Near-infrared spectroscopy of starburst galaxies

We present new K-band spectroscopy for a sample of 48 starburst galaxies, obtained using UKIRT in Hawaii. This constitutes a fair sample of the most common types of starburst galaxies found in the nearby Universe, containing galaxies with different morphologies, masses and metallicities, with far-infrared luminosity LIR<1010 L⊙. The variety of near-infrared spectral features shown by these galaxies implies different bursts characteristics, which suggests that we survey galaxies with different star formation histories or at different stages of their burst evolution. Using synthetic starburst models, we conclude that the ensemble of parameters that best describes starburst galaxies in the nearby Universe is a constant rate of star formation, a Salpeter initial mass function (IMF) with an upper mass cut-off of Mup=30 M⊙ and bursts ages between 10 Myr and 1 Gyr. The model is fully consistent with the differences observed in the optical and far-infrared (FIR) between the different types of starbursts. It suggests that H ii galaxies have younger bursts and lower metallicities than starburst nucleus galaxies (SBNGs), while luminous infrared galaxies (LIRGs) have younger bursts but higher metallicities. Although the above solution from the synthetic starburst model is fully consistent with our data, it may not constitute a strong constraint on the duration of the bursts and the IMF. A possible alternative may be a sequence of short bursts (which may follow an universal IMF) over a relatively long period of time. In favour of the multiple-burst hypothesis, we distinguish in our spectra some variations of near-infrared (NIR) features with the aperture that can be interpreted as evidence that the burst regions are not homogeneous in space and time. We also found that the burst stellar populations are dominated by early-type B stars, a characteristic which seems difficult to explain with only one evolved burst. Our observations suggest that the starburst phenomenon must be a sustained or self-sustained phenomenon: either star formation is continuous in time, or multiple bursts happen in sequence over a relatively long period of time. The generality of our observations implies that this is a characteristic of starburst galaxies in the nearby Universe.