Multi-Phase Dusty Gas in the Center of NGC 4278

Abstract: 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.