Spatial distribution of stellar populations in nearby dwarf galaxies resolved in stars by Gaia.

Abstract

Local Group of galaxies. The Local Group, our immediate neighbourhood, is the group of galaxies inwhich our Galaxy, the Milky Way, formed and is evolving. The Milky Way and the Andromeda galaxy (M31) are the two dominant spiral galaxies in the LG, and each is surrounded by an entourage of lower mass companions. It looks like an ordinary collection of dwarf galaxies dominated by two giant spirals. However for astronomers on Earth it is special. The proximity of Local Group galaxies makes them the ideal benchmarks to study galaxy formation and evolution, because they are the only systems where the accuracy and the wealth of observational data allows us to understand them in a sufficiently reliable way (Tosi 2003). In the last 25 years there has been a wealth of papers dealing with the chemical evolution of dwarfs: a rather frustrating challenge, if one considers how inconsistent with each other the results of these papers have been. Whilemost authors agreed that the initial mass function in these galaxies is fairly similar to Salpeter’s, both on the star formation regimes and on the existence of galactic winds, different groups have reached very different conclusions. These inconsistencies are due to the lack of adequate observational data. Among the Local Group dwarf galaxies the Magellanic Clouds are the nearestto our Galaxy. Their importance is many-fold but two aspects especially stand out. First, they act as a guide to our own Milky Way galaxy as to how it would appear if we could view it from a vantage point high above its dusty disk. Second, we can make use of them to understand other galaxies far too remote for any sort of detailed study. The Magellanic Clouds are fundamentally importantfor the calibration of the extragalactic distance scale. They represent one of the few opportunities we have to compare rare objects like the most luminous blue supergiant stars, variable stars, star clusters, and HII regions directly with common stars similar to the Sun, all at the same distance and all comparatively unobscured by interstellar dust. With firm calibrations in hand, we can thenconfidently proceed to more distant systems where only the very brightest objects may be accessed. The Gaia mission In 2013 the scientific community expects the launch of Gaia. It is an astrometric satellite, which was approved by ESA’s Science Programme Committee in 2000.Unlike HST and SIM, which are pointing instruments observing a preselected list of objects, Gaia is a scanning satellite that will repeatedly survey in a systematic way the whole sky, making possible broader astrophysical investigations. HST photometry and 10 m class telescope spectroscopy allowed modelling the chemical evolution of dwarf galaxies in the Local Group even more safelythan that of the solar neighbourhood. The Gaia mission might not be able to compete with these instruments, but what it will provide beyond sheer measurement accuracy is (i) its capability to perform an all-sky and sensitivity limited absolute astrometric survey, (ii) the unique combination into a single spacecraft of the three instruments, AF astrometric field, BP/RP low-resolution spectrophotometres and RVS radial-velocity spectrometre, (iii) the huge number of objects and observations which allow the accuracy on single objects to be achieved on very large samples, thus yielding statistical significance. Whereas the direct product of the Gaia mission will be a highly accurate astrometric and photometric survey to V = 20 mag, the science goals are much broader and account for the support of a large scientific community. The diversity of the science goals for Gaia requires a very complex mission and data reduction preparation. To cope with the challenge, the scientific community, together with ESA, started a joint effort to set up a data processing ground segment comprising a single processing pipeline which will deliver the intermediate and final mission science products. The development of a software system capable of covering thesimulation needs of the Gaia Data Processing and Analysis Consortium (DPAC) requires not only software engineering competence alone; a strong scientific component is also needed to ensure that the system fulfills the scientific goals. A Universe Model is developed for the simulation needs of the mission. It is a set of algorithms for computing the positions at any time, and observationalproperties of any objects expected to be observed by the Gaia instruments. The distributions of these objects and the statistics of observables should be as realistic as possible for simulations to be usable for estimating telemetry, testing software, simulating images, etc. Solar system objects (planets, satellites, asteroids, comets), Galactic objects (stars, nebulae, stellar clusters, diffuse light) and extra-galactic objects (neighbour galaxies resolved in stars, unresolved but extended remote galaxies and quasars, AGN, extra-galactic supernovae) will be simulated. For each of these simulated objects one needs to have their threedimensional spatial distribution together with their spectral characteristics (for computing photometry and spectroscopy, stable or variable in time), and their motions (for astrometric computations and for spectral corrections). Gravitational lensing for stars and galaxies are also to be