Spectroscopic and photometric studies of low-metallicity star-forming dwarf galaxies. II. HS 1442+4250

Abstract

We present broad-band V and I imaging and long-slit spectroscopy in the optical range λλ3600-7500 A°of the dwarf irregular galaxy HS 1442+4250. The oxygen abundance 12 + log(O/H) = 7.63 ± 0.02 (Z = Z⊙/19) in the brightest H II region of HS 1442+4250 places the galaxy among the most metal-deficient emission-line galaxies. The low metallicity and blue colour (V - I) ∼ 0.4 mag of the low-surface-brightness (LSB) component make HS 1442+4250 a likely rare young dwarf galaxy candidate. We use four methods to estimate the stellar population age in the LSB component of HS 1442+4250. Different star formation histories are considered. The equivalent widths of hydrogen Hα and Hβ emission lines, and of hydrogen Hγ and Hδ absorption lines, the spectral energy distribution and the observed (V-I) colours of the LSB regions are reproduced quite well by models with only young and intermediate-age stellar populations. By contrast, the observational data cannot be reproduced by a stellar population formed continuously with a constant star formation rate in the age range from 0 to ≳2 Gyr. While a faint old stellar population in HS 1442+4250 with an age ≳2 Gyr is not excluded, we find no evidence for such a population from the present data.