Near-infrared spectroscopy of a large sample of low-metallicity blue compact dwarf galaxies

TL;DR

This study uses near-infrared spectroscopy to analyze low-metallicity blue compact dwarf galaxies, revealing insights into their ionized gas properties, star formation, and molecular hydrogen behavior, with implications for understanding their physical conditions.

Contribution

It provides new NIR spectroscopic data for low-metallicity BCDs, showing that their emission lines are consistent with photoionization models and that H2 is mainly excited by fluorescence.

Findings

NIR hydrogen lines indicate similar extinction in optical and NIR ranges.

H2 emission decreases with increasing ionization, suggesting destruction by UV radiation.

[FeII] lines align with photoionization models, not shocks.

Abstract

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.