JWST catches the assembly of a $z\sim 5$ ultra-low-mass galaxy

Abstract

Using CANUCS imaging we found an apparent major merger of two $z\sim 5$ ultra-low-mass galaxies ($M_{\star }\sim 10^7{M}_{\odot }$ each) that are doubly imaged and magnified $\sim$12-15$\times$ by the lensing cluster MACS 0417. Both galaxies are experiencing young ($\sim$100 Myr), synchronised bursts of star formation with $\log (sSFR/Gy{\mathrm{r}}^{-1})\sim$1.3-1.4, yet SFRs of just $\sim 0.2M_{\odot }{\mathrm{yr}}^{-1}$. They have sub-solar ($Z\sim 0.2Z_{\odot }$) gas-phase metallicities and are connected by an even more metal-poor star-forming bridge. The galaxy that forms from the merger will have a mass of at least $M_{\star }\sim 2\times 10^7{M}_{\odot }$, at least half of it formed during the interaction-induced starburst. More than half of the ionizing photons produced by the system (before and during the merger) will have been produced during the burst. This system provides the first detailed look at a merger involving two high-$z$ ultra-low-mass galaxies of the type believed to be responsible for reionizing the Universe. It suggests that such galaxies can grow via a combination of mass obtained through major mergers, merger-triggered starbursts, and long-term in-situ star formation. If such high-$z$ mergers are common, then merger-triggered starbursts could be significant contributors to the ionizing photon budget of the Universe.

6 pages, 2 figures, and 1 table; accepted for publication in MNRAS Letter

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• Dead or Alive? How Bursty Star Formation and Patchy Dust Can Cause Temporary Quiescence in High Redshift Galaxies
• CANUCS: Constraining the MACS J0416.1-2403 Strong Lensing Model with JWST NIRISS, NIRSpec and NIRCam
• Detailed Study of Stars and Gas in a z = 8.3 Massive Merger with Extreme Dust Conditions
• Possible environmental quenching in an interacting little red dot pair at $z\sim 7$
• Unveiling the Cosmic Gems Arc at $z\sim 10$ with JWST NIRCam
• The COSMOS-Web Lens Survey (COWLS) III: forecasts versus data
• JWST's PEARLS: A z=6 quasar in a train-wreck galaxy merger system
• Gamma Analytical Modeling Evolution (GAME) I: The physical implications of deriving the stellar mass functions from z=0 to z=8