On bursty star formation during cosmological reionization -- influence on the metal and dust content of low-mass galaxies

Abstract

Observations indicate that high-redshift galaxies undergo episodic star formation bursts, driving strong outflows that expel gas and suppress accretion. We investigate the consequences for metal and dust content of galaxies at $z \geq 5$ using our semi-analytical model, ASHVINI. We track gas-phase and stellar metallicities ($Z_{\rm g}, Z_\ast$) and dust mass (M$_{\rm d}$) in dark matter haloes spanning $M_{\rm halo} = 10^6$-$10^{11}$ $M_\odot$, comparing continuous and bursty star formation scenarios - which reflect underlying assumptions of instantaneous and delayed feedback - and we allow for metallicity-dependent feedback efficiency. Delayed feedback induces oscillations in $Z_{\rm g}$ and $Z_\ast$, with $Z_{\rm g}$ declining sharply at low stellar and halo masses; the mass scale of this decline increases toward lower redshift. Reionization introduces significant scatter in $Z_{\rm g}$, producing an upturn followed by rapid decline. Metallicity-dependent feedback moderates this decline at $ z=7 - 10$, flattening the $Z_{\rm g}$-mass relation to $\simeq 0.03$-$0.04\, Z_\odot$. Dust production tracks $Z_{\rm g}$ but is sensitive to burst history, causing delayed enrichment. Our results show that burst-driven feedback decouples $Z_{\rm g}$ and $Z_\ast$, imprints intrinsic scatter in mass-metallicity relations, and delays dust growth. These effects are strongest in low-mass halos ($M_{\rm halo} \sim 10^7 M_\odot$), where shallow potentials amplify the impact of feedback. Our results are consistent with recent hydrodynamical and semi-analytical simulations and provide context for interpreting JWST (James Webb Space Telescope) metallicity and dust measurements, highlighting the importance of episodic star formation in early galaxy chemical evolution.