2006ApJ...644..914R

We present new VLA observations toward NGC 253 of the recombination line H53α (43 GHz) at an angular resolution of 1".5x1".0. The free-free emission at 43 GHz is estimated to be ∼140 mJy, implying a star formation rate of 2 M_☉/yr in the nuclear region of this starburst galaxy. A reanalysis is made for previously reported H92α observations carried out with angular resolution of 1".5x1".0 and 0".36x0".21. Based on the line and continuum emission models used for the 1".5x1".0 angular resolution observations, the RRLs H53α and H92α are tracers of the high-density (∼10⁵/cm3) and low-density (∼10³/cm3) thermally ionized gas components in NGC 253, respectively. The velocity fields observed in the H53α and H92α lines (1".5x1".0) are consistent. The velocity gradient in the central ∼18 pc of the NE component, as observed in both the H53α and H92α lines, is in the opposite direction to the velocity gradient determined from the CO observations. The enclosed virial mass, as deduced from the H53α velocity gradient over the NE component, is ∼5x10⁶ M_☉in the central ∼18 pc region. The H92α line observations at high angular resolution (0".36x0".21) reveal a larger velocity gradient, along a P.A.~-45° on the NE component, of ∼110 km/s/arcsec. The dynamical mass estimated using the high angular resolution H92α data (∼7x10⁶ M_☉) supports the existence of an accreted massive object in the nuclear region of NGC 253.