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.

Figures (8)

• Figure 1: Impact of instantaneous versus delayed feedback (fiducial model parameters): The evolution of gas and stellar mass (${M_{\rm g}}$ and $M_{\rm \star}$; solid and dotted-dashed curves), and the mass of metals in the gas phase and stars (${M_{\rm Z,g}}$ and $M_{\rm Z,\star}$; dashed and dotted curves) as a function of cosmic time (in Gyrs, lower horizontal axis) and redshift (upper horizontal axis). These predictions are based on the assembly histories of 100 halos with $M_\text{h} = 10^7 \text{M}_\odot$ ($M_\text{h} = 10^{10} \text{M}_\odot$) mass bin at $z=5$ in the upper (lower) panel, for instantaneous (delayed) feedback in the left (right) panel. Each curve represents the evolution of the median value of a given quantity, while bands indicate the range of the 10$^{\rm th}$ and 90$^{\rm th}$ percentiles.
• Figure 2: Influence of the IGM metallicity, $Z_\text{IGM}$: We show the evolution of ${M_{\rm g}}$, $M_{\rm \star}$, ${M_{\rm Z,g}}$, and $M_{\rm Z,\star}$ (solid, dot-dashed, dashed, dotted curves, respectively) with cosmic time/redshift as the metallicity of the accreted gas $(Z_{\rm IGM})$ is varied. The upper and lower panels, respectively, corresponds to $Z_\text{IGM}=10^{-5}\text{Z}_\odot$ and $10^{-1}\text{Z}_\odot$. Grey bands and curves correspond to ${M_{\rm Z,g}}$ and $M_{\rm Z,\star}$ for the fiducial $Z_\text{IGM}$.
• Figure 3: Influence of the heavy element yield, $Y_Z$: We show the evolution of ${M_{\rm g}}$, $M_{\rm \star}$, ${M_{\rm Z,g}}$, and $M_{\rm Z,\star}$ (solid, dot-dashed, dashed, dotted curves, respectively) with cosmic time/redshift as heavy elements' yield $(Y_Z)$ is varied. Upper and lower panels corresponds to $Y_Z=0.006$ and $Y_Z=0.6$. Grey bands and curves correspond to ${M_{\rm Z,g}}$ and $M_{\rm Z,\star}$ for the fiducial $Y_Z$.
• Figure 4: Influence of metal-dependent feedback: We show the impact of metal-dependent feedback (Equation \ref{['eq:sigmoid_func']}) on the evolution of ${M_{\rm g}}$, $M_{\rm \star}$, ${M_{\rm Z,g}}$, and $M_{\rm Z,\star}$ (solid, dot-dashed, dashed, dotted curves, respectively) for lower and higher halo masses at $z=5$. The upper and lower panels correspond to $M_\text{h}=10^7\text{M}_\odot$ and $10^{10}\text{M}_\odot$ respectively. As before, the grey curves correspond to the fiducial model.
• Figure 5: Stellar mass versus gas-phase metallicity relations between $z=5$ and $10$: Here we plot gas phase metallicity, $Z_{\rm g}={M_{\rm Z,g}}/{M_{\rm g}}$, as a function of stellar mass, $M_{\rm \star}$, in units of $\text{M}_\odot$ at $z=5$, 7, and 10 (blue, green and red curves respectively). The left panel shows the behaviour in our fiducial feedback model; the right shows the impact of our assumed metal-dependent feedback model. Filled symbols correspond to observational data from ArellanoCordova2022; Nakajima2023; Trump2023; Chemerynska.etal.2024; and Curti2024.