AN EXTENDED MOLECULAR GAS-DISTRIBUTION IN MARKARIAN-273 AND MERGER-LUMINOSITY EVOLUTION

We present the first interferometric measurement of CO emission (theta similar to 2 '') from the ultraluminous infrared galaxy Mrk 273. A to tal H-2 mass of 3.6 x 10(10) M. is inferred from the CO observations, half of which belongs to an extended component with deconvolved size 5 .1 x 2.5 kpc. In addition, an unresolved molecular gas complex is foun d to be coincident with the optical nucleus. The inferred H, mass, siz e (R similar to 380 pc), and mean surface mass density (Sigma(H2) grea ter than or equal to 4 X 10(4) M. pc(-3)) Of this complex, as well as the IR luminosity, are very similar to those of Arp 220. The gas in th e extended component shows a rotational velocity gradient with the kin ematic major axis aligned with the position angle of the two nuclei se en in the near-infrared. The extended molecular gas distribution and s eparation of the two nuclei suggest that Mrk 273 is a young merger sys tem. The CO emission appears confined to the edge-on system, and the s econd merger progenitor may have been gas-poor or may have had its gas transferred to the companion during the merger. The comparison of the physical properties of Mrk 273 with two other similarly gas-rich IR l uminous systems, VV 114 and Arp 220, finds a monotonic increase in ave rage gas surface density and IR luminosity efficiency (L(IR)/M(H2)) wi th decreasing projected separation of the stellar nuclei. We find that now all four nearest ultraluminous systems observed at high spatial r esolution (Arp 220, Arp 299, Mrk 231, and Mrk 273) are associated with central mass surface density in excess of 10(4) M. pc(-2).