[论文解读] Erratum: Structural properties of disk galaxies. I. The intrinsic equatorial ellipticity of bulges

J. Méndez‐Abreu, J. A. L. Aguerri+2 more|arXiv (Cornell University)|Oct 29, 2007

Astronomy and Astrophysical Research参考文献 3被引用 12

一句话总结

本研究利用一种新型的二维测光分解算法（GASP2D）对148个无棒S0–Sb星系的J波段全天红外巡天（2MASS）图像进行分析，以推导其星系核球的本征赤道椭圆度。研究发现，约80%的此类核球为非旋转对称的三轴椭球体，其平均赤道轴比⟨B/A⟩ = 0.85，表明其形成机制更可能源于并合或非旋翼演化，而非扁球形坍缩。

ABSTRACT

(Abridged) A variety of formation scenarios was proposed to explain the diversity of properties observed in bulges. Studying their intrinsic shape can help in constraining the dominant mechanism at the epochs of their assembly. The structural parameters of a magnitude-limited sample of 148 unbarred S0--Sb galaxies were derived in order to study the correlations between bulges and disks as well as the probability distribution function (PDF) of the intrinsic equatorial ellipticity of bulges. It is presented a new fitting algorithm (GASP2D) to perform the two-dimensional photometric decomposition of galaxy surface-brightness distribution. This was assumed to be the sum of the contribution of a bulge and disk component characterized by elliptical and concentric isophotes with constant (but possibly different) ellipticity and position angles. Bulge and disk parameters of the sample galaxies were derived from the J-band images which were available in the Two Micron All Sky Survey. The PDF of the equatorial ellipticity of the bulges was derived from the distribution of the observed ellipticities of bulges and misalignments between bulges and disks. Strong correlations between the bulge and disk parameters were found. About 80% of bulges in unbarred lenticular and early-to-intermediate spiral galaxies are not oblate but triaxial ellipsoids. Their mean axial ratio in the equatorial plane is <B/A> = 0.85. There is not significant dependence of their PDF on morphology, light concentration, and luminosity. The interplay between bulge and disk parameters favors scenarios in which bulges assembled from mergers and/or grew over long times through disk secular evolution. But all these mechanisms have to be tested against the derived distribution of bulge intrinsic ellipticities.

研究动机与目标

• 通过测量其本征三维形状，约束星系核球的主要形成机制。
• 解决以往研究依赖观测椭圆度所导致的投影效应和位置角错位偏差问题。
• 通过其本征形状检验核球形成是否主要由快速耗散坍缩、并合或非旋翼演化主导。
• 推导对观测偏差具有鲁棒性的本征赤道椭圆度概率密度函数（PDF）。
• 评估核棒对推断核球形状的影响，鉴于其可能被误认为三轴性。

提出的方法

• 开发了一种新型二维测光分解算法（GASP2D），用于将星系表面亮度建模为Sérsic核球与指数盘成分之和，且椭圆度和位置角保持恒定。
• 将GASP2D应用于全天红外巡天（2MASS）的J波段图像，以最小化尘埃和年轻恒星的污染。
• 采用Levenberg-Marquardt拟合算法，稳健估计结构参数（如有效半径、Sérsic指数、尺度长度）。
• 通过蒙特卡洛模拟统计校正观测椭圆度及核球与盘面位置角错位，推导出本征赤道椭圆度的PDF。
• 通过模拟30%和100%的核棒比例，评估核棒对结果的影响，并比较所得PDF。
• 通过与基本平面关系及FJ/PP关系的一致性验证结果，确保其与已知椭圆星系性质相符。

实验结果

研究问题

• RQ1 无棒S0–Sb星系中核球的本征赤道椭圆度分布为何？其如何约束形成机制？
• RQ2 观测椭圆度及核球-盘面错位在多大程度上导致对核球本征三维形状的偏差？
• RQ3 观测到的核球三轴性是否与并合、非旋翼演化或早期耗散坍缩形成机制一致？
• RQ4 核棒的存在如何影响对核球本征形状的推断？
• RQ5 核球的本征椭圆度是否依赖于形态、光度或光度集中度？

主要发现

• 在无棒透镜星系及早型至中型旋涡星系中，约80%的核球并非扁球形，而是三轴椭球体，表明其形成过程具有非球对称性。
• 赤道平面的平均轴比为⟨B/A⟩ = 0.85，对应本征椭圆度⟨E⟩ = 0.15，且该值与形态、光度或光度集中度无显著依赖关系。
• 推导出的本征赤道椭圆度概率密度函数（PDF）具有鲁棒性，即使在假设存在30%核棒比例的情况下，其结果仍保持独立。
• 核球遵循与椭圆星系相同的基底平面、FJ及PP关系，支持核球与椭圆星系具有共同的形成机制。
• 核球与盘面参数之间存在强烈相关性，如盘面尺度长度随核球有效半径和速度弥散度的增加而增大，支持非旋翼演化作为关键形成路径。
• 结果与层次并合及盘面非旋翼演化的数值模拟一致，但仅凭本征形状尚无法单独区分这两种机制；本征形状是关键判别指标。