Extremely metal-poor star-forming galaxies New detections and general morphological and photometric properties

Context. Extremely metal-deficient [12 + log(O/H) less than or similar to 7.6] emission-line galaxies in the nearby universe are invaluable laboratories of extragalactic astronomy and observational cosmology since they allow us to study collective star formation and the evolution of galaxies under chemical conditions approaching those in distant protogalactic systems. However, despite intensive searches over the last three decades, nearby star-forming (SF) galaxies with strongly subsolar metallicity remain extremely scarce. Aims. We searched the Sloan Digital Sky Survey (SDSS) and the Six-Degree Field Galaxy Redshift Survey (6dFGRS) for promising low-metallicity candidates using a variety of spectroscopic criteria. Methods. We present long-slit spectroscopy with the 3.6 m ESO telescope of eight HII regions in seven emission-line dwarf galaxies, selected from the Data Release 4 of SDSS ( six galaxies) and from 6dFGRS ( one galaxy). In addition, we use SDSS imaging data to investigate the photometric structure of the sample galaxies. Results. From the 3.6 m telescope spectra, we determine the oxygen abundance of these systems to be 7.3 less than or similar to 12 + log(O/H) less than or similar to 7.6, placing them among the most metal-poor star-forming galaxies ever discovered. Our photometric analysis reveals a moderately blue, stellar host galaxy in all sample galaxies. Conclusions. The detection of a stellar host in all galaxies studied here and all previously studied extremely metal-deficient SF galaxies implies that they are unlikely to be forming their first generation of stars. With regard to the structural properties of their host galaxy, we demonstrate that these systems are indistinguishable from blue compact dwarf (BCD) galaxies. However, in contrast to the majority (> 90%) of BCDs that are characterised by red elliptical host galaxies, extremely metal-poor SF dwarfs (hereafter XBCDs) reveal moderately blue and irregular hosts. This is consistent with a young evolutionary status and in the framework of standard star formation histories implies that several XBCDs formed most of their stellar mass in the past similar to 2 Gyr. A large fraction of XBCDs reveal a cometary morphology due to the presence of intense SF activity at one edge of an elongated host galaxy with a gradually decreasing surface brightness towards its antipodal end.