Paper detail

Update on the Cetus Polar Stream and its Progenitor

We trace the Cetus Polar Stream (CPS) with blue horizontal branch (BHB) and red giant stars (RGBs) from Data Release 8 of the Sloan Digital Sky Survey (SDSS DR8). Using a larger dataset than was available previously, we are able to refine the measured distance and velocity to this tidal debris star stream in the south Galactic cap. Assuming the tidal debris traces the progenitor&#39;s orbit, we fit an orbit to the CPS and find that the stream is confined between ~24-36 kpc on a rather polar orbit inclined 87 degrees to the Galactic plane. The eccentricity of the orbit is 0.20, and the period ~700 Myr. If we instead matched N-body simulations to the observed tidal debris, these orbital parameters would change by 10% or less. The CPS stars travel in the opposite direction to those from the Sagittarius tidal stream in the same region of the sky. Through N-body models of satellites on the best-fitting orbit, and assuming that mass follows light, we show that the stream width, line-of-sight depth, and velocity dispersion imply a progenitor of at least 10^8 solar masses. However, the density of stars along the stream requires either a disruption time on the order of one orbit, or a stellar population that is more centrally concentrated than the dark matter. We suggest that an ultra-faint dwarf galaxy progenitor could reproduce a large stream width and velocity dispersion without requiring a very recent deflection of the progenitor into its current orbit. We find that most Cetus stars have metallicities of -2.5 < [Fe/H] < -2.0, similar to the observed metallicities of the ultra-faint dwarfs. Our simulations suggest that the parameters of the dwarf galaxy progenitors, including their dark matter content, could be constrained by observations of their tidal tails through comparison of the debris with N-body simulations.