2022ApJ...935..155S

Recent developments in (sub)millimeter facilities have drastically changed the amount of information obtained from extragalactic spectral scans. In this paper, we present a feature extraction technique using principal component analysis (PCA) applied to arcsecond-resolution (1.″0-2.″0 = 72-144 pc) spectral scan data sets for the nearby type-2 Seyfert galaxy NGC 1068, using Band 3 of the Atacama Large Millimeter/submillimeter Array. We apply PCA to 16 well-detected molecular line intensity maps convolved to a common 150 pc resolution. In addition, we include the [S III]/[S II] line ratio and [C I] ³P₁-³P₀ maps in the literature, both of whose distributions show a remarkable resemblance to that of a kiloparsec-scale biconical outflow from the central active galactic nucleus. We identify two prominent features: (1) central concentration at the circumnuclear disk (CND) and (2) two peaks across the center that coincide with the biconical outflow peaks. The concentrated molecular lines in the CND are mostly high-dipole molecules (e.g., H¹³CN, HC₃N, and HCN). Line emissions from molecules known to be enhanced in an irradiated interstellar medium, CN, C₂H, and HNC, show similar concentrations and extended components along the bicone, suggesting that molecule dissociation is a dominant chemical effect of the cold molecular outflow of this galaxy. Although further investigation should be made, this scenario is consistent with the faintness or absence of emission lines from CO isotopologues, CH₃OH, and N₂H⁺ in the outflow, which are easily destroyed by dissociating photons and electrons.