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Fast and inefficient star formation due to short-lived molecular clouds and rapid feedback

Abstract

The physics of star formation and the deposition of mass, momentum and energy into the interstellar medium by massive stars (‘feedback’) are the main uncertainties in modern cosmological simulations of galaxy formation and evolution1,2. These processes determine the properties of galaxies3,4 but are poorly understood on the scale of individual giant molecular clouds (less than 100 parsecs)5,6, which are resolved in modern galaxy formation simulations7,8. The key question is why the timescale for depleting molecular gas through star formation in galaxies (about 2 billion years)9,10 exceeds the cloud dynamical timescale by two orders of magnitude11. Either most of a cloud’s mass is converted into stars over many dynamical times12 or only a small fraction turns into stars before the cloud is dispersed on a dynamical timescale13,14. Here we report high-angular-resolution observations of the nearby flocculent spiral galaxy NGC 300. We find that the molecular gas and high-mass star formation on the scale of giant molecular clouds are spatially decorrelated, in contrast to their tight correlation on galactic scales5. We demonstrate that this decorrelation implies rapid evolutionary cycling between clouds, star formation and feedback. We apply a statistical method15,16 to quantify the evolutionary timeline and find that star formation is regulated by efficient stellar feedback, which drives cloud dispersal on short timescales (around 1.5 million years). The rapid feedback arises from radiation and stellar winds, before supernova explosions can occur. This feedback limits cloud lifetimes to about one dynamical timescale (about 10 million years), with integrated star formation efficiencies of only 2 to 3 per cent. Our findings reveal that galaxies consist of building blocks undergoing vigorous, feedback-driven life cycles that vary with the galactic environment and collectively define how galaxies form stars.

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Fig. 1: Decorrelation of molecular gas and young stellar emission on sub-kiloparsec scales.
Fig. 2: Radial profiles of constrained quantities in comparison to theoretical predictions.
Fig. 3: Dependence of measured quantities on spatial resolution.

Data availability

The ALMA CO(1–0) data used in this work are from projects 2013.1.00351.S and 2015.1.00258.S (PI A. Schruba) and are publicly available through the ALMA archive (https://almascience.eso.org/alma-data/archive). The MPG/ESO 2.2-m Hα data are publicly available as raw data from the ESO archive (http://archive.eso.org/) under programme ID 065.N-0076 (PI F. Bresolin). All other (for example, derived) data that support the findings of this study are available from the corresponding author upon reasonable request.

Code availability

A dedicated publication of the analysis software used in the current study is in preparation. The code is available from the corresponding author upon reasonable request.

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Acknowledgements

J.M.D.K. and M.C. acknowledge funding from the German Research Foundation (DFG) in the form of an Emmy Noether Research Group grant no. KR4801/1-1. J.M.D.K. acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme via the ERC Starting Grant MUSTANG (grant agreement no. 714907). A.P.S.H. and D.T.H. are fellows of the International Max Planck Research School for Astronomy and Cosmic Physics at the University of Heidelberg (IMPRS-HD). We thank C. Faesi for providing his version of the MPG/ESO 2.2-m Hα map of NGC 300. We thank B. W. Keller and M. R. Krumholz for discussions.

Reviewer information

Nature thanks N. Evans and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Authors

Contributions

J.M.D.K. led the project, carried out the experiment design, developed the analysis method and wrote the text, to which A.S., M.C. and S.N.L. contributed. A.S. performed and reduced the ALMA observations and prepared all observational data for their physical analysis. J.M.D.K. and M.C. carried out the physical analysis of the data, to which A.S., S.N.L., A.P.S.H. and D.T.H. contributed. All authors contributed to aspects of the analysis, the interpretation of the results and the writing of the manuscript.

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Correspondence to J. M. Diederik Kruijssen.

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Extended data figures and tables

Extended Data Fig. 1 Radial profiles of quantities describing the galactic structure of NGC 300.

af, The surface densities of molecular gas, atomic gas, total gas and stars (a), the SFR surface density (b), the depletion times of molecular, atomic and total gas (c), the rotation curve (d), the Toomre Q stability parameter (e, where Q = 1 corresponds to equilibrium), and the metallicity (f).

Extended Data Fig. 2 Integrated properties of NGC 300 relative to other star-forming galaxies in the nearby Universe.

ac, The SFR as a function of stellar mass (a), molecular gas mass (b) and atomic gas mass (c), both for NGC 300 and nearby galaxies from the xCOLDGASS53 and xGASS54 surveys. In b and c, the arrows indicate 3σ and 5σ upper limits of non-detections in xCOLDGASS and xGASS, respectively. The solid lines represent the star-forming galaxy main sequence54 (a), the mean molecular gas depletion time of the xCOLDGASS detections (b), and the mean atomic gas depletion time of the xGASS detections (c), with the 1σ scatter shown in grey.

