Near-infrared spectra ( 0.95 – 2.4 µm ) of the Cassiopeia A and Kepler supernova remnants ( SNRs ) are presented .
Low-dispersion ( R \approx 700 ) spectra were obtained for five bright fast-moving ejecta knots ( FMKs ) at two locations on the main shell and for three bright circumstellar knots ( QSFs ) near the southwest rim of Cas A .
The main shell FMKs in Cas A exhibit a sparse near-infrared spectrum dominated by [ S ii ] 1.03 µm emission with a handful of other , fainter emission lines .
Among these are two high-ionization silicon lines , [ Si vi ] 1.96 µm and [ Si x ] 1.43 µm , which have been detected in AGNs and novae but never before in a supernova remnant .
The near-infrared spectra of circumstellar QSFs in Cas A show a much richer spectrum , with strong He i 1.083 µm emission and over a dozen bright [ Fe ii ] lines .
Observed [ Fe ii ] line ratios indicate electron densities of 5 – 9 \times 10 ^ { 4 } cm ^ { -3 } in the QSFs .
The Cas A QSF data are quite similar to the observed spectrum of a bright circumstellar knot along the northwest rim of the Kepler SNR , which also shows strong He i and [ Fe ii ] emission with a measured electron density of 2.5 – 3 \times 10 ^ { 4 } cm ^ { -3 } .
Finally , we present J - and K -band images of Cas A .
The K -band image shows faint diffuse emission which has no optical or mid-infrared counterpart but is morphologically similar to radio continuum maps and may be infrared synchrotron radiation .