New Understanding of Large Magellanic Cloud Structure, Dynamics, and Orbit from Carbon Star Kinematics

By R. P. van der Marel, D. Alves, E. Hardy, N. Suntzeff. 2002

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We formulate a new and coherent understanding of the structure and dynamics of the Large Magellanic Cloud (LMC) and its orbit around and interaction with the Milky Way. Much of our understanding of these issues hinges on studies of the LMC line-of-sight kinematics. The observed velocity field includes contributions from the LMC rotation curve V(R′), the LMC transverse velocity vector vt, and the rate of inclination change di/dt. All previous studies have assumed di/dt = 0. We show that this is incorrect and that, combined with uncertainties in vt, this has led to incorrect estimates of many important structural parameters of the LMC. We derive general expressions for the velocity field that we fit to kinematic data for 1041 carbon stars. We calculate vt by compiling and improving LMC proper-motion measurements from the literature, and we show that for known vt, all other model parameters are uniquely determined by the data. 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 in 2001. The rate of inclination change is di/dt = -0.37 ± 0.22 mas yr-1 = -103° ± 61° Gyr-1. This is similar in magnitude to predictions from N-body simulations by M. Weinberg, which predict LMC disk precession and nutation due to Milky Way tidal torques. The LMC rotation curve V(R′) has amplitude 49.8 ± 15.9 km s-1. This is 40% lower than what has previously (and incorrectly) been inferred from studies of H I, carbon stars, and other tracers. The line-of-sight velocity dispersion has an average value σ = 20.2 ± 0.5 km s-1, with little variation as a function of radius. The dynamical center of the carbon stars is consistent with the center of the bar and the center of the outer isophotes, but it is offset by 1.°2 ± 0.°6 from the kinematic center of the H I. The enclosed mass inside the last data point is MLMC(8.9 kpc) = (8.7 ± 4.3) × 109 M⊙, more than half of which is due to a dark halo. The LMC has a considerable vertical thickness; its V/σ = 2.9 ± 0.9 is less than the value for the Milky Way's thick disk (V/σ ≈ 3.9). Simple arguments for models stratified on spheroids indicate that the (out of plane) axial ratio could be ∼0.3 or larger. Isothermal disk models for the observed velocity dispersion profile confirm the finding of Alves & Nelson that the scale height must increase with radius. A substantial thickness to the LMC disk is consistent with the simulations of Weinberg, which predict LMC disk thickening due to Milky Way tidal forces. These affect LMC structure even inside the LMC's tidal radius, which we calculate to be rt = 15.0 ± 4.5 kpc (i.e., 17.°1 ± 5.°1). The new insights into LMC structure need not significantly alter existing predictions for the LMC's self-lensing optical depth, which to lowest order depends only on σ. The compiled proper-motion data imply an LMC transverse velocity vt = 406 km s-1 in the direction of position angle 78.°7 (with errors of ∼40 km s-1 in each coordinate). This can be combined with the observed systemic line-of-sight velocity, vsys = 262.2 ± 3.4 km s-1, to calculate the LMC velocity in the Galactocentric rest frame. This yields vLMC = 293 ± 39 km s-1, with radial and tangential components vLMC,rad = 84 ± 7 km s-1 and vLMC,tan = 281 ± 41 km s-1, respectively. This is consistent with the range of velocities that has been predicted by models for the Magellanic Stream. The implied orbit of the LMC has an apocenter-to-pericenter distance ratio ∼2.5 : 1, a perigalactic distance ∼45 kpc, and a present orbital period around the Milky Way of ∼1.5 Gyr. The constraint that the LMC is bound to the Milky Way provides a robust limit on the minimum mass and extent of the Milky Way dark halo: MMW ≥ 4.3 × 1011 M⊙ and rh ≥ 39 kpc (68.3% confidence). Finally, we present predictions for the LMC proper-motion velocity field, and we discuss how measurements of this may lead to kinematic distance estimates for the LMC.

Subfields: Astronomy and Astrophysics, Education, Instrumentation

View Paper: https://www.semanticscholar.org/paper/567a595c380f367e364e8c7efffad6bfd59e418c

