耗散与无耗散星系形成及星系中的暗物质含量
Dissipational versus Dissipationless Galaxy Formation and the Dark Matter Content of Galaxies
摘要：
我们研究两个极端的模型建立的恒星组成部分的发光椭圆星系。在一种情况下，我们假设恒星的形成是耗散的，中心吸积的气体辐射出其轨道和热能;暗物质晕将经历绝热收缩，中心暗物质密度分布将变陡。对于第二个模型，我们假设中央星系是由一系列远离新生星系的恒星团无耗散合并而成的。为了被吸积，这些团块通过动力学摩擦将轨道能量输给暗物质晕，从而加热中心的暗物质并平滑暗物质密度尖点。这些模型的中心暗物质密度分布有很大的不同。对于孤立的椭圆星系NGC 4494，中心暗物质密度分别在耗散和无耗散模型下遵循幂律r^（-0.2）和r^（-1.7）。通过匹配的dissipationsanddissipationless模型的观测椭圆星系的恒星组成部分，我们研究dissipationsanddissipationless合并椭圆星系的形成的相对贡献，并寻找观测测试，将区分这些模型。与强透镜最亮的星系团相比，在z=0.39时，耗散和无耗散模型的M*/L_B比值中值分别为2.1+/-0.8和5.2+/-1.7。对于NGC 4494，最佳拟合的有耗散和无耗散模型的M*/L_B分别为2.97和3.96。与被动演化和人口合成的预期恒星质光比的比较似乎排除了巨型椭圆星系中央恒星区域的纯粹耗散形成机制。
英文摘要：
We examine two extreme models for the build-up of the stellar component of luminous elliptical galaxies. In one case, we assume the build-up of stars is dissipational, with centrally accreted gas radiating away its orbital and thermal energy; the dark matter halo will undergo adiabatic contraction and the central dark matter density profile will steepen. For the second model, we assume the central galaxy is assembled by a series of dissipationless mergers of stellar clumps that have formed far from the nascent galaxy. In order to be accreted, these clumps lose their orbital energy to the dark matter halo via dynamical friction, thereby heating the central dark matter and smoothing the dark matter density cusp. The central dark matter density profiles differ drastically between these models. For the isolated elliptical galaxy, NGC 4494, the central dark matter densities follow the power-laws r^(-0.2) and r^(-1.7) for the dissipational and dissipationless models, respectively. By matching the dissipational and dissipationless models to observations of the stellar component of elliptical galaxies, we examine the relative contributions of dissipational and dissipationless mergers to the formation of elliptical galaxies and look for observational tests that will distinguish between these models. Comparisons to strong lensing brightest cluster galaxies yield median M*/L_B ratios of 2.1+/-0.8 and 5.2+/-1.7 at z=0.39 for the dissipational and dissipationless models, respectively. For NGC 4494, the best-fit dissipational and dissipationless models have M*/L_B=2.97 and 3.96. Comparisons to expected stellar mass-to-light ratios from passive evolution and population syntheses appear to rule out a purely dissipational formation mechanism for the central stellar regions of giant elliptical galaxies.
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