利用暗能量光譜儀首年資料探究鄰近宇宙的重子循環

Abstract

宇宙中的可見物質，包括恆星、星系及彌散氣體，皆由重子組成。理解重子如何透過氣體吸積、化學增豐與氣體外流，在星系內部及其周圍循環，是研究星系演化的核心課題。然而，這些過程彼此密切交織、物理機制複雜，使得純理論建模極具挑戰。因此，觀測作為約束條件對於建立正確的理論理解至關重要。

為此，本論文利用暗能量光譜儀（DESI）的首年觀測資料，探究了星系性質與星際介質（ISM）及環繞星際物質（CGM）之間的關聯。從DESI 所提供的巨量資料，我們得以偵測訊號微弱的化學豐度，並拓展星系性質的研究範圍。

我們首先利用約 90 萬組星系與類星體對，分析在 z<0.4 由 Ca II 吸收線示蹤的冷 CGM 特性，並研究其與星系性質之間的關係。通過該龐大的資料集，我們獲得可達毫埃等級的疊加光譜，進而詳細刻畫了 Ca II 吸收強度如何隨著星系質量、恆星形成率、紅移、方位角、暗物質暈質量及星系類型（包含 AGN）而改變。本研究結果涵蓋了從 10^8 到 10^11 M☉ 約三個數量級的星系質量，並顯示冷 CGM 的性質在恆星形成星系與靜謐星系間存在顯著差異。

其次，我們從 DESI 發射線星系（ELG）計畫中，發現並分析約 40,000 個低質量星系。透過疊加光譜，我們偵測到微弱的發射線，並以直接法測量其氣體金屬豐度，將質量與金屬豐度之間的關係下限延伸至約1個數量級的更低質量端。另外，與 DESI 亮星系（BGS） 的樣本相比，ELG低質量星系在此關係的低質量端呈現出較明顯的平緩趨勢。

總體而言，本研究展現了大規模光譜巡天計劃能有效捕捉微弱訊號與發掘更廣泛的星系族群，有助於增進我們對於星系演化的理解。未來新一代的光譜巡天計畫將進一步提高測量精度，並擴展星系參數空間的覆蓋範圍，從而深化我們對重子物質如何影響星系生命週期的理解。

Luminous matter in the Universe, including stars, galaxies and diffuse gas, is composed of baryons. Understanding how baryons cycle in and around galaxies via accretion, chemical enrichment, and outflows is central to the study of galaxy evolution. However, the complexity of these interconnected processes makes them challenging to model from first principles. As a result, observational constraints are essential for guiding and testing our understanding of the underlying physical mechanisms.

In this thesis, I examine how galaxy properties are linked to the properties of the interstellar medium (ISM) and the circumgalactic medium (CGM) by leveraging the Year 1 data of the Dark Energy Spectroscopic Instrument (DESI). The statistical power of DESI allows me to probe chemical abundances in low-signal regimes and expand the accessible parameter space over a wide range of galaxy properties.

First, I investigate how the properties of the cool CGM, traced by Ca II absorption lines, relate to those of galaxies at z<0.4. By using approximately 900,000 galaxy-quasar pairs and stacking this large dataset, I achieve composite spectra with sensitivity down to the mÅ level, allowing detailed characterization of Ca II absorption as a function of stellar mass, SFR, redshift, azimuthal angle, halo mass, and galaxy types, including AGNs. These relationships cover over three orders of magnitude in stellar mass from 10^8 to 10^11 M☉, and reveal that the properties of the cool CGM differ significantly between star-forming and quiescent galaxies.

Second, I identify and characterize a large population of approximately 40,000 low-mass galaxies by exploiting a serendipitous subset of the DESI Emission Line Galaxy (ELG) survey. Through stacking analysis, I detect weak auroral lines, and measure gas-phase metallicities using the direct method. This extends the mass–metallicity relation by 1 dex toward lower stellar masses compared to previous studies. A comparison with galaxies from the Bright Galaxy Survey (BGS) reveals a distinct flattening of the relation at the low-mass end.

Together, these results demonstrate the power of large-scale spectroscopic surveys in capturing faint signals and unveiling broader galaxy population, thereby advancing our understanding of galaxy evolution. Looking ahead, next-generation spectroscopic surveys will further refine these insights by enabling more precise measurements across an even broader parameter space. Ultimately, such observational advances will enhance our understanding of how baryons drive the life cycle of galaxies across cosmic time.