G. W. Roberts-Borsani, M. J. Jiménez-Donaire, M. Daprà, K. Alatalo, I. Aretxaga, J. Álvarez-Márquez, A. J. Baker, S. Fujimoto, P. A. Gallardo, M. Gralla, M. Hilton, J. P. Hughes, C. Jiménez, N. Laporte, T. A. Marriage, F. Nati, J. Rivera, A. Sievers, A. Weiß, G. W. Wilson, E. J. Wollack, M. S. Yun. 2017. Multiwavelength Characterization of an ACT-selected, Lensed Dusty Star-forming Galaxy at z=2.64. Astrophysical Journal 844, 2, DOI: 10.3847/1538-4357/aa7aa8
We present CI(2-1) and multi-transition (CO)-C-12 observations of a dusty star-forming galaxy, ACT J2029+0120, which we spectroscopically confirm to lie at z = 2.64. We detect CO(3-2), CO(5-4), CO(7-6), CO(8-7), and CI (2-1) at high significance, tentatively detect HCO+(4-3), and place strong upper limits on the integrated strength of dense gas tracers (HCN(4-3) and CS(7-6)). Multi-transition CO observations and dense gas tracers can provide valuable constraints on the molecular gas content and excitation conditions in high-redshift galaxies. We therefore use this unique data set to construct a CO spectral line energy distribution (SLED) of the source, which is most consistent with that of a ULIRG/Seyfert or QSO host object in the taxonomy of the Herschel Comprehensive ULIRG Emission Survey. We employ RADEX models to fit the peak of the CO SLED, inferring a temperature of T similar to 117 K and n(H2) similar to 10(5) cm(-3), most consistent with a ULIRG/QSO object and the presence of high-density tracers. We also find that the velocity width of the C I line is potentially larger than seen in all CO transitions for this object, and that the L'(Ci(2-1))/L'(CO(3-2)) ratio is also larger than seen in other lensed and unlensed submillimeter galaxies and QSO hosts; if confirmed, this anomaly could be an effect of differential lensing of a shocked molecular outflow.