Jovenes e Imprudentes: exceso de velocidad en la Galaxia

MARIO G. ABADI

Cordoba

Jornada; Primeras Jornadas sobre Astrofisica Estelar; 2008

Observatorio Astronomico de Cordoba

Halo stars with unusually high radial velocity {f ("hyper-velocity" stars,or HVSs)} are thought to be stars unbound to the Milky Way thatoriginate from the gravitational interaction of stellar systems withthe supermassive black hole at the Galactic center. We examine thelatest {f HVSs} compilation and find peculiarities that are unexpected inthis black hole-ejection scenario. For example, a large fraction of{f HVSs} cluster around the constellation of Leo and share a common traveltime of $sim 100$-$200$ Myr. Furthermore, their velocities are notreally extreme if, as suggested by recent galaxy formation models, theMilky Way is embedded within a $2.5 imes 10^{12} h^{-1} , M_{odot}$dark halo with virial velocity of $sim 220$ km/s. In this case, theescape velocity at {f $sim$ 50 kpc} would be $sim$ 600 km/s and very few{f HVSs} would be truly unbound. We use numerical simulations to show thatdisrupting dwarf galaxies {f may contribute to form halo stars with velocities of up to the nominal escape speed of the system and sometimes evenexceeding it.} These stars are arranged in a thinly-collimated outgoing``tidal tail´´ stripped from the dwarf during its latest pericentricpassage. We speculate that some {f HVSs} may therefore be tidal debris froma dwarf recently disrupted near the center of the Galaxy. {f According tothis interpretation,} the angular clustering of {f HVSs} results {f because, from ourperspective,} the tail is seen nearly ``end on´´, whereas the commontravel time simply reflects the fact that these stars were strippedsimultaneously from the dwarf during a single pericentricpassage. This proposal is eminently falsifiable, since it makes anumber of predictions that are {f different} from the {f black hole-ejection}mechanism and that should be testable with improved {f HVSs} datasets.