An international team led by Prof. Jing-Kun Zhao of National Astronomical Observatories of Chinese Academy of Sciences (NAOC) revealed the origin of the halo stream LAMOST-N1 using the follow up observation with SUBARU/HDS. This research proves that LAMOST-N1 is the relics of a dwarf galaxy and is one of the few halo streams clumped both in kinematic space and in chemical space. This work provides great help for our understanding merging history of Milky Way, and has been published online in the Astrophysical Journal at http://adsabs.harvard.edu/abs/2018ApJ...868..105Z.

The Lambda-CDM suggests hierarchical merging mechanism of the galaxies formation. The detection of the spatial clumped streams, such as Sagittarius stream and orphan stream, provided the direct observing evidence for this theory. The dwarf galaxies earlier accreted by Milky Way will be spread all over the sky after several orbital time. However, they still kept clumped in kinematic space and will be observed when passing the solar neighborhood. About 20 halo streams in kinematic space have been detected. LAMOST-N1 is the halo stream detected by the research team led by Gang Zhao of NAOC using LAMOST DR2 archive. With the follow up observation, the abundance pattern of LAMOST-N1 wereanalyzed. The [α/Fe] of LAMOST-N1 showed about 0.1 dex lower than the galactic stars with the same metallicity (left panel of figure 1), which is consistent with the low-α halo stars of Nissen (the purple diamonds  in figure 1, relics of dwarf galaxies from the Milky Way accretion). [Ba/Y] was 0.2dex higher than galactic stars (right panel of figure 1), and the Cr abundance was also 0.15dex lower than that of field stars of the Milky Way. All these indicated that lamost-N1 came from the extragalactic system.

Prof. Gang Zhao has contributed to this work as key co-authors. This work was supported by NSFC and Chinese Academy of Sciences.

Figure: comparisons [α/Fe] and [Ba/Y]. The red rectangles represent the member stars of LAMOST-N1; the small black dots represent the field stars of the Milky Way; the triangles represent the member stars of dwarf galaxies; the green diamonds represent the thick disk populations of Nissen; the blue diamonds represent the Nissen high-alpha halo stars of Nissen, and the purple diamonds represent the low-alpha halo stars of Nissen.