ESO PR Photo 32e/07
ESO PR Photo 33b/07

Shooting a Laser at the Galactic Centre

The Local Group

The Local Group (LG) of galaxies is an ordinary sparse group of galaxies with a zero-velocity radius of 1 Mpc. It hosts two giant spiral galaxies, our Milky Way and M31, also known as the Andromeda Galaxy. The only other ordinary galaxy in the LG is M33 (the Triangulum Galaxy), which is a late-type spiral galaxy. It is small relative to its larger neighbors such as the Milky Way Galaxy and the Andromeda Galaxy, but is about average compared to most spiral galaxies in the universe. The other Local Group members are about 40 dwarf galaxies gravitationally bound to one of the two dominating giant spirals. Dwarf galaxies are generally divided in three groups: dwarf elliptical (dE) galaxies, dwarf spheroidal (dSph) galaxies and dwarf irregular (dIrr) galaxies.
Cat's eye nebula
The cat's eye nebula

Credit: (STScI/AURA)

AGB stars

Intermediate-mass stars on the Asymptotic Giant Branch (AGB), and in particular carbon-rich (C) stars, have been known for a long time to be important tracers of intermediate-age populations in galaxies. Because of their high luminosities and distinctive spectral features, AGB stars can significantly contribute to the integrated light of galaxies. Thus, the study of their properties in nearby resolved galaxies represents an important step towards a successful intepretation of the spectra of distant unresolved galaxies. The population of C-stars is a function of the star-formation history and the chemical-enrichment of a galaxy. The corresponding observables, the ratio of C- and O-rich AGB stars (C/M), their luminosity and color/Teff distributions, just recently have become a little better understood using new models which include the effects of variable opacity in C-rich atmospheres. Because of the dependence of C-star formation on metallicity, not only is the C/M ratio globally a function of galaxy metallicity but also it can be used to map local variations in the age and metallicity of the stellar populations. Yet, the observed ratio of C- and O-rich stars is still rather controversial and dependent on the adopted technique, with narrow-band optical selection yielding sometimes contrasting results with respect to a classification based on a simple J-K color threshold.

The near-infrared

JHK bands
Broad-band JHK filters avaiable at ESO and atmospheric transmission.
A systematic census of AGB C-stars in LG galaxies has been made in last few years using the four-filter technique with two optical broad-band and two narrow-band filters (Albert et al. 2000; Bat- tinelli & Demers 2000; Nowotny et al. 2001).
A different approach (e.g. Cioni & Habing 2005; Kang et al. 2005) is based on the combination of one optical filter with J and K near infrared (IR) broad-band filters. The main advantages of using broad-band near-IR over optical broad- and narrow-band filters are the following:
(i) C and M-type stars can be easily separated in a color-color diagram;
(ii) extinction in the near-IR is much lower than in the optical, reducing the problems with differential reddening that can be present, in particular in star forming dwarf irregular galaxies;
(iii) the spectral energy distribution of AGB stars peaks in the near-IR and makes them easily distinguishable from the RGB stars;
(iv) from J-K s color and K magnitude it is possible to derive precise bolometric magnitudes; and
(v) with the development of adaptive optics at large telescopes (NACO at VLT, adaptive optics at the next generation of Extremely Large Telescopes), operating principally in the near-IR, it is important to know well the observational characteristics and have well prepared models for the interpretation of the data of more distant objects that will shortly be observed. It is important to know how these bright near-IR sources, the first to be observed in a distant galaxy, behave with age and metallicity.

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