Probing the Excitation of Extreme Starbursts: High Resolution Mid-ir Spectroscopy of Blue Compact Dwarfs

We present an analysis of the mid-infrared emission lines for a sample of 12 low metallicity Blue Compact Dwarf (BCD) galaxies based on high resolution observations obtained with Infrared Spectrograph on board the Spitzer Space Telescope. We compare our sample with a local sample of typical starburst galaxies and active galactic nuclei (AGNs), to study the ionization field of starbursts over a broad range of physical parameters and examine its difference from the one produced by AGN. The high-ionization line [OIV]25.89μm is detected in most of the BCDs, starbursts, and AGNs in our sample. We propose a diagnostic diagram of the line ratios [OIV]25.89μm/[SIII]33.48μm as a function of [NeIII]15.56μm/[NeII]12.81μm which can be useful in identifying the principal excitation mechanism in a galaxy. Galaxies in this diagram split naturally into two branches. Classic AGNs as well as starburst galaxies with an AGN component populate the upper branch, with stronger AGNs displaying higher [NeIII]/[NeII] ratios. BCDs and pure starbursts are located in the lower branch. We find that overall the placement of galaxies on this diagram correlates well with their corresponding locations in the log([NII]/Hα) vs. log([OIII]/Hβ) diagnostic diagram, which has been widely used in the optical. The two diagrams provide consistent classifications of the excitation mechanism in a galaxy. On the other hand, the diagram of [NeIII]15.56μm/[NeII]12.81μm vs. [SIV]10.51μm/[SIII]18.71μm is not as efficient in separating AGNs from BCDs and pure starbursts. Our analysis demonstrates that BCDs in general do display higher [NeIII]/[NeII] and [SIV]/[SIII] line ratios than starbursts, with some reaching values even higher than those found at the centers of AGNs. Despite their hard radiation field though, no [NeV]14.32μm emission has been detected in the BCDs of our sample. Subject headings: dust, extinction – infrared: galaxies – galaxies: dwarf – galaxies: starburst – galaxies: active – galaxies: abundances