VLA imaging of ¹²CO J = 1-0 and free-free emission in lensed submillimetre galaxies

Abstract

We present a study using the Karl G. Jansky Very Large Array (VLA) of ¹²CO J = 1-0 emission in three strongly lensed submillimetre-selected galaxies (SMM J16359, SMM J14009 and SMM J02399) at z = 2.5-2.9. These galaxies span L_IR = 10¹¹-10¹³ L_⊙, offering an opportunity to compare the interstellar medium of luminous infrared galaxies and ultraluminous infrared galaxies at high redshift. We estimate molecular gas masses in the range of 2-40 × 10⁹ M_⊙ using a method that assumes canonical underlying brightness temperature (T_b) ratios for star-forming and non-star-forming gas phases and a maximal star formation efficiency. A more simplistic method - using X_CO = 0.8 and the measured T_b ratios - yields gas masses twice as high. In SMM J14009 we find L CO 3-2'/L CO 1-0'=0.95±0.12, indicative of warm, star-forming gas, possibly influenced by the central active galactic nucleus (AGN). We set a gas mass limit of 3σ < 6 × 10⁸ M_⊙ for the Lyman break galaxy, A2218 #384, located in the same field as SMM J16359 at z = 2.515. Finally, we use the rest-frame ∼115 GHz free-free flux densities for SMM J14009 and SMM J02399 - measurements tied directly to the photoionization rate of massive stars, and made possible by VLA's bandwidth - to estimate star formation rates (SFRs) of 400-600 M_⊙ yr^-1 and to estimate the fraction of L_IR due to AGN.