基于ALMA高分辨率的多谱线数据对 NGC 1068核区中物理性质的研究

利用ALMA干涉阵望远镜(Atacama Large Millimeter/submillimeter Array)的高分辨(～0.2′′–0.7′′)的多谱线数据,包括CO (1-0)、CO (2-1)、CO (3-2)、HCN (1-0)、HCO⁺(1-0)、 HCN (3-2)、 HCO⁺(3-2)、HCN (4-3)和HCO+(4-3),并结合连续谱数据对近邻星系NGC 1068核区的物理性质进行了研究.速度积分强度图显示核周盘(CircumNuclear Disk, CND)呈现不对称的环状结构,尺度～300 pc.各分子谱线均显示, CND上的东发射结(E-knot)比西发射结(W-knot)有着更强的发射,且E-knot处表现出的分子气体的速度比W-knot更高.致密分子气体含量占比(用HCN或HCO⁺不同转动跃迁线与CO (1-0)的积分强度比值表示),以及致密分子气体比值(HCN/HCO⁺)均在CND的东部表现出更高的值,意味着CND东部和西部有着不同的物理环境或化学组成. CO ...

Physical Properties of the Nuclear Region of NGC 1068 Based on ALMA High-resolution Multi-spectral Lines

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.