Stellar populations in the Local Group

(Cioni, Ryan)

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 has been completed at UKIRT (PI Irwin, co-I Cioni). UKIRT data combine wide-field and good sensitivity improving considerably on former studies. Results on M33 (Cioni et al. 2008, 2009), the third brightest member of the Local Group, show that the ratio between C-rich (or C-type) and O-rich (or M-type) AGB stars confirms a metallicity gradient in the galaxy corresponding to spread in [Fe/H] of about 0.6 dex with substructures in the inner and outer galaxy. The gradient of the M33 disc, until ~9 kpc, is -0.078 ± 0.003 dex kpc-1 while the outer disc/halo, out to ~25 kpc, has [Fe/H] ˜ -1.7 dex (Figure 1). The metallicity of M33 supports an “inside-out” disc formation via accretion of metal poor gas from the interstellar medium (Cioni 2009). Maps showing the distribution of mean age and metallicity suggest that: the outer galaxy disc/halo is metal poorer than the nuclear region and metal-rich clumps in the inner galaxy change location with time. The average outer ring and nuclear stellar population is 6 Gyr old while central regions are a few Gyr younger. Results on NGC 6822 (Sibbons et al. 2012), a Magellanic type irregular galaxy, show that the AGB population has been detected out to a radius of 4 kpc giving a diameter of 56 arcmin. It is metal-poor ([Fe/H] = -1.29 ± 0.07 dex), but there is no obvious gradient in metallicity with either radial distance from the centre or azimuthal angle. The detected spread in the tip of the red giant branch (RGB) magnitude is consistent with that of a galaxy surrounded by a halo of old stars (Figure 2).

Fig. 1: Iron abundance in M33. Points referring to AGB abundances with σ_[Fe/H] < 0.2 dex are plotted as empty squares (magenta) and those with 0.2 < σ_[Fe/H] < 0.38 as crosses (black).The least square fit line thorough all points (blue) and those with small uncertainties (red) are indicated. Magenta points are confined within about 8 kpc, the truncation radius of M33. Beyond this radius and for [Fe/H] < -1.5 dex the least square fit (red) is shallower; this fit has been prolonged towards the centre of the galaxy with a dashed line. Filled circles (green and black) show the abundance of stellar clusters attributed to the halo of the galaxy and to the RR Lyrae stars (blue). A RGB gradient is shown in cyan..

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.