Abstract
When cosmic star formation history reaches a peak (at about redshift z ≈ 2), galaxies vigorously fed by cosmic reservoirs1,2 are dominated by gas3,4 and contain massive star-forming clumps5,6, which are thought to form by violent gravitational instabilities in highly turbulent gas-rich disks7,8. However, a clump formation event has not yet been observed, and it is debated whether clumps can survive energetic feedback from young stars, and afterwards migrate inwards to form galaxy bulges9,10,11,12. Here we report the spatially resolved spectroscopy of a bright off-nuclear emission line region in a galaxy at z = 1.987. Although this region dominates star formation in the galaxy disk, its stellar continuum remains undetected in deep imaging, revealing an extremely young (less than ten million years old) massive clump, forming through the gravitational collapse of more than one billion solar masses of gas. Gas consumption in this young clump is more than tenfold faster than in the host galaxy, displaying high star-formation efficiency during this phase, in agreement with our hydrodynamic simulations. The frequency of older clumps with similar masses13, coupled with our initial estimate of their formation rate (about 2.5 per billion years), supports long lifetimes (about 500 million years), favouring models in which clumps survive feedback and grow the bulges of present-day galaxies.
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Acknowledgements
We thank S. Juneau for discussions, and M. Cappellari for sharing his Multi-Gaussian Expansion fit software publicly. The simulations were performed at the Très Grand Centre de Calcul of the CEA (Commissariat à l’Énergie Atomique et aux énergies alternatives) under GENCI (Grand Équipement National de Calcul Intensif) allocation 2014-GEN2192. We acknowledge financial support from Agence Nationale de la Recherche (contract ANR-12-JS05-0008-01) and the European Commission through European Research Council grants StG-257720 and StG-240039.
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A.Z., E.D., E.L.F. and F.B. conceived the work, led the analysis and interpretation, and wrote the paper. R.G., F.V., V.S., A.C., M.O. and C.V. contributed key aspects of the data reduction and analysis. V.P. and F.R. contributed key aspects of the simulations development and analysis. All the authors commented on the manuscript at all stages.
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Extended data figures and tables
Extended Data Figure 1 GALFIT decomposition of the [O iii] emission line map.
a, b, [O iii] map (a) and model of the point source component (b) for the clump. c, No strong residuals or artefacts are left after removal of the point source component. The positions of the nucleus and of the clump are shown as crosses.
Extended Data Figure 2 Image ratios and mass map.
a, b, Ratio of F105W/F606W imaging in spectral flux density (Fν) scale (a), a proxy for the dust reddening of the stellar continuum, and F140W/F105W imaging, sensitive to the M/L ratio (b). The positions of the nucleus and the clump are shown as crosses. The maps show only small variations: the observed Fν ratios for the nucleus and clump positions are respectively 1.34 and 1.16 for F105W/F606W, and respectively 1.39 and 1.25 for F140W/F105W. Galaxy-wide ratios are respectively 1.27 and 1.37 for F105W/F606W and F140W/F105W. c, Mass map in units of M⊙ per pixel.
Extended Data Figure 3 Modelling of the galaxy light profile.
F140W direct image and [O iii] emission line map (a, d, g, j), GALFIT models (b, e, h, k) and residuals (c, f, i, l). The first row shows the single Sérsic profile solution, the MGE model is in the second row, and our baseline model (the sum of three Sérsic profiles, in which blue crosses mark the additional components) is in the third row. The red cross indicates the barycentre of the stellar light and the green cross marks the centre of the [O iii] off-nuclear component.
Extended Data Figure 4 The Asymmetry and M20 morphological parameters as determined from the spatial distribution of the galaxy stellar mass.
Pink and light blue triangles represent disks and mergers from MIRAGE numerical simulations42, respectively. The galaxy presented in this work (red filled circle with error bars indicating s.d.) is located in the typical region occupied by disk galaxies41; the vast majority of mergers have higher Asymmetry and/or M20 parameters. The figure shows the same number of mergers and disks even if mergers are expected to be a minority in optical samples.
Extended Data Figure 5 Upper limits to clump continuum flux.
The observed flux upper limits estimated from simulations and GALFIT modelling in the three bands are shown as black filled circles. The black horizontal lines indicate the bandpass width of each filter. Coloured curves represent reddened Starburst99 stellar population synthesis models43 with different ages (from 8 to 20 Myr), normalized to the most stringent upper limit (F105W band). The corresponding upper limits in F140W and F606W, obtained considering a spectrum with an age ∼10 Myr, are shown as grey filled circles.
Extended Data Figure 6 Emission line diagnostics.
a, BPT diagram54 showing that the emission line ratios of the whole galaxy and of the clump (red and light-blue points with error bars indicating s.d.) are consistent with being powered by star formation. The [N ii] upper limit and Hα emission of the whole galaxy are measured from the Subaru/MOIRCS longslit spectroscopy follow-up, and the [N ii]/Hα upper limit for the clump is computed by assuming the [N ii] of the whole galaxy. b, Determination of the metallicities of the whole galaxy and that of the clump from the [O iii]/[O ii] ratio64.
Extended Data Figure 7 Time evolution of physical quantities based on the clump SFR(t) from our simulations.
a, The peaks of all the curves normalized to 1 to highlight the time delay occurring between the peak of the SFR and of the luminosities LHβ, L1,500 Å and L5,000 Å. b, Peaks normalized to 1 at t = 1 Gyr to stress the relative intensity of the observables at the peak and later phases. The vertical black dotted line indicates the upper limit on the age of the clump (t = 10 Myr). The units of the plotted quantities are: SFR (M⊙ yr−1), EWHβ (Å), LHβ, L1,500 Å and L5,000 Å (erg s−1), M★/L5,000 Å (M⊙ erg−1 s) and SFR/LHβ (M⊙ yr−1 erg−1 s).
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Zanella, A., Daddi, E., Le Floc’h, E. et al. An extremely young massive clump forming by gravitational collapse in a primordial galaxy. Nature 521, 54–56 (2015). https://doi.org/10.1038/nature14409
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DOI: https://doi.org/10.1038/nature14409
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