The Magellanic Clouds
(Cioni, Napiwotzki)
Maria-Rosa Cioni is the PI of The VISTA survey of the Magellanic System - (VMC)
The Magellanic Clouds, our neighbouring galaxies, are located at a fairly well known distance (Cioni et al. 2000) towards a region of sky only lightly affected by Galactic reddening, which translates into the capability of measuring details of their stellar population and structure as well as detecting their faint stellar populations. A convenient window to look at the Magellanic Clouds is the near-infrared. At these wavelengths luminous giant stars are best investigated: asymptotic giant branch (AGB) stars and red giant branch (SGB) stars. These stars have been detected in large numbers across the galaxies by wide-field surveys like DENIS (Cioni et al. 2000) and 2MASS. Near-infrared data with unprecedented sensitivity and spatial resolution across the Magellanic Clouds, that will detect sources as faint as the main-sequence turn off point, is on-going using the VISTA telescope (The VMC survey - PI Cioni).
Left: The number density distribution of AGB stars, regardless of their spectral type, across the Large Magellanic Cloud shows a smooth outer elliptical structure embedding a thick bar and protuberances emerging from it, hinting at the existence of spiral arms (Cioni, Habing & Israel 2000)
Right: The ratio between C-rich (or C-type) and O-rich (or M-type) AGB stars (the C/M ratio) is an indicator of the iron abundance ([Fe/H]). The distribution of the C/M ratio across the Large Magellanic Cloud shows clearly the existence of the classical metallicity gradient which is present in many galaxies: the iron abundance is higher in the centre and decreases more or less radially outwards (Cioni & Habing 2003).
The Ks magnitude distribution of AGB stars holds important information on the metallicity and age of the overall stellar population. In particular, the observed magnitude distribution of C-rich and O-rich AGB stars can be interpreted using theoretical distributions constructed using stellar evolutionary models spanning a rage of metallicity and age parameters. The study of the Magellanic Clouds using this technique has showed that the star formation rate derived from localised regions within the galaxies does not apply to the galaxies as a whole but inhomogeneities in both metallicity and mean age are clearly present (Cioni et al. 2006a, 2006b, 2006c); these are according to Bekki & Cioni (2007) fossil records of clumpy pasts of galaxies.