Extended Data Fig. 3 Radial profiles of the average properties of the GMC population in NGC 300.

af, The GMC radius (a), velocity dispersion (b), luminous and virial masses (c), surface density (d), molecular hydrogen number density (e) and virial parameter (f, where αvir = 1 corresponds to virial equilibrium).

Extended Data Fig. 4 NGC 300 seen at different aperture sizes.

This figure illustrates the image processing of this work. The panels show the Hα emission (left) and CO(1–0) emission (right) from NGC 300 convolved with top-hat apertures of diameters increasing from top to bottom from 20 pc to 2,560 pc (see the annotated circles). Each panel also shows the locations of the emission peaks identified in the images at 20-pc resolution (crosses), at which the flux density measurements are made when deriving the CO-to-Hα flux ratio as a function of size scale as in Fig. 1.

Extended Data Fig. 5 PDFs of constrained quantities.

af, Normalized probability distributions of the six constrained quantities (solid lines), with best-fitting values (dashed lines) and 1σ uncertainties (dotted lines) indicated in the top-right corner of each panel.

Extended Data Fig. 6 Influence of the GMC life cycle on the decorrelation of molecular gas and young stellar emission.

Shown is the change of the CO-to-Hα flux ratio relative to the galactic average as a function of spatial scale, for apertures placed on CO emission peaks (top branch) and Hα emission peaks (bottom branch). The symbols and 1σ error bars show the CO-to-Hα flux ratios observed across the entire field of view of NGC 300 as in Fig. 1. The evolutionary timeline of the GMC life cycle is constrained by fitting the model indicated by the solid lines. Alternative models with long GMC lifetimes are shown by the dashed and dotted lines (see key). These alternatives are ruled out by the observations.

Extended Data Fig. 7 Distribution of nearest-neighbour distances of identified emission peaks.

The black solid line shows the cumulative distribution of the distances to the nearest neighbours across the combined sample of emission peaks identified in the Hα and CO maps. The median and mean distance are indicated by the vertical dashed and dotted lines, respectively. The vertical grey lines indicate lower and upper limits derived in the Methods section, the first of which is implied by the measured region separation length. The location of the median and mean nearest-neighbour distances between these limits is consistent with the measured separation lengths.

Extended Data Fig. 8 Schematic illustration of cloud-scale evolutionary cycling in the ΣΣSFR plane.

The symbols show the observed relation between the total gas surface density (Σ, where the high-redshift sample is assumed to be fully molecular) and the SFR surface density (ΣSFR) for galaxies in the local Universe and at high redshift (see key)68,139,140. For NGC 300, the error bars show the 1σ uncertainties. Dotted lines represent constant gas depletion times as indicated by the labels. The results of this work show that GMCs and star-forming regions move through this diagram. As a function of time, they increase their gas density, increase their SFR, expel gas through feedback and eventually fade by stellar evolution, as schematically illustrated by the red arrows.

Supplementary information

Video 1

Spatial de-correlation between molecular gas and ionised emission from young stars towards small spatial scales in the nearby galaxy NGC300. The top panels show the ionised emission (Hα, left) and molecular gas (CO, right) maps of NGC300, with crosses indicating the emission peaks in each of the maps. The bottom-left panel shows the gas depletion time (the ratio between the top maps). The colour of this map changes from white on large spatial scales (strong CO-Hα correlation) to bright red and blue on small spatial scales (strong CO-Hα anti-correlation). The bottom-right panel quantifies this behaviour by showing how the change of the CO-to-Hα flux ratio relative to the galactic average increases towards small (<150 pc) aperture sizes. The circle and vertical line indicate the spatial scale (‘aperture size’) at which the galaxy is observed.

Video 2

Relation between the change of the CO-to-Hα flux ratio relative to the galactic average and the physical quantities defining the cloud life cycle. The young stellar lifetime (t), the cloud lifetime (tCO), the feedback timescale (tfb), and the region separation length (λ) are initially set equal to values measured for NGC300, but are systematically varied to demonstrate their effect on the CO-to-Hα ratio. The top panels show mock ‘CO’ and ‘Hα’ maps from a numerical simulation of a disc galaxy, with an inclination and position angle mimicking NGC300. The images are generated using stellar particles in specific age intervals, yielding emission peak lifetimes as indicated in the timeline and annotation at the top of the video. The bottom-left panel shows the ‘gas depletion time’, i.e. the ratio between the top two maps. The bottom-right panel shows the change of this ratio relative to the galactic average, with λ indicated along the bottom axis. This diagram provides a non-degenerate measurement of the three measured quantities (tCO, tfb, and λ), because t is known (see Methods).

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Kruijssen, J.M.D., Schruba, A., Chevance, M. et al. Fast and inefficient star formation due to short-lived molecular clouds and rapid feedback. Nature 569, 519–522 (2019). https://doi.org/10.1038/s41586-019-1194-3

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