[论文解读] A recently quenched galaxy 700 million years after the Big Bang

Tobias J. Looser, Francesco D’Eugenio+43 more|arXiv (Cornell University)|Feb 27, 2023

Astronomy and Astrophysical Research被引用 21

一句话总结

论文报道了在 z=7.3 的一个安静星系的光谱发现与多代码建模（约距大爆炸后 700 Myr），其恒星质量约为 4–6×10^8 M☉，经历一个快速的、约 30–90 Myr 的爆发后在观测前 10–30 Myr 发生淬火，使其成为已知最早且质量最低的静止系统之一。

ABSTRACT

Local and low-redshift ($z$<$3$) galaxies are known to broadly follow a bimodal distribution: actively star forming galaxies with relatively stable star-formation rates, and passive systems. These two populations are connected by galaxies in relatively slow transition. In contrast, theory predicts that star formation was stochastic at early cosmic times and in low-mass systems: these galaxies transitioned rapidly between starburst episodes and phases of suppressed star formation, potentially even causing temporary quiescence -- so-called mini-quenching events. However, the regime of star-formation burstiness is observationally highly unconstrained. Directly observing mini-quenched galaxies in the primordial Universe is therefore of utmost importance to constrain models of galaxy formation and transformation. Early quenched galaxies have been identified out to redshift $z<5$, and these are all found to be massive ($M_{*}>10^{10}~M_{\odot}$) and relatively old. Here we report a (mini-)quenched galaxy at z$=$7.3, when the Universe was only 700~Myr old. The JWST/NIRSpec spectrum is very blue ($U$-$V$$=$0.16$\pm0.03$~mag), but exhibits a Balmer break and no nebular emission lines. The galaxy experienced a short starburst followed by rapid quenching; its stellar mass (4-6$ imes 10^8~M_\odot$) falls in a range that is sensitive to various feedback mechanisms, which can result in perhaps only temporary quenching.

研究动机与目标

• Motivate constraints on early quiescent galaxy formation and the role of feedback mechanisms in the early Universe.
• Quantify the physical properties (mass, SFR, metallicity, dust) and star-formation history of a z=7.3 quiescent galaxy.
• Assess quenching timescales and possible quenching mechanisms in the epoch of reionization.
• Compare multiple SED-fitting codes to test robustness of the inferred properties.

提出的方法

• Obtain JWST/NIRSpec prism and higher-resolution spectra of a z=7.3 candidate quiescent galaxy.
• Perform joint spectro-photometric modeling using four SED-fitting codes (ppxf, bagpipes, prospector, beagle) to derive SFH, M*, Z, A_V, and SFR.
• Use rest-frame Balmer break and absence of nebular emission lines to infer post-starburst nature and constrain SFR (<0.65 M⊙/yr from Hβ; [O III] < 6.5×10^-20 erg s^-1 cm^-2).
• Cross-validate results via multiple stellar-population templates and metallicity grids, and test for high f_esc or obscured star formation scenarios.
• Infer formation and quenching lookback times, with SFH suggesting a burst lasting 30–90 Myr and quenching 10–30 Myr before observation.

Figure 1 : NIRSpec R100/prism spectrum of JADES-GS-z7-01-QU. The clearly detected Ly \textalpha drop and the Balmer break unambiguously give a redshift of z=7.3. The absence of emission lines (together with the Balmer break) reveals that this is a – temporarily or permanently – quiescent, post-starb

实验结果

研究问题

• RQ1 Can a quiescent galaxy exist at z>7 with low stellar mass in the early Universe?
• RQ2 What are the star-formation history, metallicity, dust content, and quenching timescales of such an early quiescent system?
• RQ3 Do observations favor rapid, ejective quenching (SF/AGN-driven outflows) over starvation, and how do results constrain feedback models at high redshift?

主要发现

Metric	ppxf	bagpipes	beagle	prospector
log10(M*/M⊙)	-	8.6±0.1	8.8^{+0.1}_{-0.2}	8.7^{+0.1}_{-0.1}
log10(SFR [M⊙/yr])	-	<-1.3	-2.5^{+1.0}_{-1.0}	-2.6^{+1.5}_{-2.7}
log10(Z/Z⊙)	<-2.0	-0.7±0.1	1.9^{+0.4}_{-0.2}	1.7^{+0.2}_{-0.2}
t_quench [Myr]	~20	~10	16^{+7}_{-4}	38^{+9}_{-10}
t_form [Myr]	~100	40±10	93^{+69}_{-47}	116_{-45}^{+85}
A_V [mag]	0.4±0.1	0.32^{+0.25}_{-0.17}	0.51^{+0.03}_{-0.04}	0.1^{+0.1}_{-0.0}

• Discovery of a quiescent galaxy at z=7.29±0.01 (JADES-GS-z7-01-QU) with no detected nebular lines.
• Stellar mass constrained to M* ~ 4–6×10^8 M⊙ (dwarf-galaxy regime at z=7.3).
• Star-formation rate in last 10–Myr is extremely low; current SFR estimates range from ~10^-2.6 to 10^-1.3 M⊙/yr across codes, well below the main sequence at z~7–8.
• Star-formation history indicates a burst lasting ~30–90 Myr, forming stars at z~7.6–8.8, quenched by z~7.4–7.5, i.e., quenching within ~10–30 Myr prior to observation.
• Average stellar metallicity is inferred to be very low (log10(Z/Z⊙) ≈ -2 by three codes; one code (bagpipes) yields ≈ -0.7), suggesting limited chemical enrichment prior to quenching.
• Quenching mechanisms likely involve rapid gas ejection via feedback (SF-driven or AGN-driven outflows) rather than slow starvation; the object challenges some simulation predictions for such low-mass, early quenchers.

(a) The stellar age-metallicity grid resulting from the ppxf fit. The code reconstructs dominant metal-poor populations forming from $\sim$ 100 to $\sim$ 20 Myr prior to observation. The colour-bar represents the fractional mass distribution over the SSP grid.