Early - type galaxies in the near - infrared : 1 . 5 - 2 . 4 μ m spectroscopy ⋆ ⋆ ⋆

Context. Near-infrared (hereafter NIR) data may provide complementary information to the traditional optical population synthesis analysis of unresolved stellar populations because the spectral energy distribution of the galaxies in the 1-2.5 μm range is dominated by different types of stars than at optical wavelengths. Furthermore, NIR data are subjected to less absorption and hence could constrain the stellar populations in dust-obscured galaxies. Aims. We want to develop observational constraints on the stellar populations of unresolved stellar systems in the NIR. Methods. To achieve this goal we need a benchmark sample of NIR spectra of “simple” early-type galaxies, to be used for testing and calibrating the outputs of population synthesis models. We obtained low-resolution (R∼1000) long-slit spectra between 1.5 and 2.4 μm for 14 nearby early-type galaxies using SofI at the ESO 3.5-m New Technology Telescope and higher resolution (R∼3000) long-slit spectra, centered at the MgI at ∼1.51 μm for a heterogeneous sample of 5 nearby galaxies observed with ISAAC at Antu, one of the 8.2-m ESO Very Large Telescope. Results. We defined spectral indices for CO, NaI, CaI and MgI features and measured the strengths of these features in the sample galaxies. We defined a new global NIR metallicity index, suitable for abundance measurements in low-resolution spectra. Finally, we present an average NIR spectrum of an early-type galaxy, built from a homogenized subset of our sample. Conclusions. The NIR spectra of the sample galaxies show great similarity and the strength of some features does correlate with the iron abundance [Fe/H] and optical metal features of the galaxies. The data suggest that the NIR metal features, in combination with a hydrogen absorption feature may be able to break the age-metallicity degeneracy just like the Mg and Fe features in the optical wavelength range.