使用星系和宇宙學電腦模擬研究塵埃演化

Abstract

塵埃在星系演化中扮演了重要的角色。在星際介質中，塵埃的演化 跟非線性演化的恆星形成和恆星回饋有著強烈地關聯。恆星形成和恆 星回饋，它們推動了星系中化學成分豐富度的增長。而數值流體動力 電腦模擬是一個很好的工具來研究上述的非線性過程。 首先，我們檢驗了單一區域的塵埃演化模型是否可以重現銀河系和 鄰近星系的消光曲線。塵埃演化模型包含了，恆星噴出物中的塵埃形 成，塵埃被超新星衝擊摧毀，吸積作用造成的塵埃顆粒成長，以及塵 埃顆粒碰撞後的黏合和粉碎作用。我們用大顆粒 (≲ 0.03 μm) 和小顆粒 (≳ 0.03 μm) 來呈現塵埃顆粒尺寸的大小分佈。在合理範圍內的時間尺 度內，我們重現了銀河系的消光曲線。這表示我們的塵埃演化模型良 好的解釋了銀河系的塵埃演化過程。使用了無定型碳取代石墨來表示 炭塵埃的性質，以及考慮超新星選擇性破壞小顆粒塵埃後，大小麥哲 倫星系的消光曲線也可以重現。根據我們的結果，吸積作用造成的塵 埃顆粒成長對於重現銀河系和大小麥哲倫星系的消光曲線很重要。 接下來我們將塵埃演化模型加入數值流體動力模擬中，然後模擬單 一星系的塵埃演化。相較於單一區域模型，數值模擬提供了更真實的 物理環境來計算塵埃演化過程。在進一步的把塵埃種類分成碳塵埃與 矽塵埃後，我們可以得到消光曲線。這是第一次在模擬的星系中可以 研究星系空間上、密度上、金屬豐度上的消光曲線變化。我們呈現了 模擬的星系中的塵埃和氣體比、塵埃顆粒尺寸分佈、消光曲線的演化。 結果告訴我們，早期星系演化階段 (t ≲ 3 億年)，塵埃分布集中在星系 中間區域，因為這時候塵埃的來源主要是恆星產出的塵埃。在顆粒碰 撞的粉碎作用產出足夠的小顆粒塵埃後，吸積作用造成塵埃成長會變 得很有效率，這會快速的提升塵埃總量。在 t ≳ 30 億年後，這種塵埃 總量快速增長然後達到飽和，在那之後塵埃總量不會有太大變化。消 光曲線在 t ≲ 3 億年的時候是平坦的，因為恆星產生的塵埃是主要的 來源。經過吸積作用造成的塵埃成長後，消光曲線在紫外光波段變得 陡峭以及有一個明顯的 2175 Å 突起。在 t ≲ 30 億年後，塵埃顆粒碰撞 的黏合和粉碎作用分別主導了高密度及低密度區域。因此，在外側的 星系盤面，消光曲線有非常明顯的 2175 Å 突起和紫外光波段陡峭的斜 率，而在星系中心區域，2175 Å 突起變得緩和，紫外光波段的斜率也 變得平坦。我們發現，在 t ≲ 30 億年後的消光曲線符合了銀河系的觀 測。這說明了塵埃演化模型成功的被加入到星系數值模擬中。 最後，我們將塵埃演化模型加入到宇宙學電腦模擬，並額外的考慮 了塵埃受到星系間介質中高溫氣體的濺射作用。宇宙學模擬讓我們用 統計星系中塵埃豐度和塵埃性質。我們檢驗了塵埃質量函數和各種塵 埃性質與星系性質的比例關係。塵埃性質包含了塵埃和氣體比、塵埃 和恆星質量比、小顆粒和大顆粒豐度比。星系性質則包含恆星質量、 金屬豐度、氣體比例、恆星形成率。電腦模擬得出的塵埃質量函數與紅移 z = 0 的觀測數據大致符合，除了塵埃質量高的一端有過度產生 塵埃的現象。塵埃氣體比與金屬豐度關係與鄰近星系的觀測一致，表 示了塵埃演化模型成功與宇宙學電腦模擬結合。此外，我們探討了星 系中塵埃的紅移演化至紅移 z ∼ 5。我們發現最高的塵埃質量密度在紅 移 z = 2 和 1 之間。塵埃顆粒尺寸分布讓我們檢驗了消光曲線的紅移演 化，高紅移星系有著平坦的消光曲線，而紅移 z = 0 的星系，最陡峭的 消光曲線出現在金屬豐度 Z ∼ 0.3 Z⊙ 的星系。

Dust enrichment is one of the most important aspects in galaxy evolution. The evolution of dust is tightly coupled with the nonlinear evolution of