Extremely red galaxies: age and dust degeneracy solved?

The extremely red galaxies (ERGs) are defined in terms of their very red optical-to-near IR colours (as R-K > 5 or I-K > 4). Originally this selection was aimed at selecting old (> 1 Gyr) passively evolving elliptical galaxies at intermediate redshift (1 < z < 2), but it was soon discovered that young star-forming dusty galaxies can show similar colours and therefore be selected in the same surveys. It is crucial to distinguish between these two populations because they have very different consequences on the models of galaxy formation. Here we show that old ellipticals and dusty starbursts are expected to show different colours in the (I-K) versus (J-K) diagram for redshift range 1 < z < 2, thus providing a useful tool to classify ERGs in large samples up to K < 20. This is mainly owing to the fact that old galaxies at these redshifts have a strong 4000-Angstrom break at lambda < 1.2 mu m (J band), while dusty galaxies show smoother spectral energy distributions and therefore redder J-K colours. We discuss this difference in detail both in the framework of the stellar population synthesis models and by using observed spectra. The selection criterion is also compared with the properties of ERGs of known nature. We also show that this colour selection criterion is also useful to separate the ERGs from brown dwarf stars showing similar optical-to-IR colours.