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Near-infrared (hereafter NIR) data may provide complementary information tothe traditional optical population synthesis analysis of unresolved stellarpopulations because the spectral energy distribution of the galaxies in the1-2.5\mum range is dominated by different types of stars than at opticalwavelengths. Furthermore, NIR data are subjected to less absorption and hencecould constrain the stellar populations in dust-obscured galaxies. We want todevelop observational constraints on the stellar populations of unresolvedstellar systems in the NIR. To achieve this goal we need a benchmark sample ofNIR spectra of ``simple'' early-type galaxies, to be used for testing andcalibrating the outputs of population synthesis models. We obtainedlow-resolution (R~1000) long-slit spectra between 1.5 and 2.4\mum for 14 nearbyearly-type galaxies using SofI at NTT and higher resolution (R~3000) long-slitspectra, centered at the MgI at ~1.51\mum for a heterogeneous sample of 5nearby galaxies observed with ISAAC at VLT. We defined spectral indices for CO,NaI, CaI and MgI features and measured the strengths of these features in thesample galaxies. We defined a new global NIR metallicity index, suitable forabundance measurements in low-resolution spectra. Finally, we present anaverage NIR spectrum of an early-type galaxy, built from a homogenized subsetof our sample. The NIR spectra of the sample galaxies show great similarity andthe strength of some features does correlate with the iron abundance [Fe/H] andoptical metal features of the galaxies. The data suggest that the NIR metalfeatures, in combination with a hydrogen absorption feature may be able tobreak the age-metallicity degeneracy just like the Mg and Fe features in theoptical wavelength range.

Early-type galaxies in the near-infrared: 1.5-2.4\mum spectroscopy