Mapping the LMC Bar: A Closer Look at the Structure of a Neighboring Galaxy

Himansh Rathore and colleagues used Gaia DR3 data to closely examine the unique structure of the Large Magellanic Cloud (LMC), a nearby galaxy known for its peculiar, offset bar. The team aimed to determine the bar’s shape, size, and position by addressing crowding issues in dense star fields, which can make it challenging to count stars accurately in the LMC’s bar region. By creating a "completeness map" that accounts for crowding, they improved star counts and measured the bar’s features precisely.

Introduction

The LMC’s bar, a central feature of this satellite galaxy of the Milky Way, has long puzzled astronomers. Unlike most galaxy bars, it does not align with the galaxy's center and lacks a clear presence in the LMC’s interstellar gas. Previous studies showed the bar might be tilted, and some have suggested it is misaligned due to a past collision with the Small Magellanic Cloud (SMC). Accurately describing this bar’s structure could reveal more about how the LMC has evolved and how interactions with nearby galaxies have shaped it.

Gaia Data and Incompleteness Solution

The team used Gaia DR3 to analyze a specific group of stars called red clump stars, which are well-suited to studying the bar because they have consistent brightness, making them easy to identify. However, the LMC's crowded bar region complicates measurements by making some stars harder to detect, causing an undercount. To solve this, they introduced a correction method using the Gaia “color excess” (a measure of the difference between observed and expected light across specific filters). This approach allowed them to estimate how many stars were missed and make up for this “incompleteness.”

Completeness Map of the LMC

The researchers developed a map showing where star counts were affected by crowding across the LMC. By adjusting counts to account for missing stars, they corrected the LMC’s star distribution, revealing a clearer picture of the bar's structure. The corrected star count map confirmed that crowding significantly affected previous estimates of the bar’s size and orientation.

Measurements of the LMC’s Bar Properties

Using their corrected data, the team measured the bar’s position angle, radius, and thickness with Fourier decomposition, a mathematical approach for breaking down shapes in galaxy images. They found the bar to be approximately 2.13 kpc long with a moderate thickness, a stronger alignment than previously observed, and an offset from the LMC’s center of about 0.76 kpc.

Comparisons with Simulations

To understand how these characteristics might have arisen, Rathore and team compared their measurements to simulations of galaxies that included interactions between the LMC, SMC, and Milky Way. The results suggested the LMC’s off-center bar could indeed be a result of recent encounters with the SMC, aligning well with previous studies and supporting the idea that a recent collision influenced the LMC’s current structure.

Broader Implications

This work sheds light on the evolution of the LMC and how galaxy interactions can create unusual features like offset bars. The authors also suggest that their approach could help correct crowding issues in other crowded star systems, opening doors to improved measurements for nearby galaxies like the SMC, where dense regions also make studying star structures challenging.

Source: Rathore