| 其他摘要 | High velocity stars (HiVels) exist in the Milky Way with a total velocity (V_GSR) greater than 300 km/s. Among them, those bound by the Galactic gravitational potential with V_GSR between 300 km/s and escape velocity (V_esc) are the most reliable tracers to measure the mass distribution of the Milky Way (MW), i.e., the Galactic gravitational potential.The unbound stars with V_GSR greater than V_esc are called hyper-velocity stars (HVSs), of which the trajectories provide strong constraints on the MW's mass distribution and the shape of Galactic dark matter (DM) halo. The origins and ejection mechanisms of the extreme velocity stars are related to extreme astrophysical processes, such as black holes disrupting binary-star systems (Hills mechanism), tidal stripping the member stars of dwarf galaxies, etc. To this end, we systematically search for HiVels/HVSs, and construct large samples based on Gaia astrometric data and large-scale spectral survey data (GALAH, APOGEE, RAVE, SDSS, LAMOST); furthermore, we trace back the orbits of those stars in the Galactic potential to reveal their origins and rejection mechanisms.First, We constructed a radial velocity (RV) standard star sample containing 46,753 stars based on APOGEE DR17, which is used to detect and correct the RV zero point (RVZP) of large-scale spectroscopic survey projects. These stars cover the sky almost evenly, with 62% being red giants and 38% being main-sequence stars. These RV standard stars are stable on a baseline longer than 200 days (54% longer than one year and 10% longer than five years) with a average stability better than 215 m/s. The average observation number of those stars are 5 and each observation is required to have spectral signal-to-noise-ratio (SNR) greater than 50 and RV measurement error smaller than 500 m/s. Based on the new APOGEE RV standard star catalog, we have checked the RVZPs for current large-scale spectroscopic surveys including RAVE, LAMOST, GALAH and Gaia. By carefully analysis, we estimate their mean RVZP to be +0.149 km/s, +4.574 km/s (for LRS), -0.031 km/s and +0.014 km/s, respectively, for the four surveys. In the RAVE, LAMOST (for MRS), GALAH and Gaia surveys, RVZP exhibits systematic trend with stellar parameters (mainly [Fe/H], T_eff, log g, G_BP-G_RP and G_RVS). Correcting these small but clear RVZPs present in large-scale spectroscopic surveys are crucial for constructing high-quality HiVel/HVS samples and back-tracking analyses.Secondly, we identified 12,784 HiVels with a total velocity V_GSR >= 300 km/s by matching the spectroscopic data sets of RAVE DR5, SDSS DR12, LAMOST DR8, APOGEE DR16, and GALAH DR2 with Gaia EDR3. Of them 52 stars (40 discovered for the first time) have V_GSR exceeding the escape velocity at their location, i.e., HVS candidates. Interestingly, all these HVSs are metal-poor and late-type stars, which is significantly different from the previous HVSs identified only through extreme radial velocities that are mainly massive, metal-rich early-type stars. This finding suggests that our newly discovered HVSs may have a different ejection mechanism from the previous metal-rich early-type ones.Third, we thus systematically analyze the origins and ejection mechanisms of the HiVels/HVSs. We reconstruct the backward-integrated orbits of the 547 extreme velocity stars (HiVels or even HVSs) with V_GSR >= 0.8 V_esc in our sample. According to the orbit analysis, we found that no HVSs are ejected definitely from the Galactic center, and that there are eight late-type metal-poor HiVels having a closest distance to the Galactic center within 1 kpc. More interestingly, we found that 15 HiVels had close encounters with the satellite galaxy Sgr dSph, suggesting that they may have originated from Sgr dSph. The analysis in terms of [alpha/Fe]--[Fe/H] diagram also supports their origin from Sgr dSph. Among them the most typical is the HVS J1443+1453. By reconstructing its trajectory in the Galactic potential, we find that the orbit of J1443+1453 intersects closely with that of Sgr dSph 37.8 Myr ago, when the latter has its latest pericentric passage through the Milky Way. J1443+1453 is most likely to be tidally stripped from Sgr dSph. If there is a massive/intermediate-mass black hole harbored in Sgr dSph,then it is also possible that J1443+1453 is ejected via Hills mechanism. We then extended the back-tracking and [alpha/Fe]--[Fe/H] analyses to all the 547 extreme velocity stars, and the preliminary results support that many of them may originate from dwarf galaxies or star clusters; in particular, we discovered a HiVel (J0731) was ejected from the globular cluster M15 via Hills mechanism (see below). Based on the above observational results, we proposed a general picture that star ejection from Galactic subsystems such as dwarf galaxies and globular clusters is an important channel to produce HiVels or even HVSs, particularly the metal-poor late-type halo population.Finally, we presented the detailed analysis of the origin and ejection mechanism of J0317 from the globular cluster M15. J0731 and M15 had a close encounter on their backward-integrated orbits approximately 21 Myr ago, with the closest distance being 58 pc, which is less than M15's tidal radius 132 pc. The metal abundance [Fe/H] and [alpha/Fe] of J0731 both are highly consistent with those of M15. Furthermore, its position on the color-absolute magnitude diagram aligns precisely with the fiducial sequence of M15's member stars. These observational pieces of evidence strongly suggest an association between J0731 and M15 (with a confidence level of 5.4 sigma). The ejection velocity of J0731 is 548 km/s, and we conclude that, after excluding other common ejection mechanisms based on the observational data of J0731 and M15, it is most likely ejected by a massive black hole harbored in M15 through the Hills mechanism. To eject J0731, a intermediate-mass black hole (IMBH) with a mass of at least 100 Msun (with a confidence level of 99.999%) is required. The observational identification of IMBHs will fill the gap between stellar-mass black holes and supermassive black holes, and reveal the origin of supermassive black holes. The capacity of current observational facilities is not able to dynamically resolve the presence of IMBHs and their effect to galactic observations. Our findings suggest that it is a feasible and clear approach to identify IMBHs by finding HiVels/HVSs originating from star clusters via Hills mechanism. We predict that in the Gaia and large-scale spectroscopic survey era, more IMBHs will be find in this way. |
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