We present new Very Large Array ( VLA ) observations toward NGC 253 of the recombination line H53 \alpha ( 43 GHz ) at an angular resolution of 1 \hbox to 0.0 pt { . } { { } ^ { \prime \prime } } 5 \times 1 \hbox to 0.0 pt { . } { { } ^ { \prime% \prime } } 0 .
The free-free emission at 43 GHz is estimated to be \sim 100 mJy , implying a star formation rate of \sim 1.3 M _ { \odot } 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 \hbox to 0.0 pt { . } { { } ^ { \prime \prime } } 5 \times 1 \hbox to 0.0 pt { . } { { } ^ { \prime% \prime } } 0 and 0 \hbox to 0.0 pt { . } { { } ^ { \prime \prime } } 36 \times 0 \hbox to 0.0 pt { . } { { } ^ { \prime% \prime } } 21 .
Based on the line and continuum emission models used for the 1 \hbox to 0.0 pt { . } { { } ^ { \prime \prime } } 5 \times 1 \hbox to 0.0 pt { . } { { } ^ { \prime% \prime } } 0 angular resolution observations , the RRLs H53 \alpha and H92 \alpha are tracers of the high-density ( \sim 10 ^ { 5 } cm ^ { -3 } ) and low-density ( \sim 10 ^ { 3 } cm ^ { -3 } ) thermally ionized gas components in NGC 253 , respectively .
The velocity fields observed in the H53 \alpha and H92 \alpha lines ( 1 \hbox to 0.0 pt { . } { { } ^ { \prime \prime } } 5 \times 1 \hbox to 0.0 pt { . } { { } ^ { \prime% \prime } } 0 ) are consistent .
The velocity gradient in the central \sim 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 \sim 5 \times 10 ^ { 6 } M _ { \odot } in the central \sim 18 pc region .
The H92 \alpha line observations at high angular resolution ( 0 \hbox to 0.0 pt { . } { { } ^ { \prime \prime } } 36 \times 0 \hbox to 0.0 pt { . } { { } ^ { \prime% \prime } } 21 ) reveal a larger velocity gradient , along a P.A . \sim - 45 ^ { \circ } on the NE component , of \sim 110 km s ^ { -1 } arcsec ^ { -1 } .
The dynamical mass estimated using the high angular resolution H92 \alpha data ( \sim 7 \times 10 ^ { 6 } M _ { \odot } ) supports the existence of an accreted massive object in the nuclear region of NGC 253 .