Reconstructing the Milky Way

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The MW includes a complex mixture of stars, planets, gas and interstellar dust, radiation and ubiquitous dark matter. The stars are widely distributed in age and metallicity (reflecting the formation rate), in space (reflecting the formation place and subsequent motions), in orbits (fixed by the gravitational force). The complex distribution of chemical abundances is determined by several factors: the kind and efficiency of stellar nucleosynthesis, the ejection efficiency of nuclear processed matter, the past history of star formation, mixing of stellar ejecta with the surrounding medium inside the Galaxy, gas loss from the Galaxy, and finally gas and star accretion due to capture of dwarf galaxies capture and/or intergalactic gas. The current stereoscopic view of the MW (central bulge, halo, thick and thin disk, spiral arms in the thin disk, and everything enclosed in a big halo of dark matter) stems from decades of observations and theoretical speculations. It is essentially based on three complementary sources: (i) census of the stellar and gaseous content for increasing space volumes; (ii) determination of the geometry by means of single objects distances; (iii) determination of the total gravitational field studying the motion and the orbits in the tridimensional space. The detailed knowledge of the physical mechanisms governing formation, evolution, and present state of the MW indicates how primeval galaxies were formed. Even if the current scenario can be considered satisfactory on the whole, there are still unanswered questions. The diagnostics is hidden in the properties of the stellar populations: position, kinematics, age, and chemical composition. Positions and kinematics are the fossil records of the dynamical processes, ages and chemical compositions are the tracers of the star formation history. The population synthesis technique is a particularly useful method to study the properties of the stellar populations. The aim of these projects is to derive the properties of the Milky Way populations from CMD, luminosity functions, and kinematics, of fundamental importance for the formation of our Galaxy.

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 Theoretical tools: The Padova Galaxy Model The CMDs of Galactic stellar fields are the result of a complex game of many players, including the dynamical processes that led to the formation of bulge, halo and disc, the history of star formation, the distribution of ages chemical abundances of the component galactic stellar populations, the mass distribution along the line of sight, the extinction, the kinematics of the stars entering and leaving the volumes we examine, etc. The Padova Galaxy Model (PGM) simulates observed CMDs taking into account the geometry of the system, the spatial distribution of matter (stars of any mass, age and chemical abundance), the star formation history and the spatial variation of the extinction. The PGM has successfully been applied to reconstruct the structure of the MW (Ng et al. 1995; Bertelli et al. 1997, 1999; Vallenari et al 1999a,b, 2000, 2003; Vallenari & Ortolani 2001, Vallenari et al 2003). Recently, two major implementations have been made: the first is the self-teaching/learning genetic language to automatically decipher the CMDs and companion luminosity functions. It gives information on key parameters that define the so-called solution: star formation rate as function of time, initial mass function, mass distribution, age and metallicity distribution of the galactic stellar populations, extinction distribution. The second step forward was the inclusion of kinematics. Now PGM deals in a self-consistent fashion both with CMDs and proper motions and radial velocities for the stars in the thin/thick disc, halo (Bertelli et al. 2003, Pasetto 2005, Vallenari et al 2006, Wilkinson, Vallenari et al 2006). A major effort was spent to develop both a complete and accurate description of the gravitational potential for the various components as a function of their mass density distributions, and a kinematic model of the thin and thick discs. The method relies on the study by Amendt & Cuddeford (1991, ApJ 368,79) and takes into account the vertical tilt of the velocity ellipsoid as function of the Galactic potential. The model provides a good description of the stellar kinematics up to large distances above the Galactic Plane.
The structure of the Galaxy The model was recently applied to several problems:

 

  • THE DISK. Study of the kinematics and of the stellar properties of the Galactic disk (Vallenari et al 2006, Ragaini 2006). Main results are the determination of stellar IMF at low masses in the thin disk , implying that the initial mass function changes slope at 0.8 Mo instead than at the canonical value of 0.5 Mo; a flat IMF in the thick disk; the derivation of the kinematics of the thin and thick disk showing that no significant vertical velocity gradient in the thick disk is present. This points in favor of a formation of the Thick disk by a quick heating of the precursor disk. Finally, no significant rotation velocity is found in the Halo.
  • THE BULGE. Study of the properties of the inner Galaxy: the position angle of the Galactic Bulge (Vallenari & Ragaini 2007, Vallenari, Ragaini & Bertelli 2007), studying the interstellar extinction towards the Galactic center (Ragaini 2006) The structure of the inner bulge (|l|<17 deg) was found to be made of a thin bar located in the inner |l|<5 deg and confined on the plane (|b|<3 deg), and a bulge having at a 53 ±10 deg angle.
  • we also derived the extinction towards V838Mon and its distance (Munari et al 2005)

 

People: A. Vallenari, G. Bertelli, E. Nasi, S.Ragaini, U. Munari

Collaboration: C. Chiosi (Padova Univ.), L. Schmidtobreick (ESO, Chile)

Publications: Vallenari et al. (2008), IAUS, 245,371;

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