Research on Physical Properties in NGC 1068 Nuclear Region Based on ALMA High-resolution Multi-spectral Lines

Abstract

Using the multi-spectral lines from ALMA (Atacama Large Millimeter/submillimeter Array) with high-resolution ($\sim$ 0.2${}^{″}$–0.7${}^{″}$) and the continuum data, this work studies the physical properties of the nuclear region of nearby galaxy NGC 1068. The spectral lines include CO (1-0), CO (2-1), CO (3-2), HCN (1-0), HCO${}^{+}$ (1-0), HCN (3-2), HCO${}^{+}$ (3-2), HCN (4-3) and HCO${}^{+}$ (4-3). The CND (CircumNuclear Disk) shows an asymmetric ring structure with a size of $\sim$ 300 pc in the velocity-integrated intensity images. All the molecular lines of the CND show stronger emission at the eastern knot (E-knot) than the western knot (W-knot) of the CND. Furthermore, the E-knot shows larger velocities than the W-knot, which indicates that there is significant rotational pattern in the CND. The dense gas fraction (traced by the different transitions of HCN or HCO${}^{+}$ to CO (1-0) integrated intensity ratios) and dense gas ratio (HCN/HCO${}^{+}$) are higher at the E-knot, implying that the E- and W-knots have different physical environments or chemical compositions. The HCN emission in the CND show enhancement compared with HCO${}^{+}$, which could be affected by the AGN (Active Galactic Nucleus) radiation and starburst activity. The CO (3-2)/CO (1-0) integrated intensity ratio is a significant indicator of gas excitation. CO (3-2)/CO (1-0) ratios show much higher values at the E-knot, suggesting that there is molecular excitation enhancement caused by the extreme physical environment. Compared with the fluxes of HCN (4-3) and HCO${}^{+}$ (4-3) from the single-dish telescope JCMT (James Clerk Maxwell Telescope), the ALMA missing fluxes of dense molecular gas on 1 kpc scale are about 10%–20%. The spectral lines show that the flux ratios between E-knot and W-knot are $\sim$ 1.8–3.9. These differences shown between E-knot and W-knot may be associated with the AGN feedback. In addition, the CO (2-1), CO (1-0) and HCO${}^{+}$ (1-0) spectra show absorption features in the position of AGN. This absorption could be caused by the strong background of continuum emissions, and the gas inflow around the AGN can produce self-absorption in spectra.