simulated. Main goals The goals of the current study are:1. the investigation of the spatial distribution of stellar populations in nearby galaxies, expected to be resolved in stars by Gaia. These are part of the extra-galactic objects to be simulated in the framework of the Gaia mission preparation. The main targets are the Large and Small Magellanic Clouds, along with six dwarf irregular galaxies in the Local Group - NGC 6822, WLM, IC10, Phoenix, Sextans A and Sextans B. 2. the implementation of results from this investigation in the Gaia Universe Model. Catalogues of stars known to be part of both the Large and Small Magellanic Clouds and their characteristics (B, V , and I magnitudes) are to be provided to be used for simulations, including parameters such as Right Ascension, Declination, V magnitude, V −I colour, G magnitude, distance, radial velocities, proper motions, surface gravity log g, effective tempearture Tef f , metallicity [F e/H], α enhancement [α/F e], and extinction AV . Main resultsThe results of the current study can be divided in two categories, according to the main goals presented above: 1. The spatial distribution of stellar populations in nearby galaxies was studied.The main targets were the Large and Small Magellanic Clouds and additionally six dwarf irregular galaxies in the Local Group - NGC 6822, WLM, IC10, Phoenix, Sextans A and Sextans B. • Isopleth contour maps and radial surface density profiles of several dwarf irregular galaxies in the Local Group have been produced, based on data from the literature; • Structural parameters have been obtained for these galaxies by fitting theoretical models to the stellar radial surface density distribution; • Although the Magellanic Clouds are assumed to be irregular galaxies, our results suggest that the older populations appear to behave as tidally truncated systems, even if they do not show obvious radial symmetry; • It is confirmed that the exponential disk scale length of the LMC is about twice the SMC value; • It was shown that there is a system of young objects more concentrated in the central region of the LMC with a position angle nearly perpendicular to the other system - this is observed in the isopleth maps from both 2MASS and the carbon stars catalogue; • From the carbon stars in the LMC, it was shown that two different systems exist with a core radius of 3.3 ± 0.1 deg for the faint and 0.9 ± 0.1 deg for the bright carbon stars. An exponential model also shows such a difference in the scale height, 1.69±0.07 deg and 0.98±0.05deg, respectively, revealing a smaller central system of more massive carbon stars. 2. The results of this study were used to implement Magellanic Clouds data in the Gaia Universe Model in the framework of the Gaia mission preparation. • Catalogues of stars known to be members of both Magellanic Clouds have been obtained from the literature and their characteristics have been provided for the simulation of nearby galaxies, resolved in stars by Gaia. These are part of the extra-galactic objects to be simulated in the framework of the Gaia mission preparation; • Within the second goal of the project, we provided the requested material in appropriate format that were adopted for the simulations of the Magellanic Clouds in the framework of the Gaia Universe Model.Structure The Thesis is divided in four main parts. The structure is further described below. The introductory Part I provides an overview (Chapter 1) of the Local Group galaxies census in connection with our investigation and presents the theoretical models describing the spatial distribution of various stellar components in the galaxies (Chapter 2). A more detailed overview of the Gaia mission is given in Chapter 3. In Part II we present the results from modelling the spatial distribution of some stellar populations in the Magellanic Clouds. Chapter 4 includes an overview of the Magellanic System. Chapter 5 describes the data available at the start of the project for the Magellanic Clouds. The data sources are presented which were selected based on our requirements. In Chapter 6 are shown the results from modelling several age groups in the Large and Small Magellanic Clouds, including colour-magnitude diagrams, isopleth contour maps and radial surface density profiles. Part III contains two chapters. Chapter 7 gives a short description of the coordination unit of the Gaia Data Processing and Analysis Consortium responsible for data simulations and a description of the Gaia Universe Model itself. Chapter 8 presents our contribution to the Gaia Universe Model consisting in the implementation of Magellanic Clouds data. In some aspects Part IV is analogous to Part II. It presents the study of the stellar spatial distribution in six dwarf irregular galaxies of the Local Group. Chapter 9 gives an introduction of the galaxies, including their properties and results from previous investigations from the literature. Chapter 10 describes the survey (LGGS) which provides the photometric data for the galaxies in our sample. Chapter 11 shows the results from modelling the stellar distribution in these six dwarf irregular galaxies, including colour-magnitude diagrams, isopleth contour maps and radial surface density profiles. The final part of the Thesis contains the concluding remarks, some ideas for future work on the subject, as well as lists of all the figures and tables in the Thesis and the References.

