New tidal debris nearby the Sagittarius leading tail from the LAMOST DR2 M giant stars

摘要

We report two new sets of tidal debris nearby the Sagittarius(Sgr) tidal stream in the north Galactic cap(NGC) identified from the M giant stars in LAMOST DR2. The M giant stars located in the sky area of 210?<Λ< 290?, and having a distance of 10–20 kpc and [Fe/H]<-0.75 show clear bimodality in their velocity distribution. We denote the two peaks as Vel-3+83 for the one within a mean velocity of–3 km s^(-1) with respect to that of the well observed Sgr leading tail at the same Λ and Vel+162+26 for the other one with a mean velocity of 162 km s^(-1) with respect to the Sgr leading tail. Although the projected Λ–V_(gsr) relation of Vel-3+83 is very similar to the Sgr leading tail, the opposite trend in the Λ–distance relation as compared to the Sgr leading tail suggests Vel-3+83 has a different 3D direction of motion with any branch of the simulated Sgr tidal stream from Law & Majewski. Therefore, we propose it is new tidal debris not related to the Sgr stream. Similarly, the other substructure Vel+162+26, which is the same one as the NGC group discovered by Chou et al., also moves toward a different direction with respect to the Sgr stream, implying that it may have a different origin than the Sgr tidal stream.