A VLT VIMOS integral field spectroscopic study of perturbed blue compact
  galaxies: UM 420 and UM 462
  We report on optical integral field spectroscopy of two unrelated blue
compact galaxies mapped with the 13 x 13 arcsec^2 VIMOS integral field unit at
a resolution of 0.33 x 0.33 arcsec^2. Continuum and background subtracted
emission line maps in the light of [O III] 5007, H-alpha, and [N II] 6584 are
presented. Both galaxies display signs of ongoing perturbation and/or
interaction. UM 420 is resolved for the first time to be a merging system
composed of two starbursting components with an 'arm-like' structure associated
with the largest component. UM 462 which is a disrupted system of irregular
morphology is resolved into at least four starbursting regions. Maps of the
H-alpha radial velocity and FWHM are discussed. No underlying broad line region
was detected from either galaxy as the emission lines are well-fitted with
single Gaussian profiles only. Electron temperatures and densities as well as
the abundances of helium, oxygen, nitrogen, and sulphur were computed from
spectra integrated over the whole galaxies and for each area of recent star
formation. Maps of the O/H ratio are presented: these galaxies show oxygen
abundances that are ~20 per cent solar. No evidence of substantial abundance
variations across the galaxies that would point to significant nitrogen or
oxygen self-enrichment is found (<0.2 dex limit). Contrary to previous
observations, this analysis does not support the classification of these BCGs
as Wolf-Rayet galaxies as the characteristic broad emission line features have
not been detected in our spectra. Baldwin-Phillips-Terlevich emission line
ratio diagrams which were constructed on a pixel by pixel basis indicate that
the optical spectra of these systems are predominantly excited by stellar
photoionization.
