On the Connection Between Spiral Arm Pitch Angle and Galaxy Properties

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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