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The star-formation histories and dark-matter content of faint dwarf elliptical galaxies

Final Report Summary - HOLYDWARFS (The star-formation histories and dark-matter content of faint dwarf elliptical galaxies)

The current paradigm of Universe formation and evolution, the LCDM (Lambda cold dark matter, where "Lambda" is the cosmological constant) model, has an enormous success at predicting and explaining the large structure formation and evolution. However, we have embarrassing little knowledge about the details of baryonic matter transformations. Thought the Universe is made mostly of dark energy (about 68%), some dark matter (up to 27%) and less than 5% baryonic matter, the latest is the only one which is directly observable, mostly in galaxies. Galaxies consist of dark matter, gas, dust and stars. In stars, through thermonuclear reactions, the primordial elements (hydrogen and helium) are transformed into heavier elements, which we astronomers call “metals”. Everything we see around us on mother Earth is made in stars, even we are made from stardust! This project is centered precisely on this transformation of chemical elements and the life and death of stars in galaxies. It is not yet entirely clear what triggers and stops star formation in galaxies. Why some galaxies are star forming (blue), while other are “red” and dead objects?

To understand star-formation in particular and the evolution of galaxies in general I study dwarf galaxies. As their name suggests they have small sizes (more than 30 times smaller than our own Galaxy) and masses, which lead to few handy properties. First, they did not experience major merger events in the last half of the Universe history. Thus, their derived star-formation histories can be clearly interpreted. Second, they are more sensitive to the environmental effects, which play important role in galaxies transformation. Third, among the dwarf family are galaxies, which are the most dark matter dominated. Thus, we can study the properties of this exotic matter in dwarf galaxies and understand its impact on the galactic evolution. Forth, dwarfs' modest dimensions are easily simulated with the current high-end computers and we are able to detail the physical prescription of the star formation in dwarfs. Therefore, dwarf galaxies are ideal laboratories to study the star-formation processes and the influence of the dark matter on the galactic evolution, which are/were precisely the objectives of this project.

To reach these objectives we identified a number of tasks in particular: management, data reduction, data analyses, dynamical modelling, N-body/SPH simulations. The management of the project included scientific exploration of the observations and simulations and presenting these results in prestigious international journals and conferences. Here, I outline the main results and conclusions.

The main outcome from my work is a more unified picture of star-forming and quiescent dwarfs, with environment playing a prominent role in the ceasing of the star formation (see Fig.1). This scenario is supported by my research and by other observations and theoretical work, showing the similarities in the stellar populations between star-forming and quiescent dwarfs (Grebel et al. 2003, Weisz et al. 2011, Koleva et al. 2013, Sanchez-Almeida et al. 2009, Schroeyen et al. 2013, Verbeke 2013), their dynamics and dark matter content (Koleva et al, in prep.), the relative abundance of star forming and quiescent dwarfs on radial and tangential orbits (Conselice et al. 2003, Bouchard et al. 2009, Drinkwater et al. 2001, de Rijcke et al. 2010). These conclusions were presented as oral contributions to a number of international conferences (e.g. "Star formation in dwarf galaxies", 2012, a Lowell Observatory US workshop, and "Small Stellar Systems in Tuscany: From Globular Clusters to Dwarf Galaxies and Everything in Between", 2013, Prato, Italy) and in A1 publications (Koleva et al. 2013, Koleva et al. in prep).

Secondly, I attempted to achieve a better understanding of the star formation processes: I assisted the implementation of realistic cooling/heating curves of gas in simulations of galaxies (De Rijcke et al 2013) and through work on stellar libraries (Koleva et al 2012, Meneses-Goytia et al., subm.) which will ultimately result in better models of galaxies, which on their side will help the accuracy of the dark matter determination through better knowledge of the mass distribution of the newly formed stars; upgrade of my code for determining the kinematics, baryonic content and star-formation histories

Explanations to Fig.1 (uploaded as an attachment to this document):
Transformation of “blue”, star-forming galaxies to “red”, quiescent objects. In the left we present two types of star forming galaxies (Blue Compact Dwarfs, BCD and dwarf Irregulars, dIrr), which will stop forming stars if entering into a cluster environment. The transformation time will depend on the orbit of the galaxy within the cluster. Tangential orbit will result in keeping some star formation or/and gas while if the galaxy enters the cluster environment in a radial orbit its gas will be instantaneously stripped resulting in red and dead object (dwarf elliptical, dE). A TTD will eventually lose its gas and become a dE. If a BCD stays outside of dense environment its classification will depend on the time it is observed, during or between star formation episodes.
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