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 μm 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 ~ 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 on 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 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.