At the end of cosmic noon: Short gas depletion times in unobscured quasars at z ∼ 1
Abstract
Unobscured quasars (QSOs) are predicted to be the final stage in the evolutionary sequence from gas-rich mergers to gas-depleted, quenched galaxies. Studies of this population, however, find a high incidence of far-infrared-luminous sources-suggesting significant dust-obscured star formation-but direct observations of the cold molecular gas fuelling this star formation are still necessary. We present a NOEMA study of CO(2-1) emission, tracing the cold molecular gas, in ten lensed z = 1 − 1.5 unobscured QSOs. We detected CO(2-1) in seven of our targets, four of which also show continuum emission (λrest = 1.3 mm). After subtracting the foreground galaxy contribution to the photometry, spectral energy distribution fitting yielded stellar masses of 109 − 11 M⊙, with star formation rates of 25−160 M⊙ yr−1 for the host galaxies. These QSOs have lower LCO' than star-forming galaxies with the same LIR, and show depletion times spanning a large range (50−900 Myr), but with a median of just 90(αCO/4) Myr. We find molecular gas masses in the range ≤2−40 × 109(αCO/4) M⊙, which suggest gas fractions above ∼50% for most of the targets. Despite the presence of an unobscured QSO, the host galaxies are able to retain significant amounts of cold gas. However, with a median depletion time of ∼90 Myr, the intense burst of star formation taking place in these targets will quickly deplete their molecular gas reservoirs in the absence of gas replenishment, resulting in a quiescent host galaxy. The non-detected QSOs are three of the four radio-loud QSOs in the sample, and their properties indicate that they are likely already transitioning into quiescence. Recent cosmological simulations tend to overestimate the depletion times expected for these z ∼ 1 QSO-host galaxies, which is likely linked to their difficulty producing starbursts across the general high-redshift galaxy population.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- March 2024
- DOI:
- 10.1051/0004-6361/202347596
- arXiv:
- arXiv:2401.04475
- Bibcode:
- 2024A&A...683A.211F
- Keywords:
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- gravitation;
- gravitational lensing: strong;
- galaxies: ISM;
- quasars: emission lines;
- galaxies: starburst;
- submillimeter: ISM;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 20 pages. Accepted for publication in A&