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

Abstract

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 −Vgsr 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.