Near-infrared spectroscopy of a large sample of low-metallicity blue
  compact dwarf galaxies
  We present near-infrared (NIR) spectroscopic observations in the wavelength
range 0.90-2.40mum of eighteen low-metallicity blue compact dwarf (BCD)
galaxies and six HII regions in spiral and interacting galaxies. Hydrogen and
helium emission lines are detected in all spectra, while H2 and iron emission
lines are detected in most spectra. The NIR data for all objects have been
supplemented by optical spectra. In all objects, except perhaps for the highest
metallicity ones, we find that the extinctions A(V) in the optical and NIR
ranges are similar, implying that the NIR hydrogen emission lines in
low-metallicity BCDs do not reveal more star formation than seen in the
optical. We conclude that emission-line spectra of low-metallicity BCDs in the
0.36-2.40mum wavelength range are emitted by a relatively transparent ionized
gas. The H2 emission line fluxes can be accounted for by fluorescence in most
of the observed galaxies. We find a decrease of the H2 2.122mum emission line
relative to the Brgamma line with increasing ionization parameter. This
indicates an efficient destruction of H2 by the stellar UV radiation. The
intensities of the [FeII] 1.257mum and 1.644mum emission lines in the spectra
of all galaxies, but one, are consistent with the predictions of Cloudy stellar
photoinization models. There is thus no need to invoke shock excitation for
these lines, and they are not necessarily shock indicators in low-metallicity
high-excitation BCDs. The intensity of the HeI 2.058mum emission line is lower
in high-excitation BCDs with lower neutral gas column densities and higher
turbulent motions.
