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arXiv 2401.04475astro-ph.GA
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宇宙正午的终结：z~1处无遮蔽类星体的短气体耗尽时间

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

M. Frias Castillo, M. Rybak, J. Hodge, P. van der Werf, L. J. Abbo, F. J. Ballieux, S. Ward, C. Harrison, G. Calistro Rivera, J. P. McKean, H. R. Stacey

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20 pages. Accepted for publication in A&A
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AI总结：

本文利用NOEMA观测十个透镜化z~1无遮蔽类星体的CO(2-1)发射，发现其宿主星系分子气体耗尽时间中位数仅约90 Myr，表明强烈星暴将快速耗尽气体并使星系进入宁静状态。

摘要：

无遮蔽类星体（QSOs）被预测为从富含气体的并合到气体耗尽、熄灭星系这一演化序列的最终阶段。然而，对这一群体的研究发现其远红外明亮源的出现率很高——表明存在显著的尘埃遮蔽恒星形成——但仍需对驱动这种恒星形成的冷分子气体进行直接观测。我们利用NOEMA对十个透镜化的z=1-1.5无遮蔽QSOs进行了CO(2-1)发射研究，以追踪冷分子气体。我们在七个目标中探测到CO(2-1)，其中四个还显示出连续谱发射（λ_rest = 1.3mm）。在从测光中扣除前景星系的贡献后，光谱能量分布拟合得到宿主星系的恒星质量为10^9-11 M_⊙，恒星形成率为25-160 M_⊙ yr^-1。这些QSOs的L'_CO低于具有相同L_IR的恒星形成星系，其气体耗尽时间跨度很大（50-900 Myr），但中位数仅为90 Myr。我们发现分子气体质量在2-40 × 10^9(α_CO/4) M_⊙范围内，这表明大多数目标的气体比例高于约50%。尽管存在无遮蔽QSO，宿主星系仍能保留大量的冷气体。然而，由于气体耗尽时间的中位数约为90 Myr，这些目标中正在发生的强烈恒星形成爆发将在没有气体补充的情况下迅速耗尽它们的分子气体储备，最终导致宿主星系进入宁静状态。未探测到CO的QSOs是样本中四个射电噪QSOs中的三个，它们的性质表明它们可能已经在向宁静状态转变。最近的宇宙学模拟倾向于高估这些z~1 QSO宿主星系预期的气体耗尽时间，这可能与它们在一般高红移星系群体中难以产生星暴有关。

英文摘要：

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 = 1.3mm). After subtracting the foreground galaxy contribution to the photometry, spectral energy distribution fitting yielded stellar masses of 10^9-11 M_\odot, with star formation rates of 25-160 M_\odot yr^-1 for the host galaxies. These QSOs have lower $L'_\mathrm{CO}$ 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 Myr. We find molecular gas masses in the range 2-40 x 10^9(alpha_CO/4) M_\odot, 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.

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