Several recent studies reinforce the existence of a thick molecular disc in galaxies along with the dynamically cold thin disc. Assuming a two-component molecular disc, we model the disc of NGC 6946 as a four-component system consisting of stars, HI, thin disc molecular gas, and thick disc molecular gas in vertical hydrostatic equilibrium. Following, we set up the joint Poisson-Boltzmann equation of hydrostatic equilibrium and solve it numerically to obtain a three-dimensional density distribution of different baryonic components. Using the density solutions and the observed rotation curve, we further build a three-dimensional dynamical model of the molecular disc and consecutively produce simulated CO spectral cubes and spectral width profiles. We find that the simulated spectral width profiles distinguishably differ for different assumed thick disc molecular gas fractions. Several CO spectral width profiles are then produced for different assumed thick disc molecular gas fractions and compared with the observed one to obtain the best fit thick disc molecular gas fraction profile. We find that the thick disc molecular gas fraction in NGC 6946 largely remains constant across its molecular disc with a mean value of 0.70 ± 0.09. We also estimate the amount of extra-planar molecular gas in NGC 6946. We find ∼ 50% of the total molecular gas is extra-planar at the central region, whereas this fraction reduces to ∼ 15% at the edge of the molecular disc. With our method, for the first time, we estimate the thick disc molecular gas fraction as a function of radius in an external galaxy with sub-kpc resolution.

中文翻译：

---

NGC 6946 中厚圆盘分子气体组分

最近的几项研究加强了星系中厚分子盘以及动态冷薄盘的存在。假设有一个双组分分子盘，我们将 NGC 6946 的盘建模为一个四组分系统，由恒星、HI、薄盘分子气体和垂直流体静力学平衡的厚盘分子气体组成。接下来，我们建立了流体静力平衡的联合泊松-玻尔兹曼方程并对其进行了数值求解，以获得不同重子分量的三维密度分布。使用密度解和观察到的旋转曲线，我们进一步建立了分子盘的三维动力学模型，并连续生成模拟的 CO 光谱立方体和光谱宽度剖面。我们发现，对于不同的假设厚圆盘分子气体分数，模拟的光谱宽度轮廓明显不同。然后为不同的假设厚盘分子气体分数产生几个 CO 谱宽分布，并与观察到的进行比较以获得最佳拟合厚盘分子气体分数分布。我们发现 NGC 6946 中厚圆盘的分子气体分数在其分子圆盘上基本上保持不变，平均值为 0.70 ± 0.09。我们还估计了 NGC 6946 中超平面分子气体的数量。我们发现大约 50% 的总分子气体在中心区域是超平面的，而这一比例在分子盘边缘减少到 15%。使用我们的方法，第一次，