Early-type galaxies in the near-infrared: 1.5-2.4 μm 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 mu m range is dominated by different types of stars than at optical wavelengths. Furthermore, NIR data are less affected by dust absorption and hence could constrain the stellar populations in dust-obscured galaxies. Aims. We attempt to develop observational constraints on the stellar populations of unresolved stellar systems in the NIR. Methods. To achieve this goal, we require benchmark NIR spectra of simple early-type galaxies, to test and calibrate the outputs of population synthesis models. We obtained low-resolution (R similar to 1000) long-slit spectra between 1.5 and 2.4 mu m for 14 nearby early-type galaxies using SofI at the ESO 3.5-m New Technology Telescope and higher resolution (R similar to 3000) long-slit spectra, centered on Mgi at similar to 1.51 mu m 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 corresponding to CO, NaI, CaI and MgI absorption lines and measured their strengths in the galaxies spectrum. We defined a new global NIR metallicity index, suitable for abundance measurements in low-resolution spectra. Finally, we created an average-combined NIR spectrum of an early-type galaxy, from a homogeneous subset of our data set. Conclusions. The NIR galaxy spectra are similar to each other and some of the line index strengths correlate with both the iron abundance [Fe/H] and optical metal line indices of the galaxies. The data suggest that the NIR metal features, in combination with a hydrogen absorption feature, may break the age-metallicity degeneracy in a similar way the Mg and Fe features in the rest-frame optical wavelength range.