[Paper Review] A Population of Accreted SMC Stars in the LMC

Knut Olsen, Dennis Zaritsky+3 more|arXiv (Cornell University)|May 31, 2011

Stellar, planetary, and galactic studies48 references57 citations

TL;DR

This study identifies a population of ~5% of stars in the Large Magellanic Cloud (LMC) with kinematics inconsistent with the LMC's disk rotation, indicating they were accreted from the Small Magellanic Cloud (SMC). Using radial velocities, metallicity measurements from Ca triplet lines, and spatial distribution, the authors show these stars are counter-rotating or in a highly inclined disk, linked to H I arms previously thought to be tidal debris, suggesting the arms are instead infalling material from the SMC.

ABSTRACT

We present an analysis of the stellar kinematics of the Large Magellanic Cloud based on ~5900 new and existing velocities of massive red supergiants, oxygen-rich and carbon-rich AGB stars, and other giants. After correcting the line-of-sight velocities for the LMC's space motion and accounting for asymmetric drift in the AGB population, we derive a rotation curve that is consistent with all of the tracers used, as well as that of published HI data. The amplitude of the rotation curve is v_0=87+/-5 km s^-1 beyond a radius R_0=2.4+/-0.1 kpc, and has a position angle of the kinematic line of nodes of theta=142 degrees +/-5 degrees. By examining the outliers from our fits, we identify a population of 376 stars, or >~5% of our sample, that have line-of-sight velocities that apparently oppose the sense of rotation of the LMC disk. We find that these kinematically distinct stars are either counter-rotating in a plane closely aligned with the LMC disk, or rotating in the same sense as the LMC disk, but in a plane that is inclined by 54 degrees +/- 2 degrees to the LMC. Their kinematics clearly link them to two known HI arms, which have previously been interpreted as being pulled out from the LMC. We measure metallicities from the Ca triplet lines of ~1000 LMC field stars and 30 stars in the kinematically distinct population. For the LMC field, we find a median [Fe/H]=-0.56 +/- 0.02 with dispersion of 0.5 dex, while for the kinematically distinct stars the median [Fe/H] is -1.25 +/- 0.13 with a dispersion of 0.7 dex. The metallicity differences provide strong evidence that the kinematically distinct population originated in the SMC. This interpretation has the consequence that the HI arms kinematically associated with the stars are likely falling into the LMC, instead of being pulled out.

Motivation & Objective

• To investigate the internal kinematics of the Large Magellanic Cloud (LMC) using radial velocities of massive red supergiants and AGB stars.
• To identify and characterize a kinematically distinct stellar population within the LMC that may indicate accretion from the Small Magellanic Cloud (SMC).
• To determine the origin of H I arms in the LMC by linking their kinematics to stellar populations.
• To assess the metallicity and spatial distribution of the anomalous population to test its SMC origin.

Proposed method

• Acquired ~5900 radial velocities from new and existing spectroscopic observations of red supergiants, AGB stars, and other giants using the Hydra-CTIO multi-fiber spectrograph on the 4-m Blanco telescope.
• Corrected line-of-sight velocities for the LMC’s bulk space motion and applied asymmetric drift corrections to the AGB population to derive a consistent rotation curve.
• Identified kinematically distinct stars by analyzing velocity outliers from the rotation curve fit, using spatial and kinematic clustering to define the anomalous population.
• Measured metallicities from Ca II triplet absorption lines in ~1000 LMC field stars and 30 stars in the anomalous population to compare with SMC metallicities.
• Compared the color-magnitude diagram of the anomalous population with that of the outer SMC to assess photometric consistency.
• Modeled the kinematic geometry of the anomalous population to determine whether it is counter-rotating in the LMC plane or rotating in a highly inclined disk.

Experimental results

Research questions

• RQ1 What is the origin of the kinematically distinct stellar population in the LMC, and is it consistent with accretion from the SMC?
• RQ2 How do the metallicities of the anomalous stars compare to those of the LMC and SMC, and what does this imply about their origin?
• RQ3 Are the H I arms associated with the anomalous stars truly tidal debris, or could they be infalling material from the SMC?
• RQ4 What is the spatial and kinematic structure of the anomalous population, and how does it differ from the normal LMC disk?
• RQ5 Does the kinematic and metallicity data support a reinterpretation of the H I arms as infalling rather than tidally stripped?

Key findings

• The LMC’s rotation curve, derived from red supergiants, AGB stars, and H I data, is consistent across tracers, with a rotation speed of v₀ = 87 ± 5 km s⁻¹ beyond R₀ = 2.4 ± 0.1 kpc.
• A population of 376 stars, or ~5% of the sample, exhibits kinematics inconsistent with the LMC disk rotation, indicating they are either counter-rotating in the LMC plane or rotating in a disk inclined by 54° ± 2° to the LMC plane.
• The kinematically distinct stars are predominantly AGB stars and are spatially offset from the LMC bar, avoiding the central region where normal AGB stars are abundant.
• The median metallicity of the anomalous population is [Fe/H] = -1.25 ± 0.13 dex with a dispersion of 0.7 dex, significantly lower than the LMC field median of [Fe/H] = -0.56 ± 0.02 dex, consistent with SMC origin.
• The J - [3.6], [3.6] color-magnitude diagram of the anomalous population matches that of the outer SMC when shifted to the same distance, supporting a SMC origin.
• The H I arms associated with the anomalous stars are likely not tidally stripped from the LMC but instead represent infalling gas from the SMC, implying a revised model for the formation of the Magellanic Stream and Leading Arm.