Precise Measurements of the LMC Bar’s Geometry with Gaia DR3 and a Novel Solution to Crowding-induced Incompleteness in Star Counting

Abstract We present new measurements of the 2D geometry of the LMC’s stellar bar with precise astrometric observations of red clump stars in Gaia DR3. We develop a novel solution to tackle crowding-induced incompleteness in Gaia data sets with the Gaia BP-RP color excess. Utilizing the color excess information, we derive a 2D completeness map of the LMC’s disk. We find that incompleteness biases the bar measurements and induces large uncertainties. With the completeness-corrected 2D red clump map, we precisely measure the LMC bar’s properties using Fourier decomposition. The bar radius (semimajor axis) is <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>R</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">bar</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>2.13</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.04</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.03</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> kpc, and its position angle is 121.°26 ± 0.°21. The bar’s strength as quantified by the Fourier bi-symmetric amplitude is S bar = 0.27, indicating that the LMC has a significant bar perturbation. We find the bar has an axis ratio of 0.54 ± 0.03, and is offset with respect to the center of the outer disk isophote at R ≈ 5 kpc by 0.76 ± 0.01 kpc. These LMC bar properties agree with a hydrodynamic model where the SMC has undergone a recent direct collision with the LMC. We compare the LMC’s bar properties with other barred galaxies in the local Universe, and discover that the LMC is similar to other barred galaxies in terms of bar-galaxy scaling relations. We discuss how our completeness correction framework can be applied to other systems in the Local Group.