Submillimetre-bright galaxies at high redshift are the most luminous, heavily star-forming galaxies in the Universe1 and are characterized by prodigious emission in the far-infrared, with a flux of at least five millijanskys at a wavelength of 850 micrometres. They reside in haloes with masses about 1013 times that of the Sun2, have low gas fractions compared to main-sequence disks at a comparable redshift3, trace complex environments4,5 and are not easily observable at optical wavelengths6. Their physical origin remains unclear. Simulations have been able to form galaxies with the requisite luminosities, but have otherwise been unable to simultaneously match the stellar masses, star formation rates, gas fractions and environments7,8,9,10. Here we report a cosmological hydrodynamic galaxy formation simulation that is able to form a submillimetre galaxy that simultaneously satisfies the broad range of observed physical constraints. We find that groups of galaxies residing in massive dark matter haloes have increasing rates of star formation that peak at collective rates of about 500–1,000 solar masses per year at redshifts of two to three, by which time the interstellar medium is sufficiently enriched with metals that the region may be observed as a submillimetre-selected system. The intense star formation rates are fuelled in part by the infall of a reservoir gas supply enabled by stellar feedback at earlier times, not through major mergers. With a lifetime of nearly a billion years, our simulations show that the submillimetre-bright phase of high-redshift galaxies is prolonged and associated with significant mass buildup in early-Universe proto-clusters, and that many submillimetre-bright galaxies are composed of numerous unresolved components (for which there is some observational evidence11).
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We thank M. J. Michałowski for providing observational data. Partial support for D.N. was provided by NSF AST-1009452, AST-1442650, NASA HST AR-13906.001 and a Cottrell College Science Award. P.H., C.H., M.T. and R.T. were funded by the Gordon and Betty Moore Foundation (GBMF4561 and grant no. 776). P.H. acknowledges the Alfred P. Sloan Foundation for support. C.-A.F.-G. was supported by NASA awards PF3-140106, NNX15AB22G and NSF AST-1412836. D.K. was supported by NSF AST-1412153. R.F. was supported by NASA HF-51304.01-A, and is a Hubble fellow. The simulations here were run on Stampede at TACC through NSF XSEDE allocations TG-AST120025, TG-AST130039 and TG-AST140023, NASA Pleiades, and the Haverford College cluster.
The authors declare no competing financial interests.
Extended data figures and tables
Extended Data Figure 1 Mass of reservoir gas in the central galaxy that will be consumed during SMG starburst as a function of z.
The colour scale denotes the median scale height from the galaxy centre of mass. The gas mass consumed during the starburst is calculated by tracking the evolution of gas particles that turn into stars during the SMG phase (z ≈ 2–2.7), and is only measured for the central galaxy itself (that is, gas ejected into the halo is not included). The SMG gas reservoir follows a cycle of being pushed outward followed by re-accretion.
Extended Data Figure 2 Distribution of flux density ratio of brightest component in submillimetre-luminous region to total flux density.
The average is shown with the vertical line. Submillimetre-luminous regions often break up into multiples. The region is generally dominated by one component, although smaller subhaloes can contribute on average ∼30% of the observed flux density. The normalization of the ordinate, P, is arbitrary.
Extended Data Figure 3 Gas surface density for the central submillimetre galaxy.
The blue histogram shows the distribution of gas surface densities (Σgas) during all phases (that is, all snapshots, Snaps), while the pink histogram shows the same for the submillimetre-luminous phase. The ordinate (N) is weighted by the time a galaxy spends in a given gas surface density bin, and the normalization is arbitrary. We predict that the submillimetre-luminous phases do not have dramatically different surface density distributions compared to the non-submillimetre-luminous phases. This prediction might have been tentatively observed1,87.
Extended Data Figure 4 Molecular gas fraction as a function of galaxy stellar mass.
Blue stars show individual snapshots of the central submillimetre galaxy, while red circles with error bars (1σ) show observations of BzK galaxies and SMGs with direct CO(J = 1–0) measurements (to avoid complications in converting from higher-lying CO rotational lines to the ground state for a mass conversion). (BzK galaxies are those that have been selected on the basis of their B, z and K band luminosities.) Both the observations and our model show a declining molecular gas fraction (fgas) with increasing galaxy mass (M*), with a typical range of fgas = 0.1–0.4 for galaxies of SMG mass.
Extended Data Figure 5 Predicted spectral energy distribution (SED) for the central submillimetre galaxy.
The ordinate shows the flux density in mJy, while the abscissa shows the wavelength in μm. The blue shaded region shows the range of SEDs for all simulation snapshots that satisfy the fiducial F850μm > 5 mJy submillimetre galaxy selection criteria, while the dark grey points with error bars (1σ) are a compilation of observed data. The individual coloured lines show the SEDs for individual submillimetre-luminous snapshots. The data and models are redshifted to a common redshift z = 2. The model and data compare well, and the model suggests a diverse range of SMG SEDs.
Extended Data Figure 6 Overestimate of the SFR of high-z SMGs.
The ordinate denotes the SFR as determined from the infrared SED (SFRIR)25, while the abscissa shows the SFR averaged over the last 50 Myr in the simulations (SFR50). Up to an SFR of ∼800 M⊙ yr−1 the two correspond well. At higher SFRs, however, there is a dramatic departure owing to substantial contribution to the infrared luminosity by older stars.
Extended Data Figure 7 Resolution tests for hydrodynamic zoom simulations.
Lines show the 850 μm duty cycle above a given flux density as a function of flux density for our resolution test models presented in Methods. SR denotes our standard resolution (the resolution of our main model) while HR is a one-level-higher refinement.
Extended Data Figure 8 Stellar mass–redshift relation for the model galaxy.
The purple line shows model results, while the dark-blue filled region shows observational constraints from an abundance matching assumption22. The model and observations are in reasonable agreement, especially during the submillimetre-luminous phase (vertical shaded region). At late times, the stellar mass of the galaxy is a factor of ∼2 higher than the median observed galaxy.
Extended Data Figure 9 Tests of parameter choices for radiative transfer calculations.
The simulated galaxy for these tests is our lowest resolution cosmological simulation (m13m14). Each panel shows the 850 μm flux density light curve of the tested model, with time noted on the abscissa (redshift on the bottom, time since the Big Bang on the top). In all panels, the shaded region denotes S850 ≥ 5 mJy, which is the canonical selection criteria for SMGs. Top left, our fiducial set of parameters; top right, simulation with a 100 kpc (on a side) emission region instead of 200 kpc; bottom left, simulation with our model for PAHs turned off; bottom right, fiducial simulation run with ten times the number of photons.
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Narayanan, D., Turk, M., Feldmann, R. et al. The formation of submillimetre-bright galaxies from gas infall over a billion years. Nature 525, 496–499 (2015). https://doi.org/10.1038/nature15383
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