Oral Presentation

The nature of the progenitor of the M31 north-western stream: globular clusters as milestones of its orbit

Author(s): Takanobu Kirihara (Chiba University) Yohei Miki (University of Tokyo); Masao Mori (University of Tsukuba)

Presenter: Takanobu Kirihara (Chiba University)

The missing satellite (sub-halo) problem is well-known as a severe problem in the structure formation of the Universe based on widely accepted cold dark matter model. A long and narrow stellar stream can be a useful tracer of dark sub-halos to investigate the missing-satellite problem. The north-western stellar stream (NWS) in the halo of the Andromeda galaxy can be an excellent tool for it. For the first step, we examine the nature, possible orbits and physical properties of the progenitor of the stellar stream. We here assumed the progenitor to be an accreting dwarf galaxy with globular clusters (GCs). It is, in general, difficult to determine the progenitor's orbit precisely because of many necessary parameters. To constrain the orbit of the progenitor, we use radial velocities of five GCs whose positions and radial velocities suggest an association with the stream. We succeed to limit its orbit by test-particle simulations and estimate the efficiency of the tidal disruption. A distance measurement of the NWS using Subaru/HSC further limits possible progenitor's orbit and its properties. As a result, the progenitor has at least the mass of 2x10^6 M_sun and half-light radius of 200 pc. Moreover, brief N-body simulations successfully reproduce the basic observed features of the NWS and the GCs' line-of-sight velocities.