2018MNRAS.477.2419M

We selected blue horizontal branch (BHB) stars within the expected distance range and sky position of the Hermus Stream from Data Release 10 of the Sloan Digital Sky Survey. We identify a moving group of 19 BHB stars that are concentrated within two degrees of the Hermus Stream, between 10 and 14 kpc from the Sun. The concentration in velocity is inconsistent with a Gaussian distribution with 98 per cent confidence (2.33 sigma). The stars in the moving group have line-of-sight velocities of v_gsr ∼ 50 km s^–1, a velocity dispersion of σ_v <= 11 km s^–1, a line-of-sight depth of ∼1 kpc, and a metallicity of [Fe/H] = -2.1 ± 0.4. The best-fitting orbit has a perigalacticon of ∼4 kpc, apogalacticon of ∼17 kpc, orbital period of ∼247 Myr, eccentricity e = 0.62, and inclination i ∼ 75° from b = 90°. The BHB stars in the stream are estimated to be 12 Gyr old. An N-body simulation of a mass-follows-light ultrafaint dwarf galaxy with mass 10⁶ M_☉ and radius 40 pc is consistent with the observed properties. The properties of the identified moving group of 19 BHB stars are close enough to those of the Hermus Stream (which is traced predominantly in turnoff stars) that we find it likely that they are associated. If that is the case, then our orbit fit would imply that there is no relationship between the Hermus and Phoenix streams, as previously proposed.