The Hamburg/SAO Survey for low metallicity blue compact/H II galaxies (HSS-LM). I. The first list of 46 strong-lined galaxies

We present the description and the first results og a new project devoted to the search for extremely metal-deficient blue compact/H Ii-galaxies (BCGs) and to the creation of a well selected large BCG sample with strong emission lines. Such galaxies should be suitable for reliable determination of their oxygen abundance through the measurement of the faint [O Iii] lambda 4363 Å line. The goals of the project are two-fold: a) to discover a significant number of new extremely metal-poor galaxies (Z <~ 1/20 Z_sun), and b) to study the metallicity distribution of local BCGs. Selection of candidates for follow-up slit spectroscopy is performed on the database of objective prism spectra of the Hamburg Quasar Survey. The sky region is limited by delta >= 0° and b^II <= -30°. In this paper we present the results of the follow-up spectroscopy conducted with the Russian 6 m telescope. The list of observed candidates contained 52 objects, of which 46 were confirmed as strong-lined BCGs (EW([O Iii] lambda 5007) ge 100 Å). The remaining five lower excitation ELGs include three BCGs, and two galaxies classified as SBN (Starburst Nucleus) and DANS (Dwarf Amorphous Nucleus Starburst). One object is identified as a quasar with a strong Lyalpha emission line near lambda 5000 Å (z ~ 3). We provide a list with coordinates, measured radial velocities, B-magnitudes, equivalent widths EW([O Iii] lambda 5007) and EW(Hβ ) and for the 46 strong-lined BCGs the derived oxygen abundances 12+log (O/H). The abundances range between 7.42 and 8.4 (corresponding to metallicities between 1/30 and 1/3 Z_sun). The sample contains four galaxies with Z <~ 1/20 Z_sun, of which three are new discoveries. This demonstrates the high efficiency of the new project to find extremely metal-deficient galaxies. The radial velocities of the strong-lined ELGs range between 500 and 19 000 km s^-1 with a median value of ~ 6400 km s^-1. The typical B-magnitudes of the galaxies presented are 17fm0 -18fm0 . Tables 2 and 3 are only available in electronic form at CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/397/463. Figures A1-A6 are only available in electronic form at http://www.edpsciences.org