Constraining the Galactic bar and spiral pattern speeds with the Hyades tidal stream

Abstract

We present a suite of direct $N$-body simulations of the Hyades open cluster and its tidal stream in a Milky Way potential that includes a rotating bar and spiral arms. Using the high-resolution code PETAR and an AGAMA-based multi-component Galactic model, we vary the bar and spiral pattern speeds ($Ω_b$, $Ω_s$) on a discrete grid and quantify the resulting changes in stream orientation, length, and internal density structure. We compare the simulations to Gaia EDR3 using the convergent point (CP) and compact convergent point (CCP) methods, followed by an adaptive three-dimensional nearest-neighbor matching in Cartesian space $(x,y,z)$. The Gaia candidate members exhibit a pronounced longitudinal density peak at $Y_{\mathrm{rot}} \approx 0.1\,\mathrm{kpc}$ in a stream-aligned coordinate system. Models with $Ω_s = 22.5\,\mathrm{km\,s^{-1}\,kpc^{-1}}$ and $Ω_b \simeq 40$--$45\,\mathrm{km\,s^{-1}\,kpc^{-1}}$ best reproduce this feature, while faster-bar models fail to match the observed density structure. These models are consistent with recent constraints favoring a relatively slow Galactic bar, and they illustrate how nearby open-cluster streams can provide an independent, local constraint on non-axisymmetric Galactic dynamics.

Figures (11)

• Figure 1: Circular-velocity curves: Hunter..2024AA...692A.216H without (blue) and with (red) spiral arms; potential model used in this study without (green) and with (cyan) spiral arms. Purple dot line represents R = 8.179kpc, which is close to the Sun's position used in this work
• Figure 2: Galactocentric positions of stars in Hyades simulations with bar pattern speeds $\Omega_b$ ranging from $10$ to $55\,\mathrm{km\,s^{-1}\,kpc^{-1}}$. Models including spiral arms are shown in purple, with $\Omega_s$ fixed at $22.5\,\mathrm{km\,s^{-1}\,kpc^{-1}}$; the corresponding bar-only models are shown in green. After shifting and rotating each model into a stream-aligned frame, the blue arrow indicates the centroid velocity direction adopted as the $x$-axis.
• Figure 3: Similar to Figure \ref{['fig:diff_bar']}, but varying the spiral pattern speed $\Omega_s$ from $10$ to $55\,\mathrm{km\,s^{-1}\,kpc^{-1}}$ with $\Omega_b$ fixed at $37.5\,\mathrm{km\,s^{-1}\,kpc^{-1}}$.
• Figure 4: Time evolution of the misalignment angle (red solid lines) and the local orbital curvature (blue solid lines) for the spatially matched models. The $x$-axis represents the time since the cluster formation in Myr. The left $y$-axis indicates the misalignment angle in degrees, and the right $y$-axis shows the orbital curvature in units of $\mathrm{kpc}^{-1}$. Panel (a) shows the impact of $\Omega_b$ (with and without spiral arms), and Panel (b) focuses on the impact of $\Omega_s$ at a constant $\Omega_b$.
• Figure 5: Proper motion distributions for a representative selection of models. The first three rows contrast the kinematics with and without spiral arms ($\Omega_s = 22.5\,\mathrm{km\,s^{-1}\,kpc^{-1}}$) at bar pattern speeds of $\Omega_b = 37.5$, $50$, and $55\,\mathrm{km\,s^{-1}\,kpc^{-1}}$, respectively. The bottom row focuses on the impact of different spiral arm speeds ($\Omega_s = 10$, $37.5\,\mathrm{km\,s^{-1}\,kpc^{-1}}$) for a constant bar speed of $\Omega_b = 37.5\,\mathrm{km\,s^{-1}\,kpc^{-1}}$.