Cold Molecular Gas in Massive, Star-forming Disk Galaxies at z = 1.5

• Published in: The Astrophysical journal Vol. 718; no. 1; pp. 177 - 183
• Main Authors: Aravena, M, Carilli, C, Daddi, E, Wagg, J, Walter, F, Riechers, D, Dannerbauer, H, Morrison, G. E, Stern, D, Krips, M
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 20-07-2010; IOP
• Subjects: Astronomy; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; BRIGHTNESS; COSMOLOGY; Earth, ocean, space; ELEMENTS; EMISSION; EVOLUTION; Exact sciences and technology; GALACTIC EVOLUTION; GALAXIES; HYDROGEN; MASS; NONMETALS; OPTICAL PROPERTIES; PHYSICAL PROPERTIES; RED SHIFT; STARS; TELESCOPES; Galaxy evolution; Red shift; Cold gas; galaxies: starburst; Starburst galaxies; galaxies: evolution; Disk galaxies; Molecular gas; galaxies: high-redshift; Massive stars; galaxies: formation; Galaxy formation; Cosmology; cosmology: observations
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1088/0004-637X/718/1/177

We report the detection of the CO J = 1-0 emission line in three near-infrared selected star-forming galaxies at z {approx} 1.5 with the Very Large Array and the Green Bank Telescope. These observations directly trace the bulk of molecular gas in these galaxies. We find H{sub 2} gas masses of 8.3 {+-} 1.9 x 10{sup 10} M {sub sun}, 5.6 {+-} 1.4 x 10{sup 10} M{sub sun}, and 1.23 {+-} 0.34 x 10{sup 11} M{sub sun} for BzK-4171, BzK-21000, and BzK-16000, respectively, assuming a conversion {alpha}{sub CO} = 3.6 M {sub sun} (K km s{sup -1} pc{sup 2}){sup -1}. We combined our observations with previous CO 2-1 detections of these galaxies to study the properties of their molecular gas. We find brightness temperature ratios between the CO 2-1 and CO 1-0 emission lines of 0.80{sup +0.35}{sub -0.22}, 1.22{sup +0.61}{sub -0.36}, and 0.41{sup +0.23}{sub -0.13} for BzK-4171, BzK-21000, and BzK-16000, respectively. At the depth of our observations it is not possible to discern between thermodynamic equilibrium or sub-thermal excitation of the molecular gas at J = 2. However, the low temperature ratio found for BzK-16000 suggests sub-thermal excitation of CO already at J = 2. For BzK-21000, a large velocity gradient model of its CO emission confirms previous results of the low excitation of the molecular gas at J = 3. From a stacked map of the CO 1-0 images, we measure a CO 2-1 to CO 1-0 brightness temperature ratio of 0.92{sup +0.28}{sub -0.19}. This suggests that, on average, the gas in these galaxies is thermalized up to J = 2, has star formation efficiencies of {approx}100 L {sub sun} (K km s{sup -1} pc{sup 2}){sup -1}, and gas consumption timescales of {approx}0.4 Gyr, unlike submillimeter galaxies and quasi-stellar objects at high redshifts.