The Stellar Population Histories of Early-Type Galaxies. II. Controlling Parameters of the Stellar Populations
The Stellar Population Histories of Early-Type Galaxies. II. Controlling Parameters of the Stellar Populations
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DOI：
10.1086/301442
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发表时间：
2000-04
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影响因子：
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通讯作者：
S. C. Trager;S. Faber;G. Worthey;J. J. G. Ociw-J.;UCOLick;St. Ambrose;IA-UNAM
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文献类型：
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作者：
S. C. Trager;S. Faber;G. Worthey;J. J. G. Ociw-J.;UCOLick;St. Ambrose;IA-UNAM
本文分析了50个局域椭圆星系的单星群(SSP)当量参数与其结构参数的关系。该星系样本来自冈萨雷斯(1993)和昆茨内尔(1998)的高质量光谱测量。基本数据是文献I中得出的SSP当量年龄、t、金属度、[Z/H]和“增强”比[E/Fe]的中心值，以及包括速度色散、半径、表面亮度、质量和亮度在内的整体结构参数。星系在四维空间[Z/H]、对数t、对数σ和[E/Fe]中填充一个二维平面。SSP年龄t和速度离散度σ可以作为两个独立的参数，用来确定星系在这个超平面中的位置。超平面可以分解为两个子关系：(1)“Z平面”，其中[Z/H]是对数σ和对数t的线性函数；(2)[E/Fe]和σ之间的关系，其中[E/Fe]在高σ星系中较大。速度色散是已发现的唯一可以调节恒星群的结构参数；添加其他结构变量，如ie或Re，并不能更准确地预测[Z/H]或[E/Fe]。团簇和场椭圆遵循相同的超平面，但它们在超平面内的(σ，t)分布不同。大多数Fornax和Virgo星系团星系都很老，只有一小部分星系年龄较小。场椭圆体的SSP年龄范围较大，低σ星系有年轻化的趋势。因此，本文的样本表明，局部椭圆点在(σ，t)平面上的分布可能取决于环境。由于(σ，t)分布影响所有涉及SSP参数的二维投影，许多常见的椭圆体标度定律也可能与环境有关。在当前的样本中可以看到这一点的一些证据。例如，只有Fornax椭圆体显示出经典的质量-金属丰度关系，而其他子样本则不显示。这些椭圆的紧的mg-σ关系可以理解为金属丰度超平面的二维投影，表明它是边的。在固定的σ下，年轻的年龄往往被高的[Z/H]所抵消，保持镁几乎恒定。Mg-σ关系的紧密性并不一定意味着固定σ的年龄范围狭窄。尽管年轻恒星对SSP参数的权重很大，但对它们进行建模仍然对椭圆星系的整个恒星形成历史施加了严格的限制。[E/Fe]与σ的关系与σ较高时第二类元素的有效产额较高相一致。如果国际货币基金组织在高σ的大质量恒星中得到增强，或者如果更深的银河系势井保留了更多的SNE II富集态气体，就可能发生这种情况。无论如何，与σ相比，调整II型收益率似乎比调整Ia型收益率更符合数据。Z平面更难解释，如果它被证明是一般的，它可能是椭圆形星系中恒星形成的有力线索。目前的数据倾向于一种“结霜”模型，在该模型中，通过在较老的“基础”群体中添加一小部分较年轻恒星的结霜(假设σ不变)，产生较低的表观SSP年龄。如果结霜丰度接近或略高于基本种群，则根据需要，简单的双分量模型在Z平面上沿着恒定的σ线运行。这有利于从混合良好的预富集气体中形成恒星，而不是来自吸积物体的未混合的低金属度气体。
This paper analyzes single stellar population (SSP)–equivalent parameters for 50 local elliptical galaxies as a function of their structural parameters. The galaxy sample is drawn from the high-quality spectroscopic surveys of González (1993) and Kuntschner (1998). The basic data are central values of SSP-equivalent ages, t, metallicities, [Z/H], and "enhancement" ratios, [E/Fe], derived in Paper I, together with global structural parameters including velocity dispersions, radii, surface brightnesses, masses, and luminosities. The galaxies fill a two-dimensional plane in the four-dimensional space of [Z/H], log t, log σ, and [E/Fe]. SSP age, t, and velocity dispersion, σ, can be taken as the two independent parameters that specify a galaxy's location in this "hyperplane." The hyperplane can be decomposed into two subrelations: (1) a "Z-plane," in which [Z/H] is a linear function of log σ and log t and (2) a relation between [E/Fe] and σ in which [E/Fe] is larger in high-σ galaxies. Velocity dispersion is the only structural parameter that is found to modulate the stellar populations; adding other structural variables such as Ie or re does not predict [Z/H] or [E/Fe] more accurately. Cluster and field ellipticals follow the same hyperplane, but their (σ,t) distributions within it differ. Most Fornax and Virgo cluster galaxies are old, with a only a small sprinkling of galaxies to younger ages. The field ellipticals span a larger range in SSP age, with a tendency for lower σ galaxies to be younger. The present sample thus suggests that the distribution of local ellipticals in the (σ,t) plane may depend on environment. Since the (σ,t) distribution affects all two-dimensional projections involving SSP parameters, many of the familiar scaling laws attributed to ellipticals may also depend on environment. Some evidence for this is seen in the current sample. For example, only Fornax ellipticals show the classic mass-metallicity relation, whereas other subsamples do not. The tight Mg-σ relations of these ellipticals can be understood as two-dimensional projections of the metallicity hyperplane showing it edge-on. At fixed σ, young age tends to be offset by high [Z/H], preserving Mg nearly constant. The tightness of the Mg-σ relations does not necessarily imply a narrow range of ages at fixed σ. Although SSP parameters are heavily weighted by young stars, modeling them still places tight constraints on the total star formation history of elliptical galaxies. The relation between [E/Fe] and σ is consistent with a higher effective yield of Type II SNe elements at higher σ. This might occur if the IMF is enhanced in massive stars at high σ, or if more SNe II–enriched gas is retained by deeper galactic potential wells. Either way, modulating Type II yields versus σ seems to fit the data better than modulating Type Ia yields. The Z-plane is harder to explain and may be a powerful clue to star formation in elliptical galaxies if it proves to be general. Present data favor a "frosting" model in which low apparent SSP ages are produced by adding a small frosting of younger stars to an older "base" population (assuming no change in σ). If the frosting abundances are close to or slightly greater than the base population, simple two-component models run along lines of constant σ in the Z-plane, as required. This favors star formation from well-mixed pre-enriched gas rather than unmixed low-metallicity gas from an accreted object.