Spitzer Characterization of Dust in the Ionized Medium of the Large
  Magellanic Cloud
  A systematic investigation of dust emission associated with the ionized gas
has so far been performed only in our Galaxy and for wavelengths longer than 60
{\mu}m. Newly available Spitzer data now offer the opportunity to carry out a
similar analysis in the Large Magellanic Cloud (LMC). By cross-correlating
Spitzer SAGE (Surveying the Agents of a Galaxy's Evolution) data with the
ATCA/Parkes HI 21-cm data, the NANTEN 12CO (J=1-0) data, and both the SHASSA
H{\alpha} and the Parkes 6-cm data, we investigate the physical properties of
dust associated with the different phases of the gas (atomic, molecular and
ionized). In particular, we study the presence and nature of dust from 3.6 to
160 {\mu}m and for various regimes of the ionized gas, spanning emission
measures (EM) from \sim 1 pc cm-6 (diffuse component) to \sim 10^3 pc cm-6 (HII
regions). Using a dust emission model, and testing our results with several
radiation field spectra, we show that dust in the ionized gas is warmer than
dust associated with other phases (atomic and molecular). We also find a
decrease of the polycyclic aromatic hydrocarbons (PAH) relative abundance with
respect to big grains (BGs), as well as an increase of the near infrared (NIR)
continuum. These three results (e.g. warmer temperature, decrease of PAH
abundance and increase of the NIR continuum) are found consistently for all
regimes of the ionized gas. On the contrary, the molecular phase appears to
provide favorable conditions for the survival of PAHs. Furthermore, the very
small grain (VSG) relative abundance tends to increase in the ionized phase,
especially in bright HII regions. Last but not least, our analysis shows that
the emissivity of dust associated with the ionized gas is lower in the LMC than
in our Galaxy, and that this difference is not accounted for by the lower
metallicity of the LMC.
