来自暗物质湮灭的压力和旋涡星系的旋转曲线
Pressure from dark matter annihilation and the rotation curve of spiral galaxies
摘要：
螺旋星系的旋转曲线是冷暗物质范式的基本预测之一，在过去的几十年里，它们在最里面区域的形状一直是激烈争论的焦点。目前的工作表明，暗物质湮灭成电子-正电子对可能会影响所观察到的旋转曲线的显着金额。我们采用了一种与模型无关的方法，即所有的电子和正电子都以相同的初始能量E_0 ~ m_dm*c^2注入，注入能量范围从1 MeV到1 TeV，注入速率受INTEGRAL、Fermi和HESS数据的约束。相对论性电子-正电子气体的压强是通过求解扩散损失方程，考虑逆康普顿散射、同步辐射、库仑碰撞、韧致辐射和电离来确定的。对于代表银河系的气体密度和磁场的值，如果E_0 < 1 GeV，估计压力梯度足够强以平衡中心部分的重力。确切的数值多少取决于天体物理参数，并且随着暗物质密度分布的斜率而显著变化。对于很陡的斜率，正如绝热收缩所预期的那样，旋涡星系的自转曲线在大多数E_0值下将受到~kpc尺度的影响。通过将预测的自转曲线与矮星系和低表面亮度星系的观测结果进行比较，我们发现，在某些情况下，来自暗物质湮灭的压力可能会改善理论与观测的一致性，但它也对模型参数施加了严格的限制（最值得注意的是，晕密度剖面的内斜率，以及质量和暗物质粒子湮灭成电子-正电子对的横截面）。
英文摘要：
The rotation curves of spiral galaxies are one of the basic predictions of the cold dark matter paradigm, and their shape in the innermost regions has been hotly debated over the last decades. The present work shows that dark matter annihilation into electron-positron pairs may affect the observed rotation curve by a significant amount. We adopt a model-independent approach, where all the electrons and positrons are injected with the same initial energy E_0 ~ m_dm*c^2 in the range from 1 MeV to 1 TeV and the injection rate is constrained by INTEGRAL, Fermi, and HESS data. The pressure of the relativistic electron-positron gas is determined by solving the diffusion-loss equation, considering inverse Compton scattering, synchrotron radiation, Coulomb collisions, bremsstrahlung, and ionization. For values of the gas density and magnetic field that are representative of the Milky Way, it is estimated that pressure gradients are strong enough to balance gravity in the central parts if E_0 < 1 GeV. The exact value depends somewhat on the astrophysical parameters, and it changes dramatically with the slope of the dark matter density profile. For very steep slopes, as those expected from adiabatic contraction, the rotation curves of spiral galaxies would be affected on ~kpc scales for most values of E_0. By comparing the predicted rotation curves with observations of dwarf and low surface brightness galaxies, we show that the pressure from dark matter annihilation may improve the agreement between theory and observations in some cases, but it also imposes severe constraints on the model parameters (most notably, the inner slope of halo density profile, as well as the mass and the annihilation cross-section of dark matter particles into electron-positron pairs).
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