The Sagittarius dwarf galaxy is our nearest neighbor. Yet it has been discovered only recently, in 1994, being hidden by the stars and dust in our own Galaxy, the Milky Way. It is however possible today to better know this companion galaxy, thanks to variable stars, the RR Lyrae, in which Sgr-dw is particularly rich. In a recent paper, Patrick Cseresnjes, from Paris Observatory, shows for the first time that Sgr-dw is not typical of other satellites of the Milky Way, but reveals instead striking similarities with the Large Magellanic Cloud. He proposes and argues for the astonishing and original scenario that both systems might share a common progenitor.
The Sagittarius dwarf galaxy (Sgr hereafter) is a most interesting object. Located at only 75 000 light-years from the Sun and 50 000 light-years from the Galactic Center, it is the nearest known satellite of the Milky Way. In spite of this proximity, Sgr has been discovered only in 1994 because it was hidden to us by foreground Galactic stars.
Sgr is now in process of being swallowed by our own Galaxy after complete disruption caused by Galactic tides, showing that at least part of the stellar Halo has formed from accretion of smaller constituents. However, we still lack a clear understanding of this galaxy because the high degree of contamination by foreground Galactic stars and the varying extinction make it almost impossible to get a clean sample of stars. Fortunately, Sgr contains a fair amount of RR Lyrae stars. These variable stars have characteristic light curves and can easily be detected and separated from Galactic stars. Indeed, once their type is identified by their light curve, their absolute luminosity is derived, and the measure of their apparent luminosity gives their distance.
Patrick Cseresnjes | alphagalileo
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