Mid Infrared Properties of Low Metallicity Blue Compact Dwarf Galaxies From Spitzer

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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
• August16th, 2005

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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 (MBgt-18), small
  size and low metallicity
• Dominated by one or more recent bursts of star
  formation
• Laboratory to explore galaxies in the early
  universe

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Observation

• 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
• The remaining were observed using the peak-up
  camera at 16?m and 22?m for broadband imaging.

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SBS0335-052 and NGC7714
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MIR spectra from Spitzer IRS
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MIR slope vs metallicity
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Hardness of the radiation field measurement
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PAH EW vs NeIII/NeII
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PAH EW vs NeIII/NeIIL22?m/V

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PAH EW vs metallicity
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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/NeIIL22?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.

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Thank You