  We present a long-exposure (~10 hr) image of the supernova (SN) remnant
Cassiopeia A (Cas A) obtained with the UKIRT 3.8-m telescope using a narrow
band filter centered at 1.644 um emission. The passband contains [Fe II] 1.644
um and [Si I] 1.645 um 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 subarcsec (~0.7 arcsec 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] line not [Fe II] line 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 suggest that the
anomalous radio properties in this area could be due to the increased number of
such dense clumps. We identified 309 knots in the deep [Fe II]+[Si I] image and
classified them into QSFs and fast-moving knots (FMKs). The total H+He mass of
QSFs is ~0.23 Msun, implying that the mass fraction of dense clumps in the
progenitor's red-supergiant wind is 4--13%. The spatial distribution of QSFs
suggests that there had been a highly asymmetric mass loss $10^4$--$10^5$ yr
before the SN explosion. The mass of the [Fe II] line-emitting, shocked dense
Fe ejecta is ~3x$10^{-5}$ Msun. The comparison with the ionic S-line dominated
Hubble Space Telescope WFC3/IR image suggests that the outermost FMKs in the
southeastern area are Fe-rich.
