[Paper Review] Star formation histories of UV-luminous galaxies at $z \simeq 6.8$: implications for stellar mass assembly at early cosmic times

Lily Whitler, Daniel P. Stark+4 more|arXiv (Cornell University)|Jun 10, 2022

Galaxies: Formation, Evolution, Phenomena4 citations

TL;DR

This study analyzes the star formation histories (SFHs) of UV-luminous galaxies at z ≈ 6.8 using SED modeling with two independent codes, finding a median stellar population age of 30–70 Myr. It reveals that 15–30% of these galaxies are extremely young (≤10 Myr), while 20–25% are old (≥250 Myr), implying significant early stellar mass assembly and highlighting the need for JWST to resolve model uncertainties.

ABSTRACT

The variety of star formation histories (SFHs) of $z\gtrsim6$ galaxies provides important insights into early star formation, but has been difficult to systematically quantify. Some observations suggest that many $z\sim6-9$ galaxies are dominated by $\gtrsim200$ Myr stellar populations, implying significant star formation at $z\gtrsim9$, while others find that most reionization era galaxies are $\lesssim10$ Myr, consistent with little $z\gtrsim9$ star formation. Here, we quantify the distribution of ages of UV-bright ($-22.5\lesssim M_{UV}\lesssim-21$) galaxies colour-selected to lie at $z\simeq6.6-6.9$, an ideal redshift range to systematically study the SFHs of reionization era galaxies with ground-based observatories and Spitzer. We infer galaxy properties with two SED modelling codes and compare results, finding that stellar masses are largely insensitive to the model, but the inferred ages can vary by an order of magnitude. We infer a distribution of ages assuming a simple, parametric SFH model, finding a median age of $\sim30-70$ Myr depending on SED model. We quantify the fractions of $\leq10$ Myr and $\geq250$ Myr galaxies, finding that these systems comprise $\sim15-30$ percent and $\sim20-25$ percent of the population, respectively. With a flexible SFH model, the shapes of the SFHs are consistent with those implied by the simple model (e.g. young galaxies have rapidly rising SFHs). However, stellar masses can differ significantly, with those of young systems sometimes being more than an order of magnitude larger with the flexible SFH. We quantify the implications of these results for $z\gtrsim9$ stellar mass assembly and discuss improvements expected from JWST.

Motivation & Objective

• To quantify the distribution of stellar population ages in UV-luminous galaxies at z ≈ 6.8, a redshift range ideal for ground-based and Spitzer studies.
• To assess the robustness of inferred stellar masses and ages against different SED modeling codes and SFH parametrizations.
• To investigate the implications of these SFHs for stellar mass assembly at z ≳ 9, where direct observations are limited.
• To evaluate the impact of flexible SFH models on inferred galaxy properties, particularly stellar mass and age.

Proposed method

• SED modeling is performed using two independent codes—Prospector and BEAGLE—on photometry from Subaru/HSC, VISTA/IRAC, and Spitzer/IRAC.
• A simple parametric SFH model (exponentially rising or constant) is used to infer median ages and age distributions.
• A flexible SFH model based on the star formation rate history is applied to assess deviations from simple models.
• Photometric redshifts are constrained using multi-band photometry, with emphasis on minimizing contamination from nebular emission lines.
• Stellar masses and ages are derived via Bayesian inference, with uncertainty propagation using nested sampling (MultiNest) and dynamic nested sampling (dynesty).
• The analysis compares results from both codes to assess systematic differences in inferred properties, especially for young and old systems.

Experimental results

Research questions

• RQ1 What is the distribution of stellar population ages in UV-luminous galaxies at z ≈ 6.8, and how do these ages vary with SED modeling assumptions?
• RQ2 How do the inferred stellar masses and ages of z ≈ 6.8 galaxies depend on the choice of SFH parametrization (simple vs. flexible)?
• RQ3 What fraction of z ≈ 6.8 galaxies are consistent with being ≤10 Myr or ≥250 Myr old, and what does this imply for star formation at z ≳ 9?
• RQ4 To what extent do nebular emission line contributions affect stellar mass estimates in this redshift range?
• RQ5 How do the results constrain the total stellar mass assembly at z ≳ 9, and what improvements are expected from JWST?

Key findings

• The median stellar population age of UV-luminous galaxies at z ≈ 6.8 is estimated at 30–70 Myr, depending on the SED modeling code used.
• Approximately 15–30% of the sample are consistent with stellar populations ≤10 Myr old, indicating ongoing or very recent star formation.
• About 20–25% of the galaxies have inferred ages ≥250 Myr, suggesting significant star formation activity at z ≳ 9.
• Stellar masses inferred from flexible SFH models can differ by more than an order of magnitude from those derived with simple models, especially for young systems.
• The shapes of the SFHs from flexible models are consistent with rapidly rising star formation histories, similar to those inferred from simple models.
• The study underscores the importance of independent mass measurements—e.g., from ALMA or JWST—for validating SED modeling results and resolving current uncertainties in early galaxy evolution.