Multiphase dusty gas in the centre of NGC4278

Abstract

We present Spitzer spectroscopic mapping observations toward the central kpc region of the nearby elliptical low-ionization nuclear emission-line region (LINER) galaxy NGC4278, using the Infrared Spectrograph (IRS). These observations reveal rich mid-infrared emission features of extended ionized gas, warm molecular hydrogen and dust. Different phases of gas and dust are closely related and belong to the same elongated feature. We further study the properties of multiphase 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 the modified size distribution resulting from selective destruction. H2 S(0)-S(7) pure rotational lines of molecular hydrogen show excessive intensity and moderately high excitation temperature compared with the photon dissociation region (PDR). A strong and extended [Siii] emission line is detected, which could be a sign of the reduced depletion of silicon in interstellar dust. We also discover an extended high-ionization region associated with enhanced H2 S(1) emission. We conclude that a shock-heating component is required to account for the observed emission characteristics, which could be triggered by cloud-cloud interactions during the accretion of cold gas from the large Hi disc. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.