Early steps in the hierarchical assembly of a Milky Way-mass galaxy 1 Gyr after the Big Bang

Abstract

We report JWST observations of a $z_{\mathrm{spec}}=5.196$ compact group of three strongly lensed low-mass galaxies ($M_{\star }\sim 10^{6-7}{M}_{\odot }$ each) whose small line-of-sight velocity offsets (from $-120\pm 100$ to $+160\pm 110$ km s${}^{-1}$) and projected separations ($\sim 2$ kpc) suggest that they are undergoing merging. On the basis of abundance-matching arguments, the trio appear to be destined to evolve into a Milky Way-mass galaxy by the present day. Our spectrophotometric analysis suggests that the stellar mass growth of this system is not simply just due to the merging of its components; rather, it is being dramatically enhanced by intense bursts of star formation. Presumably initiated by tidally-induced gas inflows, these star-forming bursts boost the mass growth $2.6\pm 0.5$ times that expected in straightforward merging of the existing stellar masses. These spectroscopic observations thus not only demonstrate that hierarchical assembly remains a viable formation channel in the early assembly phases of present-day massive galaxy, but also that interaction-induced bursts of star formation are a major accelerator of this assembly process.

Submitted to ApJ