The Extreme Compact Starburst in MRK 273
  Images of neutral Hydrogen 21cm absorption and radio continuum emission at 1.4 GHz from Mrk 273 were made using the Very Long Baseline Array and Very Large Array. These images reveal a gas disk associated with the northern nuclear region with a diameter of 0.5'' (370 pc), at an inclination angle of 53deg. The radio continuum emission is composed of a diffuse component plus a number of compact sources. This morphology resembles those of nearby, lower luminosity starburst galaxies. These images provide strong support for the hypothesis that the luminosity of the northern source is dominated by an extreme compact starburst. The HI 21cm absorption shows an east-west gradient in velocity of 450 km/s across 0.3'' (220 pc), implying an enclosed mass of 2e9 M_solar, comparable to the molecular gas mass. The brightest of the compact sources may indicate radio emission from an active nucleus (AGN), but this source contributes only 3.8% to the total flux density of the northern nuclear region. The HI 21cm absorption toward the southeast radio nucleus suggests infall at 200 km/s on scales < 40 pc, and the southwest near IR nucleus is not detected in high resolution radio continuum images. 
C.L. Carilli and G.B. Taylor