Stochastic star formation activity of galaxies within the first billion years probed by JWST

Abstract

In this work, we aim at characterizing the burstiness level of high-redshift galaxy SFHs and its evolution. We implement a stochastic SFH in CIGALE using PSD, to estimate the burstiness level of star formation in galaxies at 6<z<12. We find that SFHs with a high level of stochasticity better reproduce the SEDs of z>6 galaxies, while smoother assumptions introduce biases when applied to galaxies with bursty star-formation activity. The assumed stochasticity level of the SFH also affects the constraints on galaxies' physical properties, including the main sequence. Successively assuming different levels of burstiness, we determined the best-suited SFH for each 6<z<12 galaxy in the JADES sample from a Bayes Factor analysis. Galaxies are classified according to their level of burstiness, and the corresponding physical properties are associated to them. For massive galaxies (8.8< logM*/Msun<9.5), the fraction of bursty galaxies increases from 0.38+/-0.08 to 0.77+/-0.2 at z~6 and z~12, respectively. At all redshifts, only <20% of low-mass galaxies are classified as bursty; although, this estimate is uncertain because their faintness leads to a low S/N. For bursty galaxies, the log10(SFR10/SFR100) ratio, another indicator of bursty star formation, does not evolve with redshift, but the fraction of galaxies with a high log10(SFR10/SFR100) slightly increases from 0.28+/-0.06 to 0.38+/-0.11 between z~6 and z~9. We include additional constraints from observations on sigmaUV, the dispersion of the UV magnitude distribution, and SFE, finding a maximum of 0.72+/-0.02 mag and 0.06+/-0.01 for sigmaUV and SFE, respectively. This confirms that neither alone is responsible for the weak evolution of the UVLF at z>10. Our results add further evidence that a combination with other mechanisms is likely responsible for the high-z UVLF. The stochastic SFH module is public as part of CIGALE version 2025.1.

Figures (14)

• Figure 1: Stochastic star formation histories ($\text{SFR}_{\text{smooth}} \times \text{SFR}_{\text{stochastic}}$) for different combinations of parameters generated with the StochasticSFH module. Each panel shows the impact on the SFH of one specific parameter: $\sigma$ (left panel), $\rm \tau_{break}$ (middle panel), and $\alpha$ (right panel), in all cases the age and $\rm \tau_{main}$ are fix to 500 Myr and 1000 Mys, respectively.
• Figure 2: Color-color diagram using JWST/NIRCam bands from the JADES catalog (left columns) and from the simulated catalogs (from second to last columns). These simulated catalogs are computed assuming, from left to right, $\sigma$ of 0.01, 0.1, 0.4, and 0.8. The two different rows show the color-color diagram obtained with two different band combinations chosen to take into account redshift and parameter coverage to ease the comparison. The color lines show the contours that enclose the 16$^{th}$, 50$^{th}$, 84$^{th}$, and 90$^{th}$ of the data in the observations. The implemented stochastic SFH can reproduce observed colors at $z>6$.
• Figure 3: $\chi^2$ obtained by fitting the mock catalogs with cigale. The different fits for each simulated catalog are color-coded by $\sigma_{\rm run}$ used in the fit. A high level of burstiness ($\sigma_{\rm run}\sim0.8$) provides good fit quality, whatever the intrinsic level of burstiness of the fitted galaxy, while smoother SFHs struggle to fit bursty galaxy SEDs.
• Figure 4: Comparison between the mock SFR$_{\rm 10}$ (true SFR$_{\rm 10}$) and the SFR$_{\rm 10}$ recovered by cigale (estimated SFR$_{\rm 10}$) for all mock catalogs (rows) fitted with the different values of $\sigma_{\rm run}$ (columns). The boxes span from the first quartile (Q1 = 25%) to the third quartile (Q3 = 75%), with a line indicating the median position, while the error bars extend from the 16$^{th}$ to the 84$^{th}$ percentile of the cumulative distribution. The gray background in the panels highlights when $\sigma_{\rm run}$ match $\sigma_{\rm mock}$. The SFH with the highest level of burstiness ($\sigma_{\rm run}=0.8$) provides the best estimate of SFR$_{\rm 10}$.
• Figure 5: $\chi^2$ distribution of the whole sample for each cigale run. The $\sigma_{\rm run}$ used to model the SFH is indicated by the x-axis. The black dashed line shows the median $\chi^2$ for $\sigma_{run} = 0.8$. The embedded box plot of each distribution indicates the median, interquartile range (IQR), and whiskers. A high level of burstiness accounted for in the SFH is needed to fit early galaxies' SED.