Near-infrared spectroscopy of five Blue Compact Dwarf galaxies: II Zw 40, Mrk 71, Mrk 930, Mrk 996 and SBS 0335-052E

TL;DR

This study presents near-infrared spectroscopic observations of five blue compact dwarf galaxies, revealing emission lines and ionization conditions, and compares optical, NIR, and MIR data to understand star formation and gas transparency.

Contribution

It provides new NIR spectral data for five BCDs and analyzes their emission lines, extinction, and ionization sources, integrating optical and MIR observations for comprehensive insights.

Findings

NIR hydrogen lines do not indicate more star formation than optical.

Emission-line spectra are from relatively transparent ionized gas.

Shock ionization is needed for some high-ionization lines in Mrk 930.

Abstract

We present near-infrared (NIR) spectroscopic observations of five blue compact dwarf (BCD) galaxies, II Zw 40, Mrk 71, Mrk 930, Mrk 996 and SBS 0335-052E. The NIR spectra which cover the 0.90 micron - 2.40 micron wavelength range, show hydrogen, molecular hydrogen, helium, sulfur and iron emission lines. The NIR data for all BCDs have been supplemented by optical spectra. We found the extinction coefficient in all BCDs to be very similar in both the optical and NIR ranges. The NIR hydrogen emission lines do not reveal more star formation than seen in the optical. The same conclusion is reached from Spitzer data concerning the MIR emission lines. This implies that emission-line spectra of low-metallicity BCDs in the ~ 0.36 - 25 micron wavelength range are emitted by relatively transparent ionized gas. The large extinction derived from the MIR continuum emission in some BCDs implies that the latter arises not from the visible H II regions themselves, but from locations outside these H II regions. The H2 emission line fluxes can be accounted for by fluorescence. CLOUDY stellar photoinization models of all BCDs reproduce well the fluxes of most of the observed optical and NIR emission lines, except in Mrk 930 where shock ionization is needed to account for the [Fe II] emission lines. However, some contribution of shock ionization at the level of < 10% that of stellar ionization is required to reproduce the observed fluxes of high ionization species, such as He II 0.469 micron in the optical range and [O IV] 25.89 micron in the MIR range.