Magellanic Cloud Structure from Near-Infrared Surveys. I. The Viewing Angles of the Large Magellanic Cloud

By R. P. van der Marel, M. Cioni. 2001

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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 ∼0.25 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 (∼0.03 mag). The best-fitting geometric model of an inclined plane yields an inclination angle i = 34.°7 ± 6.°2 and line-of-nodes position angle Θ = 122.°5 ± 8.°3. The quoted errors are conservative estimates that take into account the possible influence of systematic errors; the formal errors are much smaller, 0.°7 and 1.°6, respectively. There is tentative evidence for variations of ∼10° 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 Θmaj of the spatial distribution of tracers on the sky or the position angle Θmax of the line of maximum gradient in the velocity field, given that for a circular disk Θmaj = Θmax = Θ. 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 Θmaj and Θmax, for which measurements have fallen in the range 140°–190°. 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.

Subfields: Astronomy and Astrophysics, Computational Mechanics

View Paper: https://www.semanticscholar.org/paper/7ffc59d8e2c11086a43426bce9ed7b03211c242d

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Citations in the corpus

THIRD-EPOCH MAGELLANIC CLOUD PROPER MOTIONS. II. THE LARGE MAGELLANIC CLOUD ROTATION FIELD IN THREE DIMENSIONS (2013, background): “…is also seen in the PM direction; and (iii) the directions of West and North are not fixed in a zenithal projection centered on the LMC, due to the deviation of (α, δ) contours from an orthogonal grid near the South Galactic pole (see figure 4 of van der Marel & Cioni 2001, hereafter vdMC01).”
THE STAR FORMATION HISTORY OF THE LARGE MAGELLANIC CLOUD (2009, methodology): “Deprojecting the LMC Disk van der Marel & Cioni (2001) showed convincingly that the stellar populations in the LMC are distributed in a disk whose plane is inclined to the plane of the sky by 34.7◦.”
Spitzer Survey of the Large Magellanic Cloud: Surveying the Agents of a Galaxy’s Evolution (SAGE). I. Overview and Initial Results (2006, unknown): “…the LargeMagellanicCloud (LMC) is the best astrophysical laboratory for studies of the life cycle of baryonic matter, because its proximity ( 50 kpc; Feast 1999) and its favorable viewing angle (35 ; van der Marel & Cioni 2001) permit studies of the resolved stellar populations and ISM clouds.”
The role of dwarf galaxy interactions in shaping the Magellanic System and implications for Magellanic Irregulars (2012, background): “In particular, detailed analysis of the geometry (van der Marel & Cioni 2001) and kinematics (van der Marel et al. 2002) prove that the LMC is a disc galaxy.”
SPITZER SAGE SURVEY OF THE LARGE MAGELLANIC CLOUD. III. STAR FORMATION AND ∼1000 NEW CANDIDATE YOUNG STELLAR OBJECTS (2008, background): “The favorable viewing angle of this flattened galaxy (35◦, van der Marel & Cioni 2001) limits line-of-sight confusion and allows correlation of the YSOs with the interstellar medium (ISM) clouds.”
New Cepheid Period-Luminosity Relations for the Large Magellanic Cloud: 92 Near-Infrared Light Curves (2004, unknown): “Recently, van der Marel & Cioni (2001) presented a detailed study of the geometry of the LMC, based on analysis of the very extensive JHKs photometry of red giant and AGB stars from the DENIS and 2MASS surveys.”
A wide-field H I mosaic of Messier 31 - II. The disk warp, rotation, and the dark matter halo (2009, background): “A detailed description of the spherical trigonometry involved can be found in the literature (e.g. van der Marel & Cioni 2001).”
The gravitational and hydrodynamical interaction between the Large Magellanic Cloud and the Galaxy (2004, methodology): “The existence of a warp is inferred by Kim et al. (1998) from the gas velocity field in the disc of the LMC, and by van der Marel & Cioni (2001) and Olsen & Salyk (2002) using the apparent magnitude of different classes of stars as a distance indicator.”
The Magellanic Clouds Photomtric Survey: The Small Magellanic Cloud Stellar Catalog and Extinction Map (2001, background): “…variations in the mean magnitude may truly exists (for example due to a tilt of the LMC relative to a constant-distance surface; see van der Marel & Cioni (2001) for a demonstration of an analogous effects with the red giant branch tip magnitude), any localized variation, in particular one…”
A new look at the large-scale H I structure of the Large Magellanic Cloud (2002, background): “Recent AGB and RGB stellar distances give a more accurate near-side PA of 32◦ ± 8◦ (van der Marel & Cioni 2001).”
Testing LMC Microlensing Scenarios: The Discrimination Power of the SuperMACHO Microlensing Survey (2005, background): “Recent observations indicate that there might be kinematic (carbon star sample, Graff et al. 2000; RR Lyrae stars, Minniti et al. 2003) and photometric subcomponents of the LMC (Weinberg & Nikolaev 2001; van der Marel & Cioni 2001; van der Marel 2001).”
A NEW ELECTRON-DENSITY MODEL FOR ESTIMATION OF PULSAR AND FRB DISTANCES (2016, methodology): “Following van der Marel & Cioni (2001) and Haschke et al. (2012a), we model the LMC as a thick inclined disk with inclination angle = i 32 and line of nodes at astronomical position angle Q = 116 .”
New Understanding of Large Magellanic Cloud Structure, Dynamics, and Orbit from Carbon Star Kinematics (2002, background): “The position angle of the line of nodes is Θ = 129.9◦ ± 6.0◦, consistent with the value determined geometrically by van der Marel & Cioni (2001).”