciences.org Our purpose is thus to introduce the parameter “time” in the Article published by EDP Sciences and available at http://www.edpsciences.org/aa or http://dx.doi.org/10.1051/0004-6361:20065098 846 D. Fraix-Burnet et al.: Towards a phylogenetic analysis of galaxy evolution classification scheme, using all available and suitable descrip- that they could then be placed in two separate classes. In all tive characters1, and to propose an integrated way of represent- cases, one class gives birth to at least two classes. This is the ing galaxy diversity. We have named this approach “astrocladis- definition of branching evolution. Its driver is mainly the ran- tics” (Fraix-Burnet et al. 2003; Fraix-Burnet 2004). The method domness of external interactions and to a lesser extent that of and associated concepts are presented in detail elsewhere, to- internal processes. This branching evolution results in a hierar- gether with an analysis of two samples of simulated galaxies chical or tree-like organisation of the diversity. (Fraix-Burnet et al. 2006a,b). In this paper, we present the first application of astrocladistics to real galaxies. We chose to focus on the Dwarf galaxies of the Local Group for which a reason- 2.2. Data able amount of complete and homogeneous data are available. We used data from Mateo (1998) on 36 galaxies and identified According to the “hierarchical scenario”2 of galaxy formation, 25 characters. For each character, we discretized the whole range these galaxies could belong to small dark-matter halos that are of values in up to 8 bins, plus the unknown value “?”3.Theseare the building blocks of larger structures that may host big galax- assumed to be evolutionary states. For instance, if we accept that ies. In this respect, dwarf galaxies are not considered as being metallicity increases with time, then a very low value would be formed by the merging of smaller structures; but because of the an ancestral state, whereas a high value would be a derived state. relatively shallow gravitational well, they are certainly subject This has to be adapted to the sample under study. We imposed to disturbances and sweeping. However, this hierarchical sce- 15 characters to be ordered (see Table 1), i.e. changes between nario of galaxy formation has some difficulties, like predicting two adjacent states are more probable than between distant ones, too many small structures that are not seen in the form of dwarf independent of the direction of the change. The morphological galaxies (e.g. Moore et al. 1999; Venn et al. 2004). The Local character (dIrr, Intermediate, and dSph) was discarded in the Group could also represent an evolutionary microcosm in which analysis because it differs notably from the other ones by be- the environments for all the dwarfs have been somewhat similar. ing the only qualitative property and also being somewhat sub- Here we scored a variety of characters, and inferred the most par- jectively defined. It was coded into three states merely for pro- simonious history that describes the evolution of dwarf galaxies jection onto the result trees. We thus ended up with a matrix of the Local Group. of 36 objects described by 24 characters, the total percentage of unknown values being 33%). The outgroup, used to root the tree – i.e., to orientate the evo- 2. Methods lutionary processes – indicates the ancestral states of the charac- In this section, we first outline briefly some concepts of as- ters for which transformation can be hypothesized within a rea- trocladistics, then describe the data used for our analysis, and sonably accepted paradigm. Given the complicated evolutionary finally present the process of tree construction. All details on behaviour of Dwarf galaxies, for this first astrocladistics analy- astrocladistics are given in Fraix-Burnet et al. (2006a, 2006b). sis, we decided to choose a galaxy in the sample. The advantage is that the result tree can be easily rerooted with any of the sam- ple objects depending on one’s own guess, because the structure 2.1. Conceptual framework of the tree does not change. The SagDIG galaxy is chosen here / A cornerstone of phylogenetic systematics is to focus on derived as root because it has the highest ratio MHI Mofthesample.It character states to infer common ancestry relationships (Hennig is interesting to note that SagDIG seems to be at the edge of our 1965). Such characters may be viewed as evolutionary novelties Local Group (van den Bergh 2000). appearing in a particular lineage. It is assumed that two closely relative objects (also called taxa because they can be individ- 2.3. Tree construction uals, groups, or species) share derived characters, which pre- sumably originate in their common ancestor. Regarding classi- An important aspect of phylogenetic systematics is to avoid fication, phylogenetic systematics defines clades as evolutionary grouping objects based on similarities due to evolutionary con- groupings that are significant for evolution by containing all off- vergences or reversals. In phylogenetic analysis, maximum par- spring from a common ancestor, and their members share com- simony (Fitch 1971) is one optimality criterion for selecting mon derived states, that is to say, a common history.
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