Local Group Dwarf Elliptical Galaxies: II. Stellar Kinematics to Large Radii in NGC 147 and NGC 185

Abstract

We present kinematic and metallicity profiles for the M 31 dwarf elliptical (dE) satellite galaxies NGC 147 and NGC 185. The profiles represent the most extensive spectroscopic radial coverage for any dE galaxy, extending to a projected distance of 8 half-light radii (8r_(eff) ~ 14'). We achieve this coverage via Keck/DEIMOS multislit spectroscopic observations of 520 and 442 member red giant branch stars in NGC 147 and NGC 185, respectively. In contrast to previous studies, we find that both dEs have significant internal rotation. We measure a maximum rotational velocity of 17 ± 2 km s^(–1) for NGC 147 and 15 ± 5 km s^(–1) for NGC 185. While both rotation profiles suggest a flattening in the outer regions, there is no indication that we have reached the radius of maximum rotation velocity. The velocity dispersions decrease gently with radius with average dispersions of 16 ± 1 km s^(–1) and 24 ± 1 km s^(–1) for NGC 147 and NGC 185, respectively. The average metallicities for NGC 147 and NGC 185 are [Fe/H] = –1.1 ± 0.1 and [Fe/H] = –1.3 ± 0.1, respectively; both dEs have internal metallicity dispersions of 0.5 dex, but show no evidence for a radial metallicity gradient. We construct two-integral axisymmetric dynamical models and find that the observed kinematical profiles cannot be explained without modest amounts of non-baryonic dark matter. We measure central mass-to-light ratios of M/L_V = 4.2 ± 0.6 and M/LV = 4.6 ± 0.6 for NGC 147 and NGC 185, respectively. Both dE galaxies are consistent with being primarily flattened by their rotational motions, although some anisotropic velocity dispersion is needed to fully explain their observed shapes. The velocity profiles of all three Local Group dEs (NGC 147, NGC 185, and NGC 205) suggest that rotation is more prevalent in the dE galaxy class than previously assumed, but often manifests only at several times the effective radius. Since all dEs outside the Local Group have been probed to only inside the effective radius, this opens the door for formation mechanisms in which dEs are transformed or stripped versions of gas-rich rotating progenitor galaxies.

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