At the end of cosmic noon: Short gas depletion times in unobscured quasars at z ∼ 1

¹ Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
e-mail: friascastillom@strw.leidenuniv.nl
² THz Sensing Group, Department of Microelectronics, Faculty of Electrical Engineering, Mathematics and Computer Science, Mekelweg 4, 2628 CD Delft, The Netherlands
³ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany
⁴ Excellence Cluster ORIGINS, Boltzmannstraße 2, 85748 Garching bei München, Germany
⁵ Ludwig-Maximilian-Universität, Professor-Huber-Platz 2, 80539 München, Germany
⁶ School of Mathematics, Statistics and Physics, Newcastle University, Newcastle upon Tyne NE1 7RU, UK
⁷ ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
⁸ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV Groningen, The Netherlands
⁹ South African Radio Astronomy Observatory (SARAO), PO Box 443 Krugersdorp 1740, South Africa
¹⁰ Department of Physics, University of Pretoria, Lynnwood Road, Hatfield, Pretoria 0083, South Africa

Received: 28 July 2023
Accepted: 27 December 2023

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 (λ_rest = 1.3 mm). After subtracting the foreground galaxy contribution to the photometry, spectral energy distribution fitting yielded stellar masses of 10^9 − 11 M_⊙, with star formation rates of 25−160 M_⊙ yr⁻¹ for the host galaxies. These QSOs have lower L_CO^′ than star–forming galaxies with the same L_IR, and show depletion times spanning a large range (50−900 Myr), but with a median of just 90(α_CO/4) Myr. We find molecular gas masses in the range ≤2−40 × 10⁹(α_CO/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.