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Astronomer in NAOC found the chemical evidence of LAMOST-N1 to be the relics of a dwarf galaxy
Update time: 2018-12-23
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Recently, Jingkun Zhao, Gang Zhao in NAOC and professor Aoki in NAOJ performed the follow up observation for six member stars in LAMOST-N1 with SUBARU/HDS and analyzed the abundance pattern of these six stars. The [α/Fe] of LAMOST-N1 showed about 0.1 dex lower than the galactic stars with the same metallicty (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. Since the abundance pattern of LAMOST-N1 is not completely consistent with the known typical dwarf galaxies, its progenitor might be a larger dwa  rf galaxy or an earlier accreted dwarf galaxy. The work has been published in the ApJ.

Figure 1: 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.

The Lambda-CDM suggests hierarchical merging mechanism of the galaxies formation. The detection of the spatial clumped streams (such as Sagittarius stream, orphan stream, etc.) 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 such a halo stream detected with LAMOST DR2 archive. However, origins of most of these streams are unknown or remain controversial. The abundance patterns of some streams are very similar with the Milky Way. LAMOST-N1 is one of the few halo streams which are clumped in kinematic space and are different with Galactic filed stars for chemical abundances. This research will provide great help for our understanding merging history of Milky Way.

Paper links: http://adsabs.harvard.edu/abs/2018ApJ...868..105 z

news from:http://www.lamost.org/public/node/322?locale=en

Nanjing Institute of Astronomical Optics & Technology ,National Astronomical Observatories ,CAS