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Citations in the corpus

The Parkes HI survey of the Magellanic System (2004, methodology): “…projected radial velocity of the LMC as a function of position of the sky can be expressed by vLMC = vX cos l cos b + vY sin l cos b + vZ sin b, (5) where (vX , vY , vZ) = (–56 ± 39 km s−1, −219 ± 23 km s−1, 186 ± 35 km s−1) is the LMC velocity vector as determined by van der Marel et al. (2002).”
The gravitational and hydrodynamical interaction between the Large Magellanic Cloud and the Galaxy (2004, methodology): “The morphology of the MS indicates that the orbit of the LMC is nearly polar and anticlockwise, as seen from the Sun, while kinematical data (van der Marel et al. 2002) imply that the present position at ∼50 kpc from the Galactic Centre is close to the perigalacticon.”
The stellar halo of the Galaxy (2008, background): “However, the origin of this halo is less clear because the LMC appears to have been tidally perturbed by the Milky Way (its disk is asymmetric and fairly thick as shown by van der Marel 2001; van der Marel et al. 2002).”
The APOSTLE project: Local Group kinematic mass constraints and simulation candidate selection (2015, background): “For the LMC, the error bar indicates the velocity range spanned by two different estimates, 64 km s−1 from van der Marel et al. (2002), and 87 km s−1 from Olsen et al. (2011).”
Merger Histories of Galaxy Halos and Implications for Disk Survival (2007, background): “The most massive Milky Way satellite, the LMC, was likely accreted with a mass no larger than m ∼ 4 × 1010h−1M⊙ (van der Marel et al. 2002; Robertson et al. 2005).”
Exploring Halo Substructure with Giant Stars. XI. The Tidal Tails of the Carina Dwarf Spheroidal Galaxy and the Discovery of Magellanic Cloud Stars in the Carina Foreground (2006, methodology): “The actual rotation curve corresponding to these points is shown in Figure 6 of van der Marel et al. (2002).”
SIMULATIONS OF THE MAGELLANIC STREAM IN A FIRST INFALL SCENARIO (2010, methodology): “All previous models of the Magellanic system have assumed the LMC is tidally truncated to a radius of 15 kpc (van der Marel et al. 2002), resulting in a total mass estimate of 2-3 ×1010M⊙ (e.g., GN96, Murai & Fujimoto 1980; Bekki & Chiba 2005).”
Is the SMC Bound to the LMC? The Hubble Space Telescope Proper Motion of the SMC (2006, methodology): “While the radial velocities of the Clouds have been measured to high precision (van der Marel et al. 2002; Harris & Zaritsky 2006), the velocity transverse to the line of sight (the proper motion) has been harder to constrain.”
The total mass of the Large Magellanic Cloud from its perturbation on the Orphan stream (2018, unknown): “For the LMC’s present-day position and velocity, we use proper motions of μ∗α = 1.91 ± 0.02 mas yr−1, μδ = 0.229 ± 0.047 mas yr−1 (Kallivayalil et al. 2013), a radial velocity of vr = 262.2 ± 3.4 km s−1 (van der Marel et al. 2002), and a distance of 49.97 ± 1.126 kpc (Pietrzyński et al. 2013).”
Milky Way mass models for orbit calculations (2012, unknown): “(a) Edelmann et al. (2005); (b) Przybilla et al. (2008); (c) Brown et al. (2010); (d) van der Marel et al. (2002); (e) Freedman et al. (2001); ( f ) Kallivayalil et al. (2006).”
A timing constraint on the (total) mass of the Large Magellanic Cloud (2015, background): “∼ 2.9× 109M ; van der Marel et al. 2002) reside in haloes with a virial mass∼ 2×1011M (Moster et al. 2013).”
Dwarf galaxy planes: the discovery of symmetric structures in the Local Group (2013, unknown): “Following the coordinate system as introduced in van der Marel et al. (2002), this corresponds to µα cos δ = 0.045 ± 0.013 mas yr−1 and µδ = −0.032 ± 0.012 mas yr−1 at the distance of 770 kpc assumed by van der Marel et al. (2012).”
The Origin of the Magellanic Stream and Its Leading Arm (2007, background): “…for the LMC filament where the sinusoidal variation is pronounced and the filament can be reliably traced back to its origin in the LMC, which has a well-defined disk (e.g., Weinberg & Nikolaev 2001; van der Marel 2001) and rotation curve (e.g., Kim et al. 1998; van der Marel et al. 2002).”
THE M31 VELOCITY VECTOR. II. RADIAL ORBIT TOWARD THE MILKY WAY AND IMPLIED LOCAL GROUP MASS (2012, methodology): “We then correct for the effect of the viewing perspective as described in van der Marel et al. (2002) and vdM08.”
Halo Substructure and the Power Spectrum (2002, background): “…also included the Small Magellanic Cloud (SMC, with Vmax = 60 km s −1, estimated by Stanimirović 2000 to include a substantial contribution from the baryonic component) and the Large Magellanic Cloud (LMC, with Vmax = 50 km s −1, van der Marel et al. 2002) in our cumulative velocity functions.”
GASS: THE PARKES GALACTIC ALL-SKY SURVEY. I. SURVEY DESCRIPTION, GOALS, AND INITIAL DATA RELEASE (2009, methodology): “…which is close to the interaction region predicted in older models (e.g., Connors et al. 2006; Yoshizawa & Noguchi 2003) of the Magellanic System but somewhat surprising given the revised proper motions for the Large and Small Magellanic Clouds (van der Marel et al. 2002; Kallivayalil et al. 2006).”
THE STAR FORMATION HISTORY OF THE LARGE MAGELLANIC CLOUD (2009, background): “…the older stellar populations to be well-mixed throughout the LMC, its stellar disk has a solid-body rotation curve out to roughly 4 kpc (van der Marel et al. 2002) and patterns imprinted by a spatially-variable star formation rate could persist in the LMC for several rotational periods (> 1…”
DWARF GALAXIES WITH OPTICAL SIGNATURES OF ACTIVE MASSIVE BLACK HOLES (2013, background): “3 × 109 M , which is approximately the stellar mass of the LMC (van der Marel et al. 2002).”