arXiv 1812.060102018-12-14astro-ph.GA

旋臂节距角与星系性质的关系

On the Connection Between Spiral Arm Pitch Angle and Galaxy Properties

Si-Yue Yu, Luis C. Ho

备注：: 16 pages, 12 figures, 1 table; ApJ in press
DOI：: 10.3847/1538-4357/aaf895

摘要：

我们测量了79个星系样本中旋臂的俯仰角（$φ$），系统地研究了$φ$对星系形态、质量和运动学的依赖性，以探讨旋臂的物理起源。我们发现，$φ$减少（臂更紧密地缠绕），虽然有显着的分散，在早期的哈勃类型的星系，更突出的凸起，更高的浓度，和更大的总星系恒星质量（$M_*^{\rm gal}$）。对于给定的浓度，具有较大恒星质量的星系往往具有更紧密的旋臂，反之亦然。我们还发现，$φ$服从一个紧密的逆相关与中央恒星的速度色散$σ_c$ $\gtrsim$ $100$公里的$^{-1}$，而$φ$保持近似恒定的$σ_c\lesssim100$公里的$^{-1}$。我们证明了$φ$ - $σ_c$和$φ$ - $M_*^{\rm gal}$关系是一个更基本的三维$φ-σ_c-M_*^{\rm gal}$关系的投影，因此，对于大质量星系，俯仰角由$σ_c$决定，而对于小质量星系，俯仰角由$M_*^{\rm gal}$决定。与以前的研究相反，我们发现$φ$与星系的剪切速率仅松散相关。对于一个给定的剪切速率，从$N$体模拟产生的螺旋显示出比观测到的高得多的$φ$，这表明星系盘在动力学上更冷（Toomre的$Q \approx 1.2$）。相反，测得的俯仰角显示出更强的关系与中央区域的旋转曲线的形态，这样的星系与中央峰值旋转曲线有紧的手臂，而那些缓慢上升的旋转曲线有松散的武器。这些行为与密度波理论的预测定性一致。

英文摘要：

We measure the pitch angle ($φ$) of spiral arms in a sample of 79 galaxies to perform a systematic study of the dependence of $φ$ on galaxy morphology, mass, and kinematics to investigate the physical origin of spiral arms. We find that $φ$ decreases (arms are more tightly wound), albeit with significant scatter, in galaxies with earlier Hubble type, more prominent bulges, higher concentration, and larger total galaxy stellar mass ($M_*^{\rm gal}$). For a given concentration, galaxies with larger stellar masses tend to have tighter spiral arms, and vice versa. We also find that $φ$ obeys a tight inverse correlation with central stellar velocity dispersion for $σ_c$$\gtrsim$$100$ km s$^{-1}$, whereas $φ$ remains approximately constant for $σ_c\lesssim100$ km s$^{-1}$. We demonstrate that the $φ$-$σ_c$ and $φ$-$M_*^{\rm gal}$ relations are projections of a more fundamental three-dimensional $φ-σ_c-M_*^{\rm gal}$ relation, such that pitch angle is determined by $σ_c$ for massive galaxies but by $M_*^{\rm gal}$ for less massive galaxies. Contrary to previous studies, we find that $φ$ correlates only loosely with the galaxy's shear rate. For a given shear rate, spirals generated from $N$-body simulations exhibit much higher $φ$ than observed, suggesting that galactic disks are dynamically cooler (Toomre's $Q \approx 1.2$). Instead, the measured pitch angles show a much stronger relation with morphology of the rotation curve of the central region, such that galaxies with centrally peaked rotation curves have tight arms, while those with slow-rising rotation curves have looser arms. These behaviors are qualitatively consistent with predictions of density wave theory.

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