Bulges are commonly believed to form in the dynamical violence of galaxy collisions and mergers. Here we model the stellar kinematics of the Bulge Radial Velocity Assay (BRAVA), and find no sign that the Milky Way contains a classical bulge formed by scrambling pre-existing disks of stars in major mergers. Rather, the bulge appears to be a bar, seen somewhat end-on, as hinted from its asymmetric boxy shape. We construct a simple but realistic N-body model of the Galaxy that self-consistently develops a bar. The bar immediately buckles and thickens in the vertical direction. As seen from the Sun, the result resembles the boxy bulge of our Galaxy. The model fits the BRAVA stellar kinematic data covering the whole bulge strikingly well with no need for a merger-made classical bulge. The bar in our best fit model has a half-length of ~ 4kpc and extends 20 degrees from the Sun-Galactic Center line. We use the new kinematic constraints to show that any classical bulge contribution cannot be larger than ~ 8% of the disk mass. Thus the Galactic bulge is a part of the disk and not a separate component made in a prior merger. Giant, pure-disk galaxies like our own present a major challenge to the standard picture in which galaxy formation is dominated by hierarchical clustering and galaxy mergers.
Substructure depletion in the Milky Way halo by the disk
我们采用数值模拟和简单的分析估计认为，暗物质子结构的内部区域的银河系轨道可以有效地破坏磁盘冲击，一个动力学过程，已知影响球状星团。我们进行了一组基准的高分辨率无碰撞模拟，在其中，我们自动地生长一个磁盘，使我们能够检查磁盘上的子结构丰度的影响。我们还跟踪了高分辨率Aquarius模拟中的暗物质卫星的轨道，并分析估计了累积晕和磁盘冲击效应。我们的计算表明，只有10%的银河系总质量的磁盘的存在下，可以显着改变晕内部的子结构的质量函数。这具有重要的意义，特别是对于在银河系中心~30 kpc范围内看到的相对较少的卫星，在那里，盘冲击预计将使10^9 M $_{\odot}$的子结构丰度减少约2倍，10^7 M $_{\odot}$的子结构丰度减少约3倍。最大质量的10^10 M $_{\odot}$的次晕即使在盘的存在下也能存活下来。这表明不存在内部缺失卫星的问题，并质疑这些子结构是否可以在磁盘中产生瞬态特征，如多臂螺旋图案。此外，暗物质子结构通过撞击盘和中央隆起的重子结构而耗尽，这可能会加剧问题，以充分解释引力透镜系统中观察到的通量异常，并显着降低了内晕附近子结构预期的暗物质湮灭信号。
