The oldest and most metal-poor Milky Way stars form a spherical halo where they move about like atoms in a hot gas, which in turn prompts two major formation scenarios of our galaxy: extended hierarchical accretion and rapid collapse. RR Lyrae stars, which are found both in the Milky Way and the LMC, are excellent tracers of old and metal-poor populations.
By measuring the movement of 43 RR Lyrae stars in the inner regions of the LMC, the team determined that a moving hot, metal-poor, old halo also exists in the LMC, suggesting that the Milky Way and smaller, more irregular galaxies like the LMC have similar early formation histories.
The research, titled "Kinematic Evidence for an Old Stellar Halo in the Large Magellanic Cloud," is featured in the Sept. 12 issue of Science.
Kem Cook of Livermore's Institute of Geophysics and Planetary Physics and part of the Massive Compact Halo Objects (MACHO) team, which previously discovered the RR Lyrae stars in the LMC, noted that they are an easily identified tracer of an old, metal-poor population. The LMC is more than 160,000 light-years away from our galaxy.
"The bottom line is that the Large Magellanic Cloud seems to have had a similar early formation history as the Milky Way," Cook said. "It created a spherical component that is not rotationally supported, but the stars have high random velocities, like a hot gas."
Cook, along with scientists from Universidad Pontifica Catolica in Chile, the European Southern Observatory, Columbia Astrophysics Laboratory and the Mount Stromlo Observatory at The Australian National University, observed the LMC RR Lyrae stars in January 2003 and measured the radial velocity dispersion using the European Southern Obervatories VLT.
The large-velocity dispersion of the LMC RR Lyrae stars scales to the Milky Way RR Lyrae star's velocity dispersion and indicates that metal-poor old stars in the LMC are distributed in a halo population.
Models of halo formation by accretion indicate that these old objects formed in small satellite galaxies that were subsequently accreted (eaten up) by the galaxy. Meanwhile, models of halo formation by dissipational collapse indicate that the halo formed rapidly before the disk collapsed. The researchers applied these models to smaller galaxies and observed a halo population in the LMC by its oldest objects.
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