真实卫星轨道对银河系加热和隐含合并历史的根本影响
The Radical Consequences of Realistic Satellite Orbits for the Heating and Implied Merger Histories of Galactic Disks
摘要：
以前的星系盘加热模型在相互作用中引用了在现代LCDM背景下不一定有效的限制性假设：卫星和轨道是刚性和圆形的，在许多轨道时间内由于动力学摩擦而缓慢衰减。这导致磁盘加热与卫星质量的线性标度：磁盘高度和速度分散规模~M_sat/M_disk。反过来，观测到的盘厚度对合并历史提出了强有力的限制：对银河系的影响是，小于5%的质量可能来自z~2以来的合并，这与宇宙学预测相冲突。更现实的是，卫星在接近径向的轨道上合并，一旦靠近圆盘，共振相互作用有效地消除了角动量，而潮汐效应则剥离了质量，导致在几次自由落体中快速合并/破坏。在这些条件下，适当的加热效率与质量比~（M_sat/M_disk）^2呈非线性关系。我们推导出的磁盘规模的高度和速度分散体的缩放作为质量比和磁盘气体含量的函数，在这个制度，并表明这准确地描述了适当的“活”晕和磁盘的模拟结果。在现实的情况下，磁盘加热在较小的合并被抑制了一个数量级相对于以前的模型的期望。我们表明，银河系盘可以吸收约5-10 1：10的质量比合并，因为z=2，与宇宙学模型。这些区别导致显着的差异，其中质量比是最重要的磁盘加热和磁盘+凸起系统的等照度线形状。
英文摘要：
Previous models of galactic disk heating in interactions invoke restrictive assumptions not necessarily valid in modern LCDM contexts: that satellites and orbits are rigid and circular, with slow decay over many orbital times from dynamical friction. This leads to a linear scaling of disk heating with satellite mass: disk heights and velocity dispersions scale ~M_sat/M_disk. In turn, observed disk thicknesses present strong constraints on merger histories: the implication for the Milky Way is that <5% of its mass could come from mergers since z~2, in conflict with cosmological predictions. More realistically, satellites merge on nearly radial orbits, and once near the disk, resonant interactions efficiently remove angular momentum while tidal effects strip mass, leading to rapid merger/destruction in a couple of free-fall plunges. Under these conditions the proper heating efficiency is non-linear in mass ratio, ~(M_sat/M_disk)^2. We derive the scaling of disk scale heights and velocity dispersions as a function of mass ratio and disk gas content in this regime, and show this accurately describes the results of simulations with proper 'live' halos and disks. Under realistic circumstances, disk heating in minor mergers is suppressed by an order of magnitude relative to expectations of previous models. We show that the Milky Way disk could have absorbed ~5-10 1:10 mass-ratio mergers since z=2, in agreement with cosmological models. These distinctions lead to dramatic differences in which mass ratios are most important for disk heating and in the isophotal shapes of disk+bulge systems.
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