Revisiting candidate high-velocity stars associated with the Sagittarius dwarf spheroidal galaxy

Abstract

Hypervelocity stars (HVSs) are valuable tracers of extreme dynamical processes. The Sagittarius dwarf spheroidal galaxy (Sgr dSph), currently undergoing tidal disruption, offers a unique environment to search for such stars. We aim to identify candidate HVSs dynamically linked to the Sgr dSph and to assess their possible origins. Using Gaia DR3, DESI DR1, and LAMOST DR12, we selected stars with galactocentric velocities above 400 km\,s$^{-1}$ and traced their orbits in a realistic Galactic potential including the Sgr dSph and the Large Magellanic Cloud. We then tested three scenarios for their origin: the Hills mechanism, tidal disruption, and random halo star encounters. We identified 95 candidates passing within 2.5 half-mass radii of the Sgr dSph. Their kinematics are inconsistent with production by the Hills mechanism or tidal disruption but are well reproduced by halo stars that naturally cross the Sgr orbit. Furthermore, their metallicity distribution is consistent with that of the Milky Way halo rather than the Sgr stream or Sgr dSph. Our results suggest that our candidates and those in previous studies are most likely halo stars rather than genuine Sgr-origin HVSs. This highlights the need to account for the halo population when inferring stellar origins from orbital analysis and that chemical abundances will be a valuable constraint in the future. While we detect no unbound Sgr HVSs, such a discovery would directly imply extreme dynamical processes. Our results serve as a basis for future studies with upcoming surveys.

Figures (10)

• Figure 1: Distribution of galactocentric total velocity ($v_{\mathrm{GC}}$) as a function of galactocentric radius ($r_{\mathrm{GC}}$) for high-velocity stars ($P_{\mathrm{Sgr}}>0.5$). Solid, dashed, and dotted curves show the escape-velocity profiles predicted by three Milky Way potential models: Bovy2015, McMillan2017, and Cautun2020.
• Figure 2: Relative velocity $\Delta V$ with respect to the Sgr dSph at pericentric passage versus flight time $T_f$ on a logarithmic scale. Blue circles, orange triangles, and magenta squares represent results from this work, Li22, and Li23, respectively. Vertical red line indicates the 39.7 Myr flight time of the Sgr dSph since its most recent pericentric passage, with the shaded region representing the $1\sigma$ confidence interval.
• Figure 3: Distribution of high-velocity stars in Sgr Stream coordinates Vasiliev2021. Left panel: Radial velocity over Sgr stream coordinates $\Lambda_\odot$. Right panel: Tangential velocity in the $\Lambda_\odot$ direction over $\Lambda_\odot$. Blue circles, orange triangles, and magenta squares represent results from this work, Li22, and Li23, respectively.
• Figure 4: Distribution of galactocentric total velocity ($v_{GC}$) versus galactocentric radius ($r_{GC}$) for HVSs ejected via the Hills mechanism from the center of the Sgr dSph, color coded by each star's apparent magnitude in the Gaia RVS band. Results generated using the speedystar code assuming $M_{bh}=10^4 M_\odot$, ejection rate $\eta=10^{-4} yr^{-1}$ and a Salpeter IMF Salpeter1955 with $\kappa=2.35$. Solid, dashed, and dotted curves represent the escape-velocity profiles predicted by three Milky Way potential models (Bovy2015; McMillan2017; Cautun2020).
• Figure 5: Predicted number of observable HVSs, $N_\mathrm{HVS}$, in Gaia DR4 as a function of the IMF slope $\kappa$. Panels show results for different assumed central black hole masses in the Sgr dSph: $M_{bh}=10^3M_\odot$ (left), $5\times10^3M_\odot$ (center), and $10^4M_\odot$ (right). Solid orange, dashed sky-blue, and dotted bluish-green lines correspond to ejection rates of $\eta=10^{-2},10^{-3},\text{and}~10^{-4} \mathrm{yr}^{-1}$, respectively. Shaded regions indicate the 16th–84th percentile range from bootstrap resampling.