A VLT VIMOS study of the anomalous BCD Mrk 996: mapping the ionized gas kinematics and abundances

James, B. L.; Tsamis, Y. G.; Barlow, M. J.; Westmoquette, M. S.; Walsh, J. R.; Cuisinier, F. and Exter, K. M. (2009). A VLT VIMOS study of the anomalous BCD Mrk 996: mapping the ionized gas kinematics and abundances. Monthly Notices of the Royal Astronomical Society, 398(1) pp. 2–22.

DOI: https://doi.org/10.1111/j.1365-2966.2009.15172.x


A study of the blue compact dwarf (BCD) galaxy Mrk 996 based on high-resolution optical Very Large Telescope Visible Multi-Object Spectrograph integral field unit spectroscopy is presented. Mrk 996 displays multicomponent line emission, with most line profiles consisting of a narrow, central Gaussian [full width at half-maximum (FWHM) ∼ 110 km s-1] with an underlying broad component (FWHM ∼ 400 km s-1). The broad H I Balmer component splits into two separate broad components inside a 1.5-arcsec radius from the nucleus; these are attributed to a two-armed minispiral. This spiral-like nucleus rotates in the same sense as the extended narrow line ionized gas but is offset by ∼50 km s-1 from the systemic velocity of the galaxy. The rotation curve of Mrk 996 derived from the Hα narrow component yields a total mass of 5 × 108 M⊙ within a radius of 3 kpc. From the Hα luminosity we infer a global star formation rate of ∼2 M⊙ yr-1. The high excitation energy, high critical density [O III]λ4363 and [N II]λ5755 lines are only detected from the inner region and exist purely in broad component form, implying unusual excitation conditions. Surface brightness, radial velocity and FWHM maps for several emission components are presented. A separate physical analysis of the broad and narrow emission line regions is undertaken. We derive an upper limit of 10 000 K for the electron temperature of the narrow line gas, together with an electron density of 170 cm−3, typical of normal H II regions. For the broad line component, measured [O III] and [Fe III] diagnostic line ratios are consistent with a temperature of 11 000 K and an electron density of 107 cm-3. The broad line emission regions show N/H and N/O enrichment factors of ∼20 relative to the narrow line regions, but no He/H, O/H, S/H or Ar/H enrichment is inferred. Previous studies indicated that Mrk 996 showed anomalously high N/O ratios compared with BCDs of a similar metallicity. Our multicomponent analysis yields a revised metallicity of ≥0.5 Z⊙ (12 + log O/H = 8.37) for both the narrow and broad gas components, significantly higher than previous studies. As a result the narrow line region's N/O ratio is now typical for the galaxy's metallicity. The narrow line component's N/O ratio peaks outside the core region, spatially correlating with ∼3-Myr-old stellar populations. The dominant line excitation mechanism is photoionization by the ∼3000 Wolf–Rayet stars and ∼150 000 O-type stars estimated to be present in the core. This is indeed a peculiar BCD, with extremely dense zones of gas in the core, through which stellar outflows and possible shock fronts permeate contributing to the excitation of the broad line emission.

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