1. Mid Infrared Properties of Low Metallicity Blue Compact Dwarf Galaxies From Spitzer Yanling Wu, Vassilis Charmandaris, Lei Hao, Bernhard Brandl, Jeronimo Bernard-salas, Henrik Spoon and Jim Houck Cornell University Extreme Starburst Workshop, Lijiang, China August16 th, 2005

2. Introduction Blue Compact Dwarf galaxies are the class of objects where most galaxies with low metallicity abundances as dervied from HII regions have been found: faint, blue optical color (M B >-18), small size and low metallicity Blue Compact Dwarf galaxies are the class of objects where most galaxies with low metallicity abundances as dervied from HII regions have been found: faint, blue optical color (M B >-18), small size and low metallicity Dominated by one or more recent bursts of star formation Dominated by one or more recent bursts of star formation Laboratory to explore galaxies in the early universe Laboratory to explore galaxies in the early universe

3. Observation A total of 62 BCDs have been observed A total of 62 BCDs have been observed For 25 of them, we used all four Infrared Spectrograph (IRS, 5-38  m) modules to obtain spectra For 25 of them, we used all four Infrared Spectrograph (IRS, 5-38  m) modules to obtain spectra The remaining were observed using the peak- up camera at 16  m and 22  m for broadband imaging. The remaining were observed using the peak- up camera at 16  m and 22  m for broadband imaging.

4. SBS0335-052 and NGC7714

5. MIR spectra from Spitzer IRS

6. MIR slope vs metallicity

7. Hardness of the radiation field measurement

8. PAH EW vs [NeIII]/[NeII]

9. PAH EW vs [NeIII]/[NeII]*L 22  m /V

10. PAH EW vs metallicity

11. Conclusions  We detected PAH emission, at 6.2, 7.7, 8.6, 11.2 and 12.8  m, though their strength varies substantially in our sample.  The line ratios, [NeIII]/[NeII], as well as [SIV]/[SIII] indicate the hardness of the radiation field and are correlated with each other.  The PAH EW, both at 6.2 and 11.2  m, decreases with the increase of [NeIII]/[NeII] ratio, that is, as the ionization field becomes harder.  The strongest correlation we have found is the anti-correlation between the PAH EW and [NeIII]/[NeII]*L 22  m /V.  The trend that more metal poor BCDs have less PAH emission is weak.  The ratio of the 22  m/16  m flux density is ~2.5 for nearly all BCDs and shows no metallicity dependence. The 22  m/8  m (or 24  m/8  m ) ratio also shows considerable scatter, thus making prediction on the presence of PAHs based on mid-IR broad band colors challenging.

12. Thank You