Relación de lineas de trabajo "La Galaxia y el Grupo Local"

1. Poblaciones estelares e historia de la formación estelar en la Vía Láctea con Gaia
2. Moving groups as tracers of the galactic potential
3. ....

1) Poblaciones estelares e historia de la formación estelar en la Vía Láctea con Gaia

Antonio Aparicio Juan y Grupo de Poblaciones Estelares en Galaxias del IAC (29-10-2009)

El estudio de muestras significativas de poblaciones estelares resueltas en galaxias permite obtener la tasa de formación estelar en función del tiempo y la ley de metalicidad de tales sistemas con buena precisión y con una resolución temporal del orden de 1 a 2 Gyr, incluso para las épocas más tempranas de la evolución de la galaxia. Para ello estamos usando diagramas color-magnitud (DCM) de las estrellas resueltas que llegan hasta magnitudes por debajo de los “turn-offs” más viejos y nuestros códigos IAC-star, de simulación de poblaciones estelares y DCM sintético, e IAC-pop, para la solución de la historia de formación estelar. Estamos realizando este tipo de estudios en galaxias del Grupo Local utilizando datos del HST y de tierra (en el caso de galaxias satélites de la Vía Láctea).

Por su proximidad a nosotros, las poblaciones estelares de la Vía Láctea son las que ofrecen las mejores condiciones para su estudio en profundidad, incluyendo las estrellas más débiles. Sin embargo, los trabajos en la Vía Láctea se ven entorpecidos por la diversidad de distancias a las que se encuentran las estrellas de nosotros y por la dificultad de determinar esas distancias con precisión para grandes muestras. Gaia ofrecerá tales distancias hasta 2.5 kpc, por lo que abrirá la posibilidad de este tipo de análisis de máxima precisión.

Los datos de distancias y velocidades proporcionados por Gaia permitirán obtener magnitudes absolutas precisas y discriminar cinemáticamente las diferentes estructuras de la Vía Láctea. Nuestra propuesta es:

Utilizar los datos de magnitudes absolutas y los DCM profundos derivados de ellas para analizar la historia de la formación estelar de las diferentes subestructuras de la Vía Láctea. Por “historia de la formación estelar” debemos entender aquí distribución de edades y ley de metalicidad para cada población o estructura analizada. Los datos que estarán a disposición permitirán también un estudio pormenorizado de las distribuciones espaciales dentro de cada subestructura.

Utilizar los datos de cinemática para vincular las historias de formación estelar así obtenidas con la cinemática de las diferentes subestructuras.

2) Moving groups as tracers of the Galactic Potential

The study of moving groups in the Gaia Era is an example of elegant applications of robust statistical techniques to large samples of data in the N-dimensional space of parameters describing the position, kinematics, evolutionary state and chemical composition of the stars. The training on multivariate techniques are mandatory to treat rigorously the full set of both Gaia Data and on-ground observations providing accurate radial velocities and detailed chemical abundances from high resolution spectroscopy.

As mentioned, nowadays several possibilities for the origin of the moving groups are commonly considered:

• Cluster and star complex disruption
• Orbital and resonant effects of the non-axisiymmetric structure of the MW (spiral arms and bar, others?): periodic orbits, chaos, precession of periodic orbits, transients spiral structure
• Tidal debris due to accretion of satellite galaxies
• Effects on the disk due to a massive merger
• Wave in the velocity plane in an unrelaxed MW disk.

The origin of these kinematic structures is far from being resolved and a combination of chemical tagging studies, large scale galactic dynamics and cosmological simulations applied to the MW must move forward together. Up to now, available observations have constraint our moving group studies to a small volume of the galactic disk, that is the immediate solar neighborhood. Gaia capabilities to fully characterize moving groups on a wide range of galactocentric radii has to be evaluated through the use of the Gaia Universe Model Snapshot (GUMS) developed for Gaia mission preparation (DPAC).

Theoretical work on moving groups involve numerous and varied procedures and modeling which have to be developed in a parallel manner: analytical approximations (galactic potential), test particle orbit computation in a realistic galactic potential and the study of resonant streams in N-body simulations developing bar and/or spiral arms, among others. Moreover, it has also been proposed that particles in the predicted dark disk of the MW might be affected by the same resonances as stars are, triggering dark matter moving groups in the disk (Antoja et al., 2009). These kinematic structures, which up to now have been detected only in the solar neighborhood, have turn out to be a powerful tool to constrain the structural characteristics of the MW disk in its present and past form. At present, the number of spiral patterns in our MW and their shapes and speeds have not been unambiguously determined (Vallee et la., 2008). Even more, the relation between spiral and bar patterns is not well understood (Athanassoula et al., 2008), and an important work is pending to evaluate the kinematic imprints of a secular evolution of the bar and the spiral structures (Minchev et al., 2009). Though N-body simulations predict the growth of these non-axisimetric components, the signature of these models has to be explored in the stellar phase space distribution. Existing N-body simulations are limited in the number of particles in the galactic disk, less than 10^6 (see Widrow (2006, Ceverino et al. (2008), Martínez-Valpuesta (2006) as examples), thus a massively parallel computational approach is required to resolve the structure of the phase space to match Gaia observational data. although these N-body simulations can be used to explore non-equilibrium dynamical processes, other techniques such as test-particle integration methods will allow to investigate on a larger range of initial conditions (birthplaces of stars). This approach requires a good understanding of the gas dynamics (SPH) to trace the star formation history and their motion on time dependent models.

We plan to undertake this ambitious project. 1) We plan to improve existing numerical techniques, and consider critical items in our simulations as initial conditions, time step integration, number of steps and number of particles among others. 2) We shall be able to develop and run these new hybrid approaches (N-body + test particle + SPH) models improving supercomputing techniques. 3) Multiescale statistical tools will be developed to characterize the phase space properties of both, simulated model data and expected Gaia data (from Gaia Simulator). As this approach is very computationally intensive, the present project will joint efforts from researches on two complementary disciplines: theoretical dynamics and computational astronomy ( MareNostrum).