Submillimeter Galaxies at<i>z</i>∼ 2: Evidence for Major Mergers and Constraints on Lifetimes, IMF, and CO‐H₂Conversion Factor

Abstract

We report sub-arcsecond resolution IRAM PdBI millimeter CO interferometry of\nfour z~2 submillimeter galaxies (SMGs), and sensitive CO (3-2) flux limits\ntoward three z~2 UV-/optically selected star forming galaxies. The new data\nreveal for the first time spatially resolved CO gas kinematics in the observed\nSMGs. Two of the SMGs show double or multiple morphologies, with complex,\ndisturbed gas motions. The other two SMGs exhibit CO velocity gradients of ~500\nkm/s across 0.2 arcsec (1.6 kpc) diameter regions, suggesting that the star\nforming gas is in compact, rotating disks. Our data provide compelling evidence\nthat these SMGs represent extreme, short-lived 'maximum' star forming events in\nhighly dissipative mergers of gas rich galaxies. The resulting high mass\nsurface and volume densities of SMGs are similar to those of compact quiescent\ngalaxies in the same redshift range, and much higher than those in local\nspheroids. From the ratio of the comoving volume densities of SMGs and\nquiescent galaxies in the same mass and redshift ranges, and from the\ncomparison of gas exhaustion time scales and stellar ages, we estimate that the\nSMG phase duration is about 100 Myrs. Our analysis of SMGs and optically/UV\nselected high redshift star forming galaxies supports a 'universal' Chabrier\nIMF as being valid over the star forming history of these galaxies. We find\nthat the 12CO luminosity to total gas mass conversion factors at z~2-3 are\nprobably similar to those assumed at z~0. The implied gas fractions in our\nsample galaxies range from 20 to 50%.\n