Near-Infrared Knots and Dense Fe Ejecta in the Cassiopeia A Supernova Remnant

Abstract: We report the results of broadband (0.95--2.46 $\mu$m) near-infrared spectroscopic observations of the Cassiopeia A supernova remnant. Using a clump-finding algorithm in two-dimensional dispersed images, we identify 63 "knots" from eight slit positions and derive their spectroscopic properties. All of the knots emit [Fe II] lines together with other ionic forbidden lines of heavy elements, and some of them also emit H and He lines. We identify 46 emission line features in total from the 63 knots and measure their fluxes and radial velocities. The results of our analyses of the emission line features based on principal component analysis show that the knots can be classified into three groups: (1) He-rich, (2) S-rich, and (3) Fe-rich knots. The He-rich knots have relatively small, $\lesssim 200~{\rm km~s}^{-1}$, line-of-sight speeds and radiate strong He I and [Fe II] lines resembling closely optical quasi-stationary flocculi of circumstellar medium, while the S-rich knots show strong lines from O-burning material with large radial velocities up to $\sim 2000~{\rm km~s}^{-1}$ indicating that they are supernova ejecta material known as fast-moving knots. The Fe-rich knots also have large radial velocities but show no lines from O-burning material. We discuss the origin of the Fe-rich knots and conclude that they are most likely "pure" Fe ejecta synthesized in the innermost region during the supernova explosion. The comparison of [Fe II] images with other waveband images shows that these dense Fe ejecta are mainly distributed along the southwestern shell just outside the unshocked $^{44}$Ti in the interior, supporting the presence of unshocked Fe associated with $^{44}$Ti.