title: An extremely young massive clump forming by gravitational collapse in a primordial galaxy.
abstract: When cosmic star formation history reaches a peak (at about redshift z ≈ 2), galaxies vigorously fed by cosmic reservoirs are dominated by gas and contain massive star-forming clumps, which are thought to form by violent gravitational instabilities in highly turbulent gas-rich disks. 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 bulges. 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 masses, 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.
authors.affiliation: Laboratoire AIM-Paris-Saclay, CEA/DSM-CNRS-Université Paris Diderot, Irfu/Service d'Astrophysique, CEA Saclay, Orme des Merisiers, F-91191 Gif sur Yvette, France.
authors.forename: A
authors.initials: A
authors.lastname: Zanella
authors.affiliation: Laboratoire AIM-Paris-Saclay, CEA/DSM-CNRS-Université Paris Diderot, Irfu/Service d'Astrophysique, CEA Saclay, Orme des Merisiers, F-91191 Gif sur Yvette, France.
authors.forename: E
authors.initials: E
authors.lastname: Daddi
authors.affiliation: Laboratoire AIM-Paris-Saclay, CEA/DSM-CNRS-Université Paris Diderot, Irfu/Service d'Astrophysique, CEA Saclay, Orme des Merisiers, F-91191 Gif sur Yvette, France.
authors.forename: E
authors.initials: E
authors.lastname: Le Floc'h
authors.affiliation: Laboratoire AIM-Paris-Saclay, CEA/DSM-CNRS-Université Paris Diderot, Irfu/Service d'Astrophysique, CEA Saclay, Orme des Merisiers, F-91191 Gif sur Yvette, France.
authors.forename: F
authors.initials: F
authors.lastname: Bournaud
authors.affiliation: 1] Laboratoire AIM-Paris-Saclay, CEA/DSM-CNRS-Université Paris Diderot, Irfu/Service d'Astrophysique, CEA Saclay, Orme des Merisiers, F-91191 Gif sur Yvette, France [2] School of Physics, Korea Institute for Advanced Study, Heogiro 85, Seoul 130-722, Republic of Korea.
authors.forename: R
authors.initials: R
authors.lastname: Gobat
authors.affiliation: Laboratoire AIM-Paris-Saclay, CEA/DSM-CNRS-Université Paris Diderot, Irfu/Service d'Astrophysique, CEA Saclay, Orme des Merisiers, F-91191 Gif sur Yvette, France.
authors.forename: F
authors.initials: F
authors.lastname: Valentino
authors.affiliation: 1] Laboratoire AIM-Paris-Saclay, CEA/DSM-CNRS-Université Paris Diderot, Irfu/Service d'Astrophysique, CEA Saclay, Orme des Merisiers, F-91191 Gif sur Yvette, France [2] Department of Physics, Ludwig-Maximilians-Universität, Scheinerstrasse 1, 81679 München, Germany.
authors.forename: V
authors.initials: V
authors.lastname: Strazzullo
authors.affiliation: 1] Laboratoire AIM-Paris-Saclay, CEA/DSM-CNRS-Université Paris Diderot, Irfu/Service d'Astrophysique, CEA Saclay, Orme des Merisiers, F-91191 Gif sur Yvette, France [2] Astronomy Centre, Department of Physics and Astronomy, University of Sussex, Brighton BN1 9QH, UK.
authors.forename: A
authors.initials: A
authors.lastname: Cibinel
authors.affiliation: Institute for Astronomy, ETH Zürich, Wolfgang-Pauli-strasse 27, 8093 Zürich, Switzerland.
authors.forename: M
authors.initials: M
authors.lastname: Onodera
authors.affiliation: Aix Marseille Université, CNRS, LAM (Laboratoire dAstrophysique de Marseille), F-13388 Marseille, France.
authors.forename: V
authors.initials: V
authors.lastname: Perret
authors.affiliation: 1] Laboratoire AIM-Paris-Saclay, CEA/DSM-CNRS-Université Paris Diderot, Irfu/Service d'Astrophysique, CEA Saclay, Orme des Merisiers, F-91191 Gif sur Yvette, France [2] Department of Physics, University of Surrey, Guildford GU2 7XH, UK.
authors.forename: F
authors.initials: F
authors.lastname: Renaud
authors.affiliation: 1] Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy [2] INAF Osservatorio Astronomico di Bologna, Via Ranzani 1, 40127 Bologna, Italy.
authors.forename: C
authors.initials: C
authors.lastname: Vignali