银河系核球中垂向金属丰度梯度的一种可能的动力学起源机制

A Possible Dynamical Origin of the Vertical Metallicity Gradient in the Milky Way Bulge

研究发现,即使在银河系长期缓慢演化的过程中,其核球中依然可能存在垂直的金属丰度梯度。该结果反驳了早期研究中认为的银河系核球中长期缓慢演化的过程会消除任何存在的化学丰度梯度的观点。通过使用一个模拟银河系薄盘的多体数值模拟模型得出该研究结果,该模型能自洽地通过盘的棒不稳定性和屈曲不稳定性形成一个盒状/花生状的核球,并结合化学-动力学模型,分析银河系核球中化学成分的演化。在该演化模型中设定了一个初始的径向金属丰度梯度(-0.3 dex/kpc),通过长期缓慢的演化过程,其核球部分呈现出与观测相符的垂直金属丰度梯度。对此一种可能的解释是,星系盘的金属丰度分布在长期缓慢演化的过程中经历了“两步”演化过程,首先盘星系初始半径较大的贫金属粒子在径向混合的过程中产生了更大的径向速度弥散,然后由于核球的屈曲不稳定性演化,垂向速度弥散最终会保持与径向速度弥散的一定比值(约0.8)。这个机制导致贫金属粒子在演化过程中能占据更大的垂向范围,从而形成核球的垂直金属丰度梯度。银河系核球动力学模型通过简单的化学-动力学过程揭示的核球金属丰度演化机制,应在包含更复杂的核球演化机制的模型中同样存在。此外,还通过化学动力学模型讨论了核球观测中存在的争议,如金属丰度分布函数中产生多个“峰”的起源问题,以及该模型构建中存在的一些的不足。

Observations show a clear vertical metallicity gradient in the Galactic bulge.It is often regarded as a counter-argument against the`secular evolution'of the Milky Way bulge,as the strong mixing of the internal instabilities in the secular evolution may erase the pre-existing vertical chemical gradient.Inspired by some previous work,we use a thin disk model that self-consistently forms a boxy/peanut bulge via the bar and buckling insta-bilities,to explore the origin of this chemical gradient.We nd the vertical gradient can be established if the initial disk is set up with a radial metallicity gradient,as in the evolution the metal-poor particles in the outer region move inward and get more vertically thickened than the inner metal-rich particles.This is because the formation of the bar always tend to heat the in-plane random motions(σR)of the metal-poor population at larger radius more,therefore when theσZ/σR of all populations nally reach a similar value about 0.8 in the buckling instabilities,the vertical random motion(σZ)of metal-poor population is also more heated.This formation mechanism revealed by our simplest secular evolution disk model must also be in action in other more complicated disk models that experienced similar internal instabilities.Our chemo-dynamical model can produce the multiple\peaks"in the metallicity distribution functions found in many recent bulge observations.We also discuss other implications and the weaknesses of this model.