Update on the Cetus Polar Stream and its progenitor

Abstract

We trace the Cetus Polar Stream (CPS) with blue horizontal branch and red giant stars from Data Release 8 of the Sloan Digital Sky Survey. Using a larger data set 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 that the tidal debris traces the progenitor's orbit, we fit an orbit to the CPS and find that the stream is confined between ∼24 and 36 kpc on a rather polar orbit inclined 87° to the Galactic plane. The eccentricity of the orbit is 0.20, and the period is ∼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 ≳ 108 M ⊙. 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 ultrafaint 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 ultrafaint 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. © 2013. The American Astronomical Society. All rights reserved..