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ABSTRACT It remains a major challenge to derive a theory of cloud-scale ($$\lesssim100$$ pc) star formation and feedback, describing how galaxies convert gas into stars as a function of the galactic environment. Progress has been hampered by a lack of robust empirical constraints on the giant molecular cloud (GMC) lifecycle. We address this problem by systematically applying a new statistical method for measuring the evolutionary timeline of the GMC lifecycle, star formation, and feedback to a sample of nine nearby disc galaxies, observed as part of the PHANGS-ALMA survey. We measure the spatially resolved (∼100 pc) CO-to-H α flux ratio and find a universal de-correlation between molecular gas and young stars on GMC scales, allowing us to quantify the underlying evolutionary timeline. GMC lifetimes are short, typically $$10\!-\!30\,{\rm Myr}$$, and exhibit environmental variation, between and within galaxies. At kpc-scale molecular gas surface densities $$\Sigma _{\rm H_2}\ge 8\,\rm {M_\odot}\,{{\rm pc}}^{-2}$$, the GMC lifetime correlates with time-scales for galactic dynamical processes, whereas at $$\Sigma _{\rm H_2}\le 8\,\rm {M_\odot}\,{{\rm pc}}^{-2}$$ GMCs decouple from galactic dynamics and live for an internal dynamical time-scale. After a long inert phase without massive star formation traced by H α (75–90 per cent of the cloud lifetime), GMCs disperse within just $$1\!-\!5\,{\rm Myr}$$ once massive stars emerge. The dispersal is most likely due to early stellar feedback, causing GMCs to achieve integrated star formation efficiencies of 4–10 per cent. These results show that galactic star formation is governed by cloud-scale, environmentally dependent, dynamical processes driving rapid evolutionary cycling. GMCs and H ii regions are the fundamental units undergoing these lifecycles, with mean separations of $$100\!-\!300\,{{\rm pc}}$$ in star-forming discs. Future work should characterize the multiscale physics and mass flows driving these lifecycles.
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Molecular gas in AzTEC/C159: a star-forming disk galaxy 1.3Gyr after the Big Bang
We studied the molecular gas properties of AzTEC/C159, a star-forming disk galaxy at $z=4.567$. We secured $$^{12}$$CO molecular line detections for the $$J=2\to1$$ and $$J=5\to4$$ transitions using the Karl G. Jansky VLA and the NOEMA interferometer. The broad (FWHM$$\sim750\,{\rm km\,s}^{-1}$$) and tentative double-peaked profiles of both $$^{12}$$CO lines are consistent with an extended molecular gas reservoir, which is distributed in a rotating disk as previously revealed from [CII] 158$$\mu$$m line observations. Based on the $$^{12}$$CO(2$$\to$$1) emission line we derived $$L'_{\rm{CO}}=(3.4\pm0.6)\times10^{10}{\rm \,K\,km\,s}^{-1}{\rm \,pc}^{2}$$, that yields a molecular gas mass of $$M_{\rm H_2 }(\alpha_{\rm CO}/4.3)=(1.5\pm0.3)\times 10^{11}{\rm M}_\odot$$ and unveils a gas-rich system with $$\mu_{\rm gas}(\alpha_{\rm CO}/4.3)\equiv M_{\rm H_2}/M_\star=3.3\pm0.7$$. The extreme star formation efficiency (SFE) of AzTEC/C159, parametrized by the ratio $$L_{\rm{IR}}/L'_{\rm{CO}}=(216\pm80)\, {\rm L}_{\odot}{\rm \,(K\,km\,s}^{-1}{\rm \,pc}^{2})^{-1}$$, is comparable to merger-driven starbursts such as local ultra-luminous infrared galaxies (ULIRGs) and SMGs. Likewise, the $$^{12}$$CO(5$$\to$$4)/CO(2$$\to$$1) line brightness temperature ratio of $$r_{52}= 0.55\pm 0.15$$ is consistent with high excitation conditions, similar to that observed in SMGs. We constrained the value for the $$L'_{\text{CO}}-{\rm H}_2$$ mass conversion factor in AzTEC/C159, i.e. $$\alpha_{\text{CO}}=3.9^{+2.7}_{-1.3}{\rm \,M}_{\odot}{\rm \,K}^{-1}{\rm \,km}^{-1}{\rm \,s\,pc}^{-2}$$, that is consistent with a self-gravitating molecular gas distribution as observed in local star-forming disk galaxies. Cold gas streams from cosmological filaments might be fueling a gravitationally unstable gas-rich disk in AzTEC/C159, which breaks into giant clumps forming stars as efficiently as in merger-driven systems and generate high gas excitation.
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- Award ID(s):
- 1614213
- PAR ID:
- 10058275
- Author(s) / Creator(s):
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Date Published:
- Journal Name:
- Astronomy & astrophysics
- ISSN:
- 0004-6361
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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