# The emergence of galactic thin and thick discs across cosmic history

**Authors:** Takafumi Tsukui, Emily Wisnioski, Joss Bland-Hawthorn, Ken Freeman  
**arXiv:** [2409.15909](https://arxiv.org/abs/2409.15909)  
**Date:** 2024-09-24  
**Summary page:** https://www.summarizepaper.com/en/paper/2409.15909v1/

## Key points

- Study of galactic thin and thick discs is a constantly evolving topic in astronomy
- Modern disc galaxies exhibit distinct thin and thick discs differentiated by kinematics, structure, chemistry, and age
- Formation mechanisms and timing of onset for thin and thick discs are open questions
- Research using edge-on galaxies from JWST programs identified thick and thin discs dating back over 10 billion years
- Galaxies classified into two categories: requiring two distinct discs or well-fitted by a single disc
- Strong correlation between disc sizes/heights and galaxy mass across all cosmic times
- Transition from single to double discs occurred earlier in high-mass galaxies compared to low-mass ones
- Delayed formation of thin discs in low-mass galaxies attributed to Toomre Q-regulated disc formation
- Thick discs may continue to accumulate mass alongside thin-disc counterparts
- Use of long filter bands in JWST observations allowed study up to z ≲ 2 with unprecedented sensitivity and resolution

## Summary

The study of galactic thin and thick discs across cosmic history is a constantly evolving topic in the field of astronomy. Modern disc galaxies exhibit distinct thin and thick discs that can be differentiated based on their kinematics, radial structure, chemistry, and age. However, the formation mechanisms and timing of onset for these two components remain open questions. To address this gap in knowledge, researchers conducted a study using edge-on galaxies selected from flagship James Webb Space Telescope (JWST) programs. By examining the disc structures in rest-frame near-infrared bands, they were able to identify thick and thin discs at cosmological distances dating back over 10 billion years. This study classified galaxies into two categories: those requiring two distinct discs (thin and thick) and those well-fitted by a single disc. The results showed a strong correlation between disc radial sizes/vertical heights and total galaxy mass/disc mass across all cosmic times. Interestingly, the structure of thick discs resembled those found in single-disc galaxies, suggesting that galaxies may first form a thick disc before developing a thin one. This transition from single to double discs occurred earlier (around 8 billion years ago) in high-mass galaxies ($10^{9.75} - 10^{11}M_\odot$) compared to low-mass ones ($10^{9.0} - 10^{9.75}M_\odot$), indicating a sequential formation process following a "downsizing" trend. The delayed formation of thin discs in low-mass galaxies was attributed to Toomre $Q$-regulated disc formation, resulting in an observed anti-correlation between the ratio of thick-to-thin disc masses and total galaxy mass. Despite this dominant sequential pattern observed, it was noted that thick discs may continue to accumulate mass alongside their thin-disc counterparts. Previous studies at high redshifts did not require two distinct disc components due to limitations such as using short rest-frame wavebands affected by recent star formation. However, with the use of long filter bands in JWST observations, researchers were able to study cosmic epochs up to 𝑧 ≲ 2 (∼ 10 Gyr ago) with unprecedented sensitivity and resolution. In conclusion, this study sheds light on the evolution of galactic thin and thick discs across cosmic history, providing valuable insights into their formation mechanisms and timing of onset. Further research is needed to fully understand the complexities involved in the assembly of these two distinct structures within galaxies.