Very large array H53α and H92α line observations of the central region of NGC 253

We present new VLA observations toward NGC 253 of the recombination line H53 alpha ( 43 GHz) at an angular resolution of 1.'' 5 x 1.'' 0. The free-free emission at 43 GHz is estimated to be similar to 140 mJy, implying a star formation rate of 2 M-circle dot yr(-1) in the nuclear region of this starburst galaxy. A reanalysis is made for previously reported H92 alpha observations carried out with angular resolution of 1.'' 5 x 1.'' 0 and 0.'' 36 x 0.'' 21. Based on the line and continuum emission models used for the 1.'' 5 x 1.'' 0 angular resolution observations, the RRLs H53 alpha and H92 alpha are tracers of the high-density ( similar to 10(5) cm(-3)) and low-density ( similar to 103 cm(-3)) thermally ionized gas components in NGC 253, respectively. The velocity fields observed in the H53 alpha and H92 alpha lines ( 1.'' 5; 1.'' 0) are consistent. The velocity gradient in the central similar to 18 pc of the NE component, as observed in both the H53 alpha and H92 alpha lines, is in the opposite direction to the velocity gradient determined from the CO observations. The enclosed virial mass, as deduced from the H53 alpha velocity gradient over the NE component, is similar to 5 x 10(6) M-circle dot in the central similar to 18 pc region. The H92 alpha line observations at high angular resolution ( 0.'' 36 x 0.'' 21) reveal a larger velocity gradient, along a P: A: similar to - 45 degrees on the NE component, of similar to 110 km s(-1) arcsec(-1). The dynamical mass estimated using the high angular resolution H92 alpha data ( similar to 7; 106 M-circle dot) supports the existence of an accreted massive object in the nuclear region of NGC 253.