2013MNRAS.431.1823M

Recent models show that thermally pulsing asymptotic giant branches (TP-AGB) stars should dominate the near-infrared (NIR) spectra of populations 0.3-2Gyr old, leaving unique signatures that can be used to detect young/intermediate stellar population in galaxies. However, no homogeneous data base of star-forming galaxies is available in the NIR to fully explore and apply these results. With this in mind, we study the NIR spectra of a sample of 23 Hii and starburst galaxies, aimed at characterizing the most prominent spectral features (emission and absorption) and continuum shape in the 0.8-2.4µm region of these objects. Five normal galaxies are also observed as a control sample. Spectral indices are derived for the relevant absorption lines/bands and a comparison with optical indices of the same sample of galaxies available in the literature is made. We found no correlation between the optical and the NIR indices. This is probably due to the differences in aperture between these two sets of data. That result is further supported by the absence or weakness of emission lines in the NIR for a subsample of galaxies, while in the optical the emission lines are strong and clear, which means that the ionization source in many of these galaxies is not nuclear, but circumnuclear or located in hotspots outside the nucleus. We detected important signatures predicted for a stellar population dominated by the TP-AGB, like CN 1.1 µm and CO 2.3 µm. In at least one galaxy (NGC 4102), the CN band at 1.4µm was detected for the first time. We also detect TiO and ZrO bands in the region 0.8-1µm that have never been reported before in extragalactic sources. The shape of the continuum emission is found to be strongly correlated to the presence/lack of emission lines. An observational template for the star-forming galaxies is derived to be used as a benchmark of stellar population(s) in starburst galaxies against which to compare NIR spectroscopy of different types of galaxies, especially those with active galactic nucleus activity and/or those at high redshift.