Precise Measurements of the LMC Bar's Geometry With Gaia DR3 and a Novel Solution to Crowding Induced Incompleteness in Star Counting

TL;DR

To precisely measure the LMC bar geometry, the authors use Gaia DR3 red clump stars and a color-excess based completeness correction to account for crowding in the bar region. They perform a Fourier decomposition of the completeness-corrected 2-D stellar density to extract bar parameters ($R_{bar}$, $PA_{bar}$, $S_{bar}$, $(b/a)_{bar}$, and $\Delta_{bar}$) and compare with hydrodynamic models of a recent LMC-SMC collision. The work shows that crowding biases can dramatically inflate uncertainties and understate bar strength, but after correction the LMC bar aligns with scaling relations of barred galaxies and is consistent with a recent direct collision with the SMC as a plausible origin. They also propose a general completeness-correction framework applicable to other Local Group systems.

Abstract

We present new measurements of the two-dimensional (2-D) 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 datasets with the Gaia BP-RP color excess. Utilizing the color excess information, we derive a 2-D completeness map of the LMC's disk. We find that incompleteness biases the bar measurements and induces large uncertainties. With the completeness-corrected 2-D red clump map, we precisely measure the LMC bar's properties for the first time using Fourier decomposition. The bar radius is $R_{bar} = 2.13^{+0.03}_{-0.04}$ kpc, and its position angle is $121.26^{\circ} \pm 0.21^{\circ}$. 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 $0.54 \pm 0.03$, and is offset with respect to the center of the outer disk isophote at R $\approx$ 5 kpc by $0.76 \pm 0.01$ 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.

Figures (10)

• Figure 1: Left Panel: The probability density distribution of the corrected Gaia BP-RP color excess ($C^\ast$) for the sample of LMC red clump stars obtained by applying the selection criteria of C22 as shown in Appendix A. The region enclosed by the dashed line, dash-dot line and the solid line represent one, two and three standard deviations from the mean $C^\ast$ respectively. Middle Panel: The spatial distribution of stars residing beyond the $3-\sigma$ distribution as shown in the left panel, which we remove from our sample. These excluded stars constitute around $1\%$ of the initial count and trace the crowded bar region of the LMC's disk. This indicates that $C^\ast$ can be used as a representative of crowding induced incompleteness in the LMC. Right Panel: The spatial distribution of the sample of red clump stars that we obtain after applying the selection criteria based on $C^\ast$. The star counts are significantly underestimated in the central region, which is an effect of crowding. We refer to this as the "Incomplete sample".
• Figure 2: Left Panel: The spatial distribution of the spread in the corrected Gaia color excess ($\sigma_{C^\ast}$), which traces crowded regions in the LMC's disk. Middle Panel: The radial profile of the spatial distribution of $\sigma_{C^\ast}$. The profile approaches a constant value in the outer disk, which indicates an intrinsic scatter to $C^\ast$ ($\sigma_{C^\ast}^{int}$) presumably due to instrumental and astrophysical effects independent of crowding. We find this constant value to be 0.04 (depicted by the black dashed line) by fitting the radial profile data with a tangent hyperbolic function. Right Panel: The spatial distribution of $\sigma_{C^\ast}^{crowd}$, which we obtain by subtracting the intrinsic scatter in $C^\ast$ from the total scatter in quadrature. $\sigma_{C^\ast}^{crowd}$ directly probes crowding in the LMC's disk.
• Figure 3: The completeness map of the LMC's disk derived from the spread in the Gaia BP-RP color excess. The map shows the median completeness in spatial bins of $(\Delta \xi \times \Delta \eta) = (0.02^\circ \times 0.02^\circ)$. A completeness of 1 indicates $100\%$ of the stars are being counted in that spatial bin, while a completeness of 0 indicates most of the stars are not being counted.
• Figure 4: Completeness corrected LMC disk obtained by weighting the incomplete disk in Figure \ref{['fig:cuts']} ( right panel) by the inverse of the completeness map (Figure \ref{['fig:comp_man']}). Central region becomes much more prominent after the completeness correction.
• Figure 5: Red clump number density profile for the incomplete (red dashed) and the completeness corrected (black solid) LMC disks. The number density in the central regions is significantly underestimated in the incomplete disk, which is corrected for in the completeness corrected disk. The dash-dot grey line represents the best fit exponential profile to the completeness corrected data. The agreement of the data with the exponential profile in the inner disk ($R < 5^\circ$) further corroborates our completeness correction framework, where no prior information on the density profile is utilized.