But it is 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 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.

Using two series of photographic plates, taken at La Silla (European Southern Observatory) and digitized by the MAMA (operated at the Centre d’Analyse des Images, Observatoire de Paris), Patrick Cseresnjes and his collaborators detected about 2,000 RR Lyrae stars in Sgr spread over 50 square degrees.

The spatial distribution of these stars allows them to map the northern extension of Sgr, where the galactic stars outnumber those of Sgr by a factor up to a thousand. Compared to other satellites of the Milky Way, Sgr seems to be much more massive and extended.

Stellar evolution theory indicates that RR Lyraes are more than 10 Gigayears old. A catalog of such stars offers therefore a unique opportunity to determine the progenitor of Sgr. The most obvious information available is the period, which is very accurate and independent of crowding and extinction, allowing robust comparisons between different systems.

Patrick Cseresnjes and his collaborators compared the period distribution of RR Lyrae stars in Sgr with those of all other dwarf galaxies with a known RR Lyrae population. The similarity with the Large Magellanic Cloud (LMC) clearly stands out.

This similarity is even more striking when one considers that there are no other two distributions showing such a high correlation. Statistical tests show that an identical parent distribution for Sgr and the LMC cannot be ruled out, in spite of the high resolution provided by the large size of the samples in both systems.

The similarity between Sgr and the LMC is not restricted to RR Lyrae stars, but has also been observed through other populations, such as carbon stars, in 1998, or red giant branch stars, in 2001. These similarities strongly suggest that both systems have similar stellar populations.

This means that Sgr could be a debris pulled out of the LMC after a collision and has been injected on its present orbit only recently. Possible configurations are a collision between the LMC and the galaxy or the Small Magellanic Cloud.

This scenario, though attractive, raises many questions which need to be addressed. When did the collision occur? What happened to the gas? How can the present orbital planes of Sgr and the LMC seem to be perpendicular to each other?

Future numerical simulations will assess the feasibility of each of these scenarios.

(References: Cseresnjes P., 2001, "RR Lyrae stars in the Sagittarius dwarf galaxy: Period analysis", Astronomy & Astrophysics 375 909; Cseresnjes P., Alard C. & Guibert J., 2001, "Structure of the Sagittarius dwarf galaxy at low Galactic latitudes"; Astronomy & Astrophysics 357 871 Cseresnjes P., Ph.D. Thesis, Observatoire de Paris.)

26-Feb-2002