Stellar population of Local Group galaxies: near-infrared diagnostics
(Cioni)
The star formation history in galaxies is influenced by the environment in which they reside. Thus, it is important to study statistically significant samples of galaxies in different environments as well as galaxies of different types and in different stages of evolution. The first step towards increasing the parameter space for these studies is to investigate the stellar content of galaxies in the Local Group (dwarfs and irregulars) which differ in luminosity, mass and metallicity.
A programme to observe the luminous red stars in Local Group galaxies is taking place at UKIRT (PI Irwin, co-I Cioni). UKIRT data combine wide-field and good sensitivity improving considerably on former studies. Preliminary results on M33, the third brightest member of the Local Group, show that asymptotic giant branch stars trace a smooth structure with hints of the major galaxy spiral arms. The ratio between C-rich (or C-type) and O-rich (or M-type) AGB stars suggests a metallicity ([Fe/H]) spread of about 0.6 dex across the galaxy. Stars in the NW of the galaxy are fainter than stars in the SE. The mean age and metallicity of the stellar population are inhomogeneously distributed across the galaxy with central regions metal richer than the overall disk.
Deep and wide-field observations of a given system are also powerful tools to testify the presence of tidal streams as well as past accretions or merger events. These effects are usually traced by the distribution of HI gas, if present, or by the distribution of stars such as giant stars which are likely more metal poor and have a spatially relaxed distribution.
Instruments currently available allow us to resolve individual AGB stars in galaxies out to a few Mpc, while to reach giant stars in galaxies beyond this limit, for example out to the Virgo or Fornax clusters of galaxies as well as in the intra-cluster region new facilities like JWST and Extremely Large Telescopes (ELTs) are necessary.