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 {\rm 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]25.89$\mu$m is
detected in most of the BCDs, starbursts, and AGNs in our sample. We propose a
diagnostic diagram of the line ratios [OIV]25.89$\mu$m/[SIII]33.48$\mu$m as a
function of [NeIII]15.56$\mu$m/[NeII]12.81$\mu$m 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]/H$\alpha$) vs. log([OIII]/H$\beta$) 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]15.56$\mu$m/[NeII]12.81$\mu$m vs.
[SIV]10.51$\mu$m/[SIII]18.71$\mu$m is not as efficient in separating AGNs from
BCDs and pure starbursts. (abridged)
