"The Origin and The Evolution of Dust in the Hot Interstellar gas of Elliptical Galaxies"
Detection of elliptical galaxies at 60 and 100 microns with the IRAS satellite showed conclusively that these galaxies contain truly interstellar dust, which is mixed into hot interstellar gas at the virial temperature, typically 10e7 K. Dust grains in this harsh environment are progressively destroyed by collisions with thermal protons that dislodge atomic fragments from the grain surface (sputtering). However, as the dust grains are slowly reduced in size, they are heated both by UV stellar radiation from post-AGB stars and by inelastic collisions with thermal electrons and this energy is radiated in the far-IR. New dust grains are continuously injected into the interstellar gas by dusty stellar winds from red giant stars. Dusty stellar winds are verified by mid-IR observations of ellipticals where the emission follows a de Vaucouleurs stellar profile. The total far-IR emission from internally produced grains during their sputtering lifetime is roughly consistent with the far-IR luminosities observed so far, but large discrepancies between expectation and observation have recently become evident. For example, in one giant Virgo elliptical the far-IR emission observed with ISO is about 40 times larger than expected, while another similar Virgo elliptical was undetected. The detected galaxy may have recently received a contribution of new dust from a merger with a gas-rich galaxy, but the merger must have been very recent indeed since the dust destruction time is only about 10e8 years. We propose to observe a small number of nearby spatially-resolved and optically luminous elliptical galaxies with SPITZER in order to calibrate our theoretical models of dust creation, destruction and radiation. In addition we wish to evaluate the alternative hypothesis that mergers are an important source of interstellar dust. Almost nothing is currently known about the radial distribution of far-IR emission from ellipticals, but we expect SPITZER to change this soon.