With the aim of investigating galaxies with two strong simultaneous
starbursts, we have extracted a sample of galaxies with double-peaked emission
lines in their global spectra from the SDSS spectral database. We then fitted
the emission lines Halpha, Hbeta, [OIII]5007, [NII]6584, [SII]6717 and
[SII]6731 of 129 spectra by two Gaussians to separate the radiation of the two
(blue and red) components. A more or less reliable decomposition of the all
those emission lines have been found for 55 spectra. Using a standard BPT
classification diagram, we have been able to divide the galaxies from our
sample into two subsamples: Sample A consisting of 18 galaxies where both
components belong to the photoionised class of objects, and Sample B containing
37 galaxies which show non-thermal ionisation (AGNs). We have examined the
properties of the blue and red components, and found that the differences
between radial velocities of components lie within 200 - 400 km/s for galaxies
of both subsamples. The equivalent number of ionising stars is in the range
10^4 - 10^5 O7V stars for each component in the galaxies of Sample A. We have
estimated the oxygen and nitrogen abundances as well as the electron
temperatures for each component using the recent NS-calibration and from global
spectra for galaxies from Sample A using both the NS and ON-calibration. We
have found that the global oxygen abundance is typically in between the
measured abundances of individual components for our sample of galaxies, and
that both calibrations provide consistent global abundances. Finally, we
suggest the classical O/H -- N/O diagram is used to test the reliability of the
dividing lines between starburst-like objects and AGNs in the so-called BPT
diagram.Comment: 13 pages, 13 figures, 1 table, accepted for publication in MNRA