Το Τοπικό Σμήνος γαλαξιών, η άμεση γειτονιά μας, είναι η ομάδα των γαλαξιών στην οποία σχηματίστηκε και εξελίσσεται ο Γαλαξίας μας. Ο Γαλαξίας μας και η Ανδρομέδα (Μ31) είναι οι δύο κυρίαρχοι σπειροειδείς γαλαξίες στο Τοπικό Σμήνος, και κάθε ένας από αυτούς περιβάλλεται από δορυφόρους γαλαξίες μικρότερης μάζας.Tο 2013 η επιστημονική κοινότητα αναμένει την εκτόξευση του δορυφόρου Gaia. Πρόκειται για έναν αστρομετρικό δορυφόρο, η έγκριση του οποίου πραγματοποιήθηκε το 2000 από την επιτροπή επιστημονικών προγραμμάτων του Ευρωπαiκού Οργανισμού Διαστήματος (ESA). Παρόλο που το άμεσο προiόν της αποστολής Gaia θα είναι μία αστρομετρική και φωτομετρική έρευνα υψηλής ακρίβειας μέχρι το μέγεθος V=20 mag, οι επιστημονικοί στόχοι είναι πολύ ευρύτεροι και υποστηρίζονται από μία τεράστια επιστημονική κοινότητα. Το Μοντέλο του Σύμπαντος (“Universe model”) που έχει ήδη αναπτυχθεί για τις ανάγκες των προσομοιώσεων της αποστολής, aποτελείται από ένα σύνολο αλγορίθμων για τον υπολογισμό της θέσης ανά πάσα στιγμή, καθώς και των ιδιοτήτων, κάθε αντικειμένου που πρόκειται να παρατηρηθεί από τα όργανα του δορυφόρου Gaia. Οι κυριότεροι στόχοι της παρούσας διατριβής είναι οι εξής: Η διερεύνηση της χωρικής κατανομής των αστρικών πληθυσμών των γειτονικών γαλαξιών, και ειδικώτερα μέρος του αστρικού πληθυσμού ο οποίος αναμένεται να παρατηρηθεί από το δορυφόρο Gaia. Πιο συγκεκριμένα, μελετώνται το Μικρό και το Μεγάλο Νέφος του Μαγγελάνου, καθώς και έξι ακόμα νάνοι ανώμαλοι γαλαξίες του Τοπικού Σμήνους γαλαξιών: Ο NGC 6822, ο WLM, ο IC10, ο Phoenix, ο Sextans A και ο Sextans B. Η εφαρμογή της παραπάνω διερεύνησης στο Μοντέλο του Σύμπαντος. Στο πλαίσιο αυτής της συνεργασίας θα παραδοθούν προς προσομοίωση αστρικοί κατάλογοι των Νεφών του Μαγγελάνου. Οι κατάλογοι αυτοί θα περιέχουν μία σειρά παραμέτρων για κάθε αστέρα, όπως για παράδειγμα τα μεγέθη B, V, Ι και G, η ορθή αναφορά και η απόκλιση, το χρώμα V I, η απόσταση, η ακτινική ταχύτητα, η ιδία κίνηση, η επιφανειακή βαρύτητα (log g), η ενεργός θερμοκρασία (Teff), η μεταλλικότητα ([Fe/H]), η επαύξηση σωματιδίων α (α enhancement – [α/Fe]) και η εξάλειψη του φωτός (extinction - AV).