[Paper Review] The Burstiness of Star Formation at $z\sim6$: A Huge Diversity in the Recent Star Formation Histories of Very UV-faint Galaxies

Ryan Endsley, John Chisholm+3 more|arXiv (Cornell University)|Oct 2, 2024

Astronomy and Astrophysical Research4 citations

TL;DR

This study analyzes the star formation histories (SFHs) of 368 z∼6 Lyman-break galaxies using JWST and Hubble imaging, revealing that the faintest UV galaxies (M_UV ≈ -17.4) exhibit far greater diversity in recent SFHs—especially strong SFR downturns and low Hα/UV luminosity ratios—than brighter ones. The results show bursty star formation is common, causing rapid, 1–2 mag fluctuations in UV magnitude over 10–30 Myr, explaining the abundance of high-redshift galaxies at z>10.

ABSTRACT

IRAC data have long implied that early ($z\gtrsim6$) galaxies often have very high specific star formation rates (sSFR$\gtrsim$30 Gyr$^{-1}$), but JWST data have shown that at least some early galaxies are forming stars far less vigorously. Here, we systematically analyze the recent star formation histories (SFHs) of a large ($N=368$) sample of $z\sim6$ Lyman-break galaxies (LBGs) spanning $-22\lesssim M_\mathrm{UV}\lesssim-16$ assembled from ACS+NIRCam imaging in the GOODS and Abell 2744 fields. We find that very low H$α$-to-UV luminosity ratios ($L_\mathrm{Hα}/L_\mathrm{UV}$) and strong recent downturns in star formation rate (SFR) are $\approx$5$ imes$ more common among the UV-faintest subset of our sample ($\langle M_\mathrm{UV} angle=-17.4$) compared to the brightest subset ($\langle M_\mathrm{UV} angle=-20.0$). The frequency of high $L_\mathrm{Hα}/L_\mathrm{UV}$ and strong recent SFR upturns is approximately constant with UV luminosity. We discuss how bursty SFHs naturally reproduce this much greater diversity in recent SFHs among very UV-faint galaxies. Using public NIRSpec/prism data, we newly confirm recent strong SFR downturns among three LBGs in our sample, and validate our photometric inferences on key SFH signatures among $z\sim6$ LBGs in general. Our results imply that early galaxies frequently cycle through phases of rapid stellar mass assembly and other periods of much slower growth. This yields huge ($\gtrsim$1-2 mag) fluctuations in $M_\mathrm{UV}$ on rapid ($\sim$10-30 Myr) timescales, helping explain the surprising abundance of $z>10$ galaxies. Finally, we caution that this burstiness causes all existing high-redshift samples (particularly line-selected samples) to be far less complete to galaxies with long recent phases of low sSFR than those currently undergoing a burst.

Motivation & Objective

• To understand the diversity of recent star formation histories (SFHs) in very UV-faint galaxies at z∼6.
• To resolve the discrepancy between IRAC-based sSFR estimates and JWST observations showing lower star formation activity in some early galaxies.
• To investigate why the faintest z∼6 galaxies show stronger SFR downturns and lower L_Hα/L_UV ratios than brighter ones.
• To validate photometric SFH inferences using public NIRSpec/prism data on a subset of galaxies.
• To assess the implications of bursty SFHs for the observed abundance of high-redshift galaxies (z>10).

Proposed method

• The authors analyze ACS+NIRCam imaging from the GOODS and Abell 2744 fields to assemble a sample of 368 z∼6 Lyman-break galaxies with M_UV between -22 and -16.
• They use photometric SED fitting to infer recent star formation histories (SFHs), focusing on SFR evolution over the last ~100 Myr.
• They compute Hα-to-UV luminosity ratios (L_Hα/L_UV) as a diagnostic of recent star formation activity.
• They compare the frequency of SFR downturns and upturns across UV luminosity bins, particularly contrasting the faintest (⟨M_UV⟩ = -17.4) and brightest (⟨M_UV⟩ = -20.0) subsets.
• They validate key SFH signatures using publicly available NIRSpec/prism spectroscopy on three LBGs in the sample.
• They model the impact of bursty SFHs on UV magnitude evolution, estimating timescale and amplitude of fluctuations.

Experimental results

Research questions

• RQ1 Why do the faintest z∼6 galaxies show significantly higher frequencies of SFR downturns and low L_Hα/L_UV ratios compared to brighter galaxies?
• RQ2 How do bursty star formation histories explain the observed diversity in recent SFHs among UV-faint z∼6 galaxies?
• RQ3 To what extent do photometric SFH inferences for z∼6 LBGs align with direct spectroscopic measurements from NIRSpec/prism data?
• RQ4 How do rapid, bursty SFHs contribute to the observed abundance of galaxies at z>10?
• RQ5 What are the implications of burstiness for the completeness of high-redshift galaxy surveys, especially line-selected samples?

Key findings

• Among z∼6 Lyman-break galaxies, the UV-faintest subset (⟨M_UV⟩ = -17.4) shows ≈5× higher frequency of strong recent SFR downturns and low L_Hα/L_UV ratios than the brightest subset (⟨M_UV⟩ = -20.0).
• The frequency of strong recent SFR upturns and high L_Hα/L_UV ratios remains approximately constant across UV luminosity bins.
• Bursty star formation histories naturally explain the observed diversity in recent SFHs among the faintest galaxies, with rapid mass assembly phases alternating with quiescent periods.
• Three LBGs from the sample are newly confirmed via NIRSpec/prism data to have strong recent SFR downturns, validating the photometric SFH inference method.
• Burstiness leads to rapid, 1–2 mag fluctuations in M_UV over timescales of 10–30 Myr, which helps explain the unexpected abundance of z>10 galaxies.
• Existing high-redshift samples, especially line-selected ones, are significantly incomplete for galaxies with long recent phases of low specific star formation rate (sSFR), favoring bursty systems.