Freshly formed dust in the cassiopeia a supernova remnant as revealed by the Spitzer space telescope

We performed Spitzer Infrared Spectrograph mapping observations covering nearly the entire extent of the Cassiopeia A supernova remnant (SNR), producing mid-infrared (5.5-35 mu m) spectra every 5 ''-10 ''. Gas lines of Ar, Ne, O, Si, S, and Fe, and dust continua were strong for most positions. We identify three distinct ejecta dust populations based on their continuum shapes. The dominant dust continuum shape exhibits a strong peak at 21 mu m. A line-free map of 21 mu m peak dust made from the 19-23 mu m range closely resembles the [Ar II], [O IV], and [Ne II] ejecta-line maps, implying that dust is freshly formed in the ejecta. Spectral fitting implies the presence of SiO2, Mg protosilicates, and FeO grains in these regions. The second dust type exhibits a rising continuum up to 21 mu m and then flattens thereafter. This weak 21 mu m'' dust is likely composed of Al2O3 and C grains. The third dust continuum shape is featureless with a gently rising spectrum and is likely composed of MgSiO3 and either Al2O3 or Fe grains. Using the least massive composition for each of the three dust classes yields a total mass of 0.020 M-circle dot. Using the most massive composition yields a total mass of 0.054 M-circle dot. The primary uncertainty in the total dust mass stems from the selection of the dust composition necessary for fitting the featureless dust as well as 70 mu m flux. The freshly formed dust mass derived from Cas A is sufficient from SNe to explain the lower limit on the dust masses in high-redshift galaxies.