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Exploring evolutionary links between dwarf galaxy types using distant Local Group late-type dwarfs

Final Report Summary - DWARFGALAXIES (Exploring evolutionary links between dwarf galaxy types using distant Local Group late-type dwarfs)

One of the most important questions in astrophysics is how to explain the great variety of galaxy types that we observe in the Universe - spirals, ellipticals, irregulars, and their scaled down version, such as the dwarf spheroidals and the dwarf irregular galaxies - and to do so within a well-motivated cosmological framework.

In our current understanding of the composition of the Universe, only a very small fraction of the whole mass/energy density budget is in the form of matter whose properties we understand (i.e. ``baryons'', essentially atoms) - about 5%. The rest appears to be made of what is called ``dark matter'' (~25%) and ``dark energy" (~70%). Dark matter is elusive, as in astrophysics we can infer its presence only via its
gravitational interaction with the baryons. Yet, the nature of dark matter, i.e. what type of particle(s) is made of and what are its properties, greatly influences the formation and evolution of the baryonic structures (galaxies, clusters of galaxies and so on), which is what we can observe and study.

The presently favoured cosmological framework in a dark matter context for the formation and evolution of structures in the Universe predicts that smallest structures are those that form first, while larger structures are formed by the accretion and merging of smaller units. In this model, the smallest galaxies, i.e. dwarf galaxies, would be the most ancient galactic systems, those that have witnessed the early phases of the Universe, and have then had a very important role in the formation of larger galaxies like our own Milky Way. Dwarf galaxies are also those that appear to contain the largest ratio of dark matter
with respect to luminous matter, as indicated by the kinematics of the stars within the dwarf galaxies that orbit around the Milky Way: potentially these are some of the best ``astrophysical laboratories'' that we can use to gain insights into the nature of dark matter. Dwarf galaxies are also the most common type of galactic system that we observe, which implies that the favourite way of forming galaxies in the Universe
is indeed in the form of dwarf galaxies. For all these reasons, great attention is given to these systems; in particular, of great importance are the dozens of dwarf galaxies which inhabit the Local Group, because these are the systems that we have the possibility to study in the greatest detail, using the best telescope facilities world-wide.

The project of the researcher G.Battaglia focused on the population of dwarf galaxies of different types that are found in the Local Group, i.e. dwarf irregulars, dIrr and dwarf spheroidals, dSphs. These have presently different properties: while the former are still actively forming stars and contain neutral gas
(the raw material for star formation), the latter stopped forming stars several billion years ago, are devoid of gas and are now passively evolving.

Of special interest to the researcher were the questions: are dIrrs and dSphs intrinsically different objects? Or can they be considered as descending from similar progenitors which some mechanism made evolve differently? Fundamental to the investigation of these questions is the comparison of the properties of the individual stars in these different galaxy types, as they carry information on the star formation history, the chemical enrichment history and the dark matter potential well. One of the
main aims of the proposal was hence to improve our observational understanding of the Local Group dIrrs and of the so-called transition-types dTs (dwarf galaxies which have intermediate properties to dIrrs and dSphs, and so thought to be caught in the act of transforming from one type to the other one), for which
very little information on the properties of their stellar component was available with respect to the Milky Way dSphs. The researcher has very actively pursued the acquisition of such data-set by proposing observations on some of the best and competitive telescope facilities world-wide and succeeded in the task. The observations of the galaxies are planned to run through the next year and analysis of the data-set acquired to date is planned to start soon. Results from a sub-set of the galaxies show striking similarities between the way stars of different ages are distributed in a dT and in the most luminous dSphs, as well as an internal kinematics for the stars in a dIrr much more similar to the one in dSphs with respect to what could have been inferred by analysing the kinematics of the dIrr gas.

The hypothesis that dIrrs and dSphs may descend from similar progenitors which were put on a different evolutionary path via some mechanism receives support from the general observation that dSphs are found to orbit much larger galaxies (like the Milky Way), whilst dIrrs are relatively isolated systems. The environment in which dwarf galaxies live in is then clearly playing a role in their evolution and in determining their present morphological types. But how strong does this environmental interaction need be? Can it simply remove the gas from dIrrs and transform them into dSphs? or does it have a much more profound impact? Understanding this point is important also because the tidal interaction with the large spirals can in principle affect the way dark matter is distributed within dwarf galaxies and
can alter the kinematics of their stars, hence of the properties we derive. Strong tidal disturbance from the Milky Way can leave its imprint in the stellar component of the affected dwarf galaxy. By analyzing a recent data-set of exquisite quality, the researcher has found strong hints of tidal disturbance in the structure of a Milky Way dSph, where previously other possible signs of tidal effects had been detected mainly from the kinematics of its stars. Plans are under way to make an accurate model of this galaxy to assess the impact of tidal disturbance on its dark matter properties and to exploit existing data-sets (or acquire new ones if needed) to explore the presence of tidal disturbance also on the other dSphs satellites of the Milky Way.

The future avenues to follow to progress in our understanding of the possible evolutionary links between dIrrs and dSphs, and the way environment may transform one into the other one, will depend on the results that the acquired data-set will provide. Certainly, the information extracted on the metallicities of individual stars and on the dark matter content of dIrrs and dTs (as derived from the kinematics of the stars and possibly the gas), will make it possible to run models of the evolution of these galaxies which will give insight on how the stellar component was built in time, and how similar or different this may be in comparison to the evolution of dSphs. The ``seed'' data-set acquired by the researcher will indicate the interesting questions that may need follow-up, providing targets for exploitation of future approved instrumental facilities like VLT/MOONS and in a longer run with the E-ELT. Wide-area, deep photometric surveys of the closer-by dSphs are crucial to the exploration of possible signs of tidal disturbance in this class of galaxies, while simulations aimed at reproducing in detail their properties (structure, internal kinematics) will tell us how much their orbiting around the Milky Way may have affected their dark matter content. Such studies will benefit enormously by the data that the Gaia ESA mission will acquire,
for example allowing us to reconstruct their orbit around the Milky Way.