Determining the Orientation of the Large Magellanic Cloud’s Stellar Disk Using Gaia Parallax

Observations of the Large Magellanic Cloud (LMC) indicate that it has a slightly elliptical stellar disk and is set at an angle relative to the plane of the Milky Way described by line of nodes and inclination. All previous measurements of these parameters rely on photometry of stellar population that make assumptions about the distribution of stellar age, stellar metallicity, and dust which could bias the results. However, utilizing parallax measurements removes these inherent biases and assumptions by relying on purely geometrical information. While the uncertainty of a Gaia parallax measurement for a single LMC star is high, accurate distance information can be obtained by averaging over many stars. We are able to measure the line of nodes position angle and inclination of the stellar disk directly by averaging Gaia DR2 parallax measurements of thousands of LMC stars at each spatial position. As previously reported by other authors, we find a systematic offset in the Gaia parallaxes. We add +0.0355 mas to the Gaia parallaxes to adjust the mean LMC distance to the literature value of ∼49 kpc. This value for the LMC provides an additional constraint on the Gaia parallax offset at the magnitudes of the LMC stars (G∼18-19 mag) and far from the Milky Way midplane. Our best fit LMC stellar disk distance model yields a line of nodes position angle of 145.3 ± 5.0 deg and an inclination angle of 52.7 ± 2.4 deg, with uncertainties derived from a Monte Carlo simulation. While this line of nodes orientation falls within the range of previously determined values 140−190 deg, our inclination falls well above the previously determined range of 20−30 deg. Our results show a central clump of stars behind the plane of the stellar disk (where crowding is a concern),which may be responsible for skewing the inclination.