Multi-Phase Dusty Gas in the Center of NGC 4278
  We present the Spitzer spectroscopic mapping observations toward the central
kpc region of the nearby elliptical LINER galaxy, NGC 4278, by using the
Infrared Spectrograph (IRS). These observations reveal rich mid-IR emission
features of extended ionized gas, warm molecular hydrogen and dust. Different
phases of gas and dust are closely related and belong to a same elongated
feature. We further study properties of multi-phase dusty gas to uncover the
underlying mechanism of ionization and excitation. The band ratio and intensity
of PAH features in the central region might reflect modified size distribution
resulted from selective destruction. H$_{2}$ S(0)-S(7) pure rotational lines of
molecular hydrogen show excessive intensity and moderately high excitation
temperature comparing with photon dissociation region (PDR). A strong and
extended [SiII] emission line is detected, which could be a sign of reduced
depletion of silicon in interstellar dust. We also discover an extended high
ionization region associated with enhanced H$_{2}$ S(1) emission. We conclude
that a shock-heating component is required to account for observed emission
characteristics, which could be triggered by cloud-cloud interactions during
accretion of cold gas from the large HI disk.
