2018ApJ...866..139K

We present a long-exposure (∼10 hr), narrowband image of the supernova (SN) remnant Cassiopeia A (Cas A) centered at 1.644 µm emission. The passband contains [Fe II] 1.644 µm and [Si I] 1.645 µm lines, and our "deep [Fe II]+[Si I] image" provides an unprecedented panoramic view of Cas A, showing both shocked and unshocked SN ejecta, together with shocked circumstellar medium at subarcsecond (∼0.''7 or 0.012 pc) resolution. The diffuse emission from the unshocked SN ejecta has a form of clumps, filaments, and arcs, and their spatial distribution correlates well with that of the Spitzer [Si II] infrared emission, suggesting that the emission is likely due to [Si I] not [Fe II] as in shocked material. The structure of the optically invisible western area of Cas A is clearly seen for the first time. The area is filled with many quasi-stationary flocculi (QSFs) and fragments of the disrupted ejecta shell. We identified 309 knots in the deep [Fe II]+[Si I] image and classified them into QSFs and fast-moving knots (FMKs). The comparison with previous optical plates indicates that the lifetime of most QSFs is >=60 yr. The total H+He mass of QSFs is ≃0.23 M_☉, implying that the mass fraction of dense clumps in the progenitor's mass ejection immediately prior to the SN explosion is about 4%-6%. FMKs in the deep [Fe II]+[Si I] image mostly correspond to S-rich ejecta knots in optical studies, while those outside the southeastern disrupted ejecta shell appear Fe-rich. The mass of the [Fe II] line emitting, shocked dense Fe ejecta is ∼3 x 10^–5 M_☉.