2014A&A...561A..33I

We studied a large sample of ∼14000 dwarf star-forming galaxies with strong emission lines. These low-metallicity galaxies with oxygen abundances of 12+logO/H ∼7.4-8.5 are selected from the Sloan Digital Sky Survey (SDSS) and distributed in the redshift range of z ∼0-0.6. We modelled spectral energy distributions (SED) of all galaxies, which were based on the SDSS spectra in the visible range of 0.38 µm-0.92 µm and included both the stellar and ionised gas emission. These SEDs were extrapolated to the UV and mid-infrared ranges to cover the wavelength range of 0.1 µm-22 µm. The SDSS spectroscopic data were supplemented by photometric data from the GALEX, SDSS, 2MASS, WISE, IRAS, and NVSS all-sky surveys. Using these data, we derived global characteristics of the galaxies, such as their element abundances, luminosities, and stellar masses. The luminosities and stellar masses range within the sample over ∼5 orders of magnitude, thereby linking low-mass and low-luminosity blue compact dwarf galaxies to luminous galaxies, which are similar to high-redshift Lyman-break galaxies. It was found that the luminosity L(Hβ) of the Hβ emission line, a characteristic of the youngest stellar population with an age of a few Myr, is correlated with luminosities in other wavelength ranges. This implies that the most recent burst of star formation makes a significant contribution to the emission in the visible range and dominates in other wavelength ranges. It was also found that the contribution of the young population to the galaxy luminosity is higher for galaxies with higher L(Hβ) and higher equivalent widths EW(Hβ). We found 20 galaxies with very red WISE mid-infrared m(3.4 µm)-m(4.6 µm) colour (≥2 mag), which suggests the important contribution of the hot (with a temperature of several hundred degree) dust emission in these galaxies. Our analysis of the balance between the luminosity in the WISE bands that covered a wavelength range of 3.4 µm-22 µm and the luminosity of the emission absorbed at shorter wavelengths showed that the luminosity of the hot dust emission is increased with increasing L(Hβ) and EW(Hβ). We demonstrated that the emission emerging from young star-forming regions is the dominant dust-heating source for temperatures to several hundred degrees in the sample star-forming galaxies.