Bar pattern speed modulation across LMC stellar populations

TL;DR

This study addresses whether the Large Magellanic Cloud (LMC) has a single bar pattern speed or if $\Omega_p$ depends on the stellar population. Using Gaia DR3 astrometry for ~11.5 million LMC stars, it defines five CMD-based evolutionary phases and applies the Dehnen method (via patternSpeed.py) to extract $\Omega_p$ and the bar phase $\phi_b$ across both free and fixed bar-region definitions. The key finding is a strong modulation of $\Omega_p$ with evolutionary phase, spanning roughly $-0.9$ to $6.6$ km s$^{-1}$ kpc$^{-1}$, with an equilibrium value near $-1.3$ km s$^{-1}$ kpc$^{-1}$ for the total sample and no corotation inside the disc; this demonstrates that the LMC bar cannot be characterized by a single global speed. The results imply population-dependent bar dynamics and highlight selection biases in integrated measurements, supporting the notion of an extremely slow bar likely influenced by recent LMC–SMC interactions. Future Gaia releases are expected to reduce systematic errors and enable deeper chemo-dynamical connections (age/metallicity) that will refine the understanding of bar evolution, with potential implications for the Milky Way bar as well.

Abstract

The bar pattern speed of the LMC has been measured using Gaia data, suggesting the presence of a slow pattern, perhaps not rotating at all. Numerical simulations of interacting LMC-SMC systems were able to reproduce a bar stoppage. Here, we report on the first measurement of the bar pattern speed of the LMC as a function of the evolutionary phase of its stellar populations. Astrometric and photometric data of 11 million LMC stars from Gaia DR3 were used to build five evolutionary phases, from less to more evolved stars. The Dehnen method, a new procedure to derive bar pattern speeds from kinematics of particles in N-body simulations, is applied to the LMC stellar populations. We observe a modulation of the bar pattern speed with the evolutionary phase, meaning that different LMC stellar populations exhibit different pattern speeds, ranging from -0.9 to 6.6 km/s/kpc. Moreover, less evolved stars have a lower pattern speed while the bar of more evolved phases tends to rotate faster. The LMC bar is thus extremely slow, ruling out the presence of bar corotation within the disc, in agreement with a previous claim, but this time observed with various stellar populations. It is the first time that a pattern speed is measured separately for different stellar populations in any galaxy. The LMC pattern speed cannot be simply resumed to a singular value, but instead is an overlay of different patterns depending on the evolutionary phase of the stars. Future Gaia releases will be crucial to investigate more deeply the relations of the pattern speed with important astrophysical parameters of stars, like their age and metallicity, which will be helpful to constrain the chemo-dynamical evolution of the LMC bar.

Figures (2)

• Figure 1: Modulation of $\Omega_p$ with the evolutionary phase. Filled symbols (open, respectively) are results with free (fixed) bar region. The dashed line $\Omega_p = 0.1 \pm 0.7$$\mathrm{km~s^{-1}\,kpc^{-1}}$ is for the Total sample, the dotted line $\Omega_p = -1.0 \pm 0.5$$\mathrm{km~s^{-1}\,kpc^{-1}}$ is for the NN Complete sample of 2024ajimenez.
• Figure 2: Maps of LMC star count (left), radial ($v_R - \bar{v}_R$, middle) and tangential ($v_\phi - \bar{v}_\phi$, right) velocities for the Young, RC, RGB, BL and AGB+RRL stellar evolutionary phases, from top to bottom respectively. The dashed circle marks the bar radius (Tab. \ref{['tab:res']}). A dashed line shows the direction of the bar phase angle. The $x$ and $y$ axes refer to the Cartesian coordinates in the LMC disc. The pixel scale of the grid is chosen at 200 pc.