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

Abstract

We present new VLA observations toward NGC 253 of the recombination line H53a (43 GHz) at an angular resolution of 1B5 ; 1B0. The free-free emission at 43 GHz is estimated to be 140 mJy, implying a star formation rate of 2 M yr1 in the nuclear region of this starburst galaxy. A reanalysis is made for previously reported H92 observations carried out with angular resolution of 1B5 ; 1B0 and 0B36 ; 0B21. Based on the line and continuum emission models used for the 1B5 ; 1B0 angular resolution observations, the RRLs H53 and H92 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 H53 and H92 lines (1B5 ; 1B0) are consistent. The velocity gradient in the central 18 pc of the NE component, as observed in both the H53 and H92 lines, is in the opposite direction to the velocity gradient determined from the CO observations. The enclosed virial mass, as deduced from the H53 velocity gradient over theNE component, is5 ; 106 M in the central18 pc region. The H92 line observations at high angular resolution (0B36 ; 0B21) reveal a larger velocity gradient, along a P:A: 45 on the NE component, of 110 km s1 arcsec1. The dynamical mass estimated using the high angular resolution H92 data (7 ; 106 M)110 km s1 arcsec1. supports the existence of an accreted massive object in the nuclear region of NGC 253.