Magellanic cloud structure from near-infrared surveys. I. The viewing angles of the large Magellanic Cloud

We present a detailed study of the viewing angles of the LMC disk plane. We find that our viewing direction differs considerably from the commonly accepted values, which has important implications for the structure of the LMC. The discussion is based on an analysis of spatial variations in the apparent magnitude of features in the near-IR color-magnitude diagrams extracted from the Deep Near-Infrared Southern Sky Survey (DENIS) and Two Micron All-Sky Survey (2MASS). Sinusoidal brightness variations with a peak-to-peak amplitude of similar to0.25 mag are detected as a function of position angle. The same variations are detected for asymptotic giant branch stars (using the mode of their luminosity function) and for red giant branch stars (using the tip of their luminosity function), and these variations are seen consistently in all of the near-IR photometric bands in both DENIS and 2MASS data. The observed spatial brightness variations are naturally interpreted as the result of distance variations because of one side of the LMC plane being closer to us than the opposite side. There is no evidence that any complicating effects, such as possible spatial variations in dust absorption or the age/metallicity of the stellar population, cause large-scale brightness variations in the near-IR at a level that exceeds the formal errors (similar to0.03 mag). The best-fitting geometric model of an inclined plane yields an inclination angle i = 34 degrees .7 +/- 6 degrees .2 and line-of-nodes position angle Theta = 122 degrees .5 +/- 8 degrees .3. The quoted errors are conservative estimates that take into account the possible influence of systematic errors; the formal errors are much smaller, 0 degrees .7 and 1 degrees .6, respectively. There is tentative evidence for variations of similar to 10 degrees in the viewing angles with distance from the LMC center, suggesting that the LMC disk plane may be warped. Traditional methods to estimate the position angle of the line of nodes have used either the major-axis position angle Theta (maj) of the spatial distribution of tracers on the sky or the position angle Theta (max) of the line of maximum gradient in the velocity field, given that for a circular disk Theta (maj) = Theta (max) = Theta. The present study does not rely on the assumption of circular symmetry and is considerably more accurate than previous studies of its kind. We find that the actual position angle of the line of nodes differs considerably from both Theta (maj) and Theta (max), for which measurements have fallen in the range 140 degrees -190 degrees. This indicates that the intrinsic shape of the LMC disk is not circular but elliptical. Paper II of this series explores the implications of this result through a detailed study of the shape and structure of the LMC. The inclination angle inferred here is consistent with previous estimates, but this is to some extent a coincidence, given that also for the inclination angle most previous estimates were based on the incorrect assumption of circular symmetry.