An Alternative Origin for Hypervelocity Stars

MARIO G. ABADI, JULIO F. NAVARRO, MATTHIAS STEINMETZ

ASTROPHYSICAL JOURNAL

Institute of Physics

Año: 2009 vol. 691 p. 63 - 66

0004-637X

Halo stars with unusually high radial velocity ("hypervelocity" stars,or HVS) 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 HVS compilation and find peculiarities that are unexpected inthis black hole-ejection scenario. For example, a large fraction ofHVS 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 thiscase, the escape velocity at $sim 50$ kpc would be $sim 600$ km/sand very few HVS would be truly unbound. We use numerical simulationsto show that disrupting dwarf galaxies may contribute halo stars withvelocities up to and sometimes exceeding the nominal escape speed ofthe system. These stars are arranged in a thinly-collimated outgoing``tidal tail´´ stripped from the dwarf during its latest pericentricpassage. We speculate that some HVS may therefore be tidal debris froma dwarf recently disrupted near the center of the Galaxy. In thisinterpretation, the angular clustering of HVS results 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 distinct from the black-hole ejectionmechanism and that should be testable with improved HVS datasets.