2022A&A...657L..15H

We use a combination of new NOrthern Extended Millimeter Array (NOEMA) observations of the pair of [CI] transitions, the CO(7-6) line, and the dust continuum, in addition to ancillary CO(1-0) and CO(3-2) data, to study the molecular gas properties of Q1700-MD94. This is a massive, main-sequence galaxy at z ≃ 2. We find that for a reasonable set of assumptions for a typical massive star-forming galaxy, the CO(1-0), the [CI](1-0) and the dust continuum yield molecular gas masses that are consistent within a factor of ∼2. The global excitation properties of the molecular gas as traced by the [CI] and CO transitions are similar to those observed in other massive star-forming galaxies at z ∼ 2. Our large velocity gradient modeling using RADEX of the CO and [CI] spectral line energy distributions suggests the presence of relatively warm (T_kin = 41 K), dense (n_H2 = 8 x 10³ cm^–3) molecular gas, comparable to the high-excitation molecular gas component observed in main-sequence star-forming galaxies at z ∼ 1. The galaxy size in the CO(1-0) and CO(7-6) line emission is comparable, which suggests that the highly excited molecular gas is distributed throughout the disk, powered by intense star formation activity. A confirmation of this scenario will require spatially resolved observations of the CO and [CI] lines, which can now be obtained with NOEMA upgraded capabilities.