A Gemini/GMOS study of the physical conditions and kinematics of the blue compact dwarf galaxy Mrk 996

Aims. We present an integral field spectroscopic study with the Gemini Multi-Object Spectrograph (GMOS) of the unusual blue compact dwarf (BCD) galaxy Mrk 996. Methods. We show through velocity and dispersion maps, emission-line intensity and ratio maps, and by a new technique of electron density limit imaging that the ionization properties of different regions in Mrk 996 are correlated with their kinematic properties. Results. From the maps, we can spatially distinguish a very dense high-ionization zone with broad lines in the nuclear region, and a less dense low-ionization zone with narrow lines in the circumnuclear region. Four kinematically distinct systems of lines are identified in the integrated spectrum of Mrk 996, suggesting stellar wind outflows from a population of Wolf-Rayet (WR) stars in the nuclear region, superposed on an underlying rotation pattern. From the intensities of the blue and red bumps, we derive a population of ~473 late nitrogen (WNL) stars and ~98 early carbon (WCE) stars in the nucleus of Mrk 996, resulting in a high N(WR)/N(O+WR) of 0.19. We derive, for the outer narrow-line region, an oxygen abundance 12 + log (O/H) = 7.94 ± 0.30 (~0.2 Z⊙) by using the direct Te method derived from the detected narrow [O iii]λ4363 line. The nucleus of Mrk 996 is, however, nitrogen-enhanced by a factor of ~20, in agreement with previous CLOUDY modeling. This nitrogen enhancement is probably due to nitrogen-enriched WR ejecta, but also to enhanced nitrogen line emission in a high-density environment. Although we have made use here of two new methods – principal component analysis (PCA) tomography and a method for mapping low- and high-density clouds – to analyze our data, new methodology is needed to further exploit the wealth of information provided by integral field spectroscopy.