An extremely young massive clump forming by gravitational collapse in a primordial galaxy

TL;DR

This study presents direct observational evidence of a very young, massive star-forming clump in a distant galaxy, formed by gravitational collapse, supporting theories of long-lived clumps that contribute to galaxy bulge growth.

Contribution

First direct observation of a very young, massive star-forming clump formed by gravitational collapse in a high-redshift galaxy, confirming long-lived clump survival.

Findings

The clump is younger than 10 Myr and contains over 10^9 solar masses of gas.

Star formation efficiency in the clump is over 10 times higher than in the host galaxy.

Clump formation rate and frequency suggest they can survive feedback for hundreds of millions of years.

Abstract

When the cosmic star formation history peaks (z ~ 2), galaxies vigorously fed by cosmic reservoirs are gas dominated and contain massive star-forming clumps, thought to form by violent gravitational instabilities in highly turbulent gas-rich disks. However, a clump formation event has not been witnessed yet, and it is debated whether clumps survive energetic feedback from young stars, thus migrating inwards to form galaxy bulges. Here we report spatially resolved spectroscopy of a bright off-nuclear emission line region in a galaxy at z = 1.987. Although this region dominates the star formation in the galaxy disk, its stellar continuum remains undetected in deep imaging, revealing an extremely young (age < 10 Myr) massive clump, forming through the gravitational collapse of > 10$^9$ M$_{\odot}$ of gas. Gas consumption in this young clump is > 10 times 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 (~ 2.5 Gyr$^{-1}$) supports long lifetimes (~ 500 Myr), favouring scenarios where clumps survive feedback and grow the bulges of present-day galaxies.