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

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

Galaxies: Formation, Evolution, Phenomena参考文献 2被引用 9

一句话总结

本研究使用一种新型的二维测光分解算法（GASP2D）对来自银河系两微米全天巡天（2MASS）的J波段图像进行分析，测量了148个无棒S0–Sb星系的星系核球内在赤道椭圆度。研究发现，约80%的这些核球为非旋转对称的三轴体，而非扁球体，其平均赤道轴比⟨B/A⟩ = 0.85，表明其可能通过并合或长时间尺度的自洽演化形成，而非快速耗散坍缩。

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> = 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.</b>

研究动机与目标

• 通过测量核球的内在三维形状，约束星系核球的主要形成机制。
• 探究核球形态、光度或集中度是否与内在椭圆度相关。
• 评估潜在的核棒对推导出的内在形状分布的影响。
• 检验核球与盘面参数之间观测到的相关性是否更支持自洽演化或并合驱动的形成机制。
• 从观测到的椭圆度和核球-盘面错位角分布中，推导出内在赤道椭圆度的概率密度函数（PDF）

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拟合为Sérsic核球与具有恒定椭圆度和位置角的指数盘面之和。
• 将GASP2D应用于来自两微米全天巡天（2MASS）的148个无棒S0–Sb星系的J波段图像，构成一个星等限制样本。
• 使用稳健的Levenberg–Marquardt拟合算法，可靠地估计了包括核球和盘面有效半径、Sérsic指数、尺度长度、椭圆度和位置角在内的结构参数。
• 通过利用核球-盘面错位角分布对观测椭圆度进行去投影，推导出内在赤道椭圆度的概率密度函数（PDF）。
• 通过模拟核棒对推导PDF的影响来检验结果的稳健性，假设核棒比例为30%和100%。
• 将推导出的核球性质与分层并合及自洽演化的数值模拟预测进行比较。

实验结果

研究问题

• RQ1 无棒S0–Sb星系中核球的内在赤道椭圆度分布是什么？该分布如何约束其形成机制？
• RQ2 核球与盘面参数之间观测到的相关性是否与自洽演化或并合驱动形成一致？
• RQ3 核棒的存在是否显著影响核球内在形状的推导结果？
• RQ4 核球的内在椭圆度是否依赖于星系形态、光度或光度集中度？
• RQ5 观测到的核球性质与分层并合和盘面自洽演化模拟的预测相比如何？

主要发现

• 在无棒透镜星系及早型至中型旋涡星系中，约80%的核球为三轴体，而非扁球体，表明其并非轴对称球状体。
• 核球的平均赤道轴比为⟨B/A⟩ = 0.85，对应平均内在椭圆度⟨E⟩ = 0.15。
• 内在赤道椭圆度的概率密度函数（PDF）在星系形态、光度或光度集中度方面均无显著依赖性。
• 即使在考虑30%的星系含有核棒的情况下，推导出的PDF依然保持一致，表明核棒不会显著偏倚内在形状的测量结果。
• 核球遵循与椭圆星系相同的 Fundamental Plane、FJ关系和PP关系，支持其具有共同的形成机制。
• 盘面尺度长度与核球有效半径及中心速度 dispersion 之间存在强相关性，表明存在协同演化过程，与自洽演化或并合驱动增长一致。