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 {\bf ("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 {\bf HVSs} compilation and find peculiarities that are unexpected inthis black hole-ejection scenario. For example, a large fraction of{\bf 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 $2.5\times 10^{12} h^{-1} \, M_{\odot}$dark halo with virial velocity of $\sim 220$ km/s. In this case, theescape velocity at {\bf $\sim$ 50 kpc} would be $\sim$ 600 km/s and very few{\bf HVSs} would be truly unbound. We use numerical simulations to show thatdisrupting dwarf galaxies {\bf 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 {\bf HVSs} may therefore be tidal debris froma dwarf recently disrupted near the center of the Galaxy. {\bf According tothis interpretation,} the angular clustering of {\bf HVSs} results {\bf 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 {\bf different} from the {\bf black hole-ejection}mechanism and that should be testable with improved {\bf HVSs} datasets.