Spitzer/IRS spectroscopy of Blue Compact Dwarf Galaxies

Blue Compact Dwarfs (BCD), is a subclass of dwarf galaxies which are dominated by a recent burst of star formation, responsible for their blue optical colors. Recently,mid-IR imaging and spestroscopy of a handful such systems revealed the presence of dusty embedded sources as well as ionic line emission in their spectra consistent with a hard radiation field from massive young stars. However, the shape and strength the Polycyclic Aromatic Hydrocarbons (PAH) bands, which is a nearly ubiquitous tracer of starformation, varied substantially.

Using the unprecedented sensitivity of the Infrared Spectrograph (IRS)* on the Spitzer Space Telescope we have observed, as part of the IRS GTO program, a sample of BCDs with metallicities ranging from 1/40th to nearly solar. Based on the 5-38micron IRS spectra, as well as the 16 and 22micron peak-up images we compare the spectral slope and mid-IR luminosity of our targets with known values of typical starburst galaxies. Measuring the PAH equivalent width (EW) at 6.2micron as well as various ionic lines, we explore a number of correlations between the mid-IR colors the PAH EW and the metallicity of the galaxies. We will present preliminary results of our findings.

*The IRS was a collaborative venture between Cornell University and Ball Aerospace Corporation funded by NASA through the Jet Propulsion Laboratory and the Ames Research Center.