The near-IR spectra of 28 nearby galaxies were used to define the "average" J, H and K spectra of normal galaxies along the Hubble diagram between E and Sc. The target objects and the aperture were chosen to be similar to those by K96 in order to merge the two data sets and obtain self-consistent spectra between 0.1 and 2.4 µm. The average effective colours of the galaxies of the various classes were used to calibrate the final spectra. The overall precision is about 2%.
The spectra were used to compute the k-corrections in the near-IR bands and to check the predictions of several spectrophotometric models of galaxy evolution. We find that most model can accurately reproduce the observed continuum shape of the elliptical galaxies, if solar metallicities are assumed. In this case the derived SSP ages at above 8 Gyr. Nevertheless, large differences are found between the models, for example in terms of the derived age of the dominant stellar population.
On the other hand, larger discrepancies are found between the observed spectral features and the predictions of the models. We note that the spectra of cool giant stars from some accurate libraries (as KH86 and M98) can reproduce the observed features in great detail, and conclude that the disagreement is probably due to the stellar libraries used in the models.
The spectral features are also used to study the stellar populations by using several stellar classification recipes based on near-IR absorption lines, some of them applied to galaxies for the first time. The dominant contribution to the near-IR emission of all the galaxies between E and Sc is confirmed to be due to giant stars of spectral type about K5, with solar or slightly subsolar metallicity.
The final spectra for each galaxy type, the relative k-corrections (both in the Bessel & Brett and in the CIT systems) and the profiles of the used filters can be downloaded from the web site www.arcetri.astro.it/~filippo/spectra. In some cases it could be useful to simulate the spectra of galaxies with colours different from those in Table 2, for example to reproduce individual galaxies or match total colours. In the same web site it is possible to define a new set of colours, compute the corresponding spectra and download the results.
We are grateful to Tino Oliva, Livia Origlia and Valentin Ivanov, for enlightening discussions about near-IR spectra and stellar classification, and to Stephan Charlot and Gustavo Bruzual for having provided us a recent version of their model. We are also grateful to the TIRGO staff for the support during the observations. LP acknowledges a partial support from the ASI grant ASI-ARS99-44.