ALMA Reveals the Molecular Gas Properties of Five Star-forming Galaxies across the Main Sequence at 3
Schinnerer, Eva; Tasca, Lidia; Liu, Daizhong; Groves, Brent; Rodighiero, Giulia; Sargent, Mark; Lemaux, Brian C.; Enia, Andrea; Talia, Margherita; Cassata, Paolo; Morselli, Laura; Romano, Michael; Rodríguez-Muñoz, Lucía; Smolcic, Vernesa; Ribeiro, Bruno; Karim, Alexander; Ibar, Eduardo; Fèvre, Olivier Le; Fiore, Stefano; Mancini, Chiara
Italy, Germany, Australia, Chile, United Kingdom, France, United States, Netherlands, Croatia
Abstract
We present the detection of CO (5-4) with signal-to-noise ratio (S/N) > 7-13 and a lower CO transition with S/N > 3 (CO (4-3) for four galaxies, and CO (3-2) for one) with the Atacama Large Millimeter/submillimeter Array in bands 3 and 4 in five main-sequence (MS) star-forming galaxies with stellar masses (3-6) × 1010M⊙ at 3 < z < 3.5. We find a good correlation between the total far-infrared luminosity LFIR and the luminosity of the CO (5-4) transition L_{CO(5-4)}^{'}, where L_{CO(5-4)^{'} increases with star formation rate (SFR), indicating that CO (5-4) is a good tracer of the obscured SFR in these galaxies. The two galaxies that lie closer to the star-forming MS have CO spectral line energy distribution (SLED) slopes that are comparable to other star-forming populations, such as local submillimeter galaxies and BzK star-forming galaxies; the three objects with higher specific star formation rates have far steeper CO SLEDs, which possibly indicates a more concentrated episode of star formation. By exploiting the CO SLED slopes to extrapolate the luminosity of the CO (1-0) transition and using a classical conversion factor for MS galaxies of α_{CO}=3.8 M_{⊙}(K km s01 pc-2)-1, we find that these galaxies are very gas-rich, with molecular gas fractions between 60% and 80% and quite long depletion times, between 0.2 and 1 Gyr. Finally, we obtain dynamical masses that are comparable to the sum of stellar and gas mass (at least for four out of five galaxies), allowing us to put a first constraint on the αCO parameter for MS galaxies at an unprecedented redshift.