[論文レビュー] Building the First Galaxies -- Chapter 2. Starbursts Dominate The Star Formation Histories of 6 < z <12 Galaxies

Alan Dressler, Marcia Rieke+13 more|arXiv (Cornell University)|Jun 4, 2023

Astronomy and Astrophysical Research被引用数 9

ひとこと要約

The study analyzes 894 galaxies at 6<z<12 from JWST/JADES GOODS-S with SEDz* to derive star formation histories, finding that starbursts dominate the histories (~70%) while longer SFHs contribute about one-third of stellar mass by z~6.

ABSTRACT

We use SEDz* -- a code designed to chart star formation histories (SFHs) of 6

研究の動機と目的

• Characterize the star formation histories of first galaxies in the 6<z<12 interval using JWST/NIRCam SEDs.
• Test and validate SEDz* as a tool to recover epoch-by-epoch stellar mass buildup from A-star dominated populations.
• Classify SFHs into distinct types and quantify their contributions to stellar mass over cosmic time.

提案手法

• Apply SEDz*, a NNLS-based tool, to fit 9-band NIRCam photometry with templates built from Bruzual & Charlot models including emission/continuum.
• Divide redshifts 6<z<12 into seven integer steps (z=12 to z=6) representing 42–186 Myr epochs and test two template sets (10 Myr bursts and constant star formation CSF).
• Reconstruct SFHs by adding template epochs from z=12 downward, selecting epoch of observation by minimum chi-squared, and summing mass contributions per epoch to form the SFH.
• Mitigate z<4/low-z interlopers using a multi-step validation: minima shallow/steepness of chi-squared, HST detections in visible bands, and EAZY photo-z comparisons to build a gold sample.
• Assess robustness of SFH types against dust extinction and confirm no strong extinction in the sample (AV ~0.1–0.4 ruled out).

Figure 1: Examples of the four types of SFHs $5<z<12$ found with SEDz* . The text describes how, considered together, these examples demonstrate the fidelity of SEDz* -derived SFHs. The left box shows the observed SED (black points with error bars) from 9-band NIRCam imaging and the NNLS fit of SEDz

実験結果

リサーチクエスチョン

• RQ1 What are the dominant star formation histories of galaxies in the 6<z<12 epoch?
• RQ2 Can SED-based templates using A-star dominated populations reliably recover epoch-by-epoch stellar mass buildup in the first galaxies?
• RQ3 What is the distribution of SFH types (burst, stochastic, contiguous, continuous) in this redshift range?
• RQ4 How do SFH types correlate with redshift, mass, and environment within the JADES GOODS-S field?
• RQ5 What fraction of stellar mass by z~6 is assembled through short bursts versus longer star-formation histories?

主な発見

• SFH types are four: SFH1 (burst), SFH2 (stochastic), SFH3 (contiguous 3-epoch), SFH4 (continuous 4–6 epochs).
• Starbursts dominate the SFHs, comprising ~70% of the sample, while longer SFHs contribute about one-third of the accumulated stellar mass by z~6.
• The 894 SFHs imply integrated masses of 1.34e11, 1.09e11, 1.00e11, and 3.16e10 Msun for SFH1–SFH4 respectively, totaling ~4e11 Msun by z=6 in the field.
• No strong dependencies of SFH type on large-scale environment were found, though a compact group with early star formation at z=11–12 suggests long SFHs may dominate in rare dense environments.
• The sample is broadly dust-poor, with AV constraints ruling out significant extinction; yet a dust-enshrouded early burst is proposed as a possible precursor to measured SFHs.

Figure 2: Examples of the identified four types of SFHs of this study observed at redshifts $z\sim 6$ ( $5.75<z<6.75$ ), starting with three examples of single starbursts in the top row and continuing in order, across the rows and and down, to multi-burst “stochastic” histories, three-epoch contiguo