Abstract
(abridged) We present deep archival VLT/FORS1+UVES spectroscopic observations of the system of two blue compact dwarf (BCD) galaxies SBS 0335-052E and SBS 0335-052W. Our aim is to derive element abundances in different HII regions of this unique system of galaxies and to study spatial abundance variations. We determine abundances of helium, nitrogen, oxygen, neon, sulfur, chlorine, argon and iron. The oxygen abundance in the brighter eastern galaxy varies in the range 7.11 to 7.32 in different HII regions supporting previous findings and suggesting the presence of oxygen abundance variations on spatial scales of ~1-2 kpc. The oxygen abundance in the brightest region No.1 of SBS 0335-052W is 7.22+/-0.07, consistent with previous determinations.Three other HII regions are much more metal-poor with an unprecedently low oxygen abundance of 12+logO/H=7.01+/-0.07 (region No.2), 6.98+/-0.06 (region No.3), and 6.86+/-0.14 (region No.4). These are the lowest oxygen abundances ever derived in emission-line galaxies. Helium abundances derived for the brightest HII regions of both galaxies are mutually consistent. We derive weighted mean He mass fractions of 0.2485+/-0.0012 and 0.2514+/-0.0012 for two different sets of HeI emissivities. The N/O abundance ratio in both galaxies is slightly higher than that derived for other BCDs with 12+logO/H<7.6. This implies that the N/O in extremely metal-deficient galaxies could increase with decreasing metallicity.
Highlights
The pair of extremely metal-deficient blue compact dwarf (BCD) galaxies SBS 0335–052E and SBS 0335–052W plays a key role in understanding physical conditions in the interstellar medium, star formation, and stellar evolution at very low metallicity
The electron temperatures Te(O iii), Te(O ii), and Te(S iii), electron number density Ne(S ii), the ionization correction factors (ICFs), and the ionic and total N, O, Ne, S, Cl, Ar, and Fe abundances derived from the forbidden emission lines are given in Tables 5 and 6, respectively and in Table 7 (UVES observations)
We have presented our analysis of archival Very Large Telescope (VLT)/FORS and VLT/UVES spectroscopic observations of a system of two extremely low-metallicity blue compact dwarf (BCD) galaxies, SBS 0335–052E and SBS 0335–052W
Summary
The electron temperature Te (O iii) in all H ii regions, except for one, is derived from the [O iii] λ4363/(λ4959+λ5007). We determine ionic abundances of helium, nitrogen, oxygen, neon, sulfur, chlorine, argon and iron. The empirical relations for ionization correction factors are used to derive total abundances of these elements
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