Investigating the Chemical Homogeneity of Low-Metallicity Blue Compact Dwarf Galaxies Using Integral Field Spectroscopy

¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
² Space Telescope European Co-ordinating Facility, ESO, Karl-Schwarzschild Strasse 2, 85748 Garching, Germany
³ Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UK

Abstract

It has been claimed in the past that in low-metallicity Blue Compact Dwarf (BCDs) galaxies, the N/O value is independent of metallicity (O/H ratio), implying the need to invoke a primary production of nitrogen in intermediate-mass stars, in addition to the secondary nitrogen produced from the CNO cycle in high-mass stars. In order to better understand this controversial issue we undertook an integral field spectroscopic study of the nebular gas within a sample of BCDs previously thought to have anomalously high N/O values. Here we present the results of this study for 3 BCDs: two with anomalous N/O values (Mrk 996 and UM 420) and one with more normal N/O values (UM 462). We describe in detail how we derived the physical conditions (Te, Ne) as a function of position within the galaxy, and as a consequence, how this revealed both revised metallicities and normal N/O ratios and uncovered one of the first clear evidences of nitrogen self-enrichment of an H ii  region from WR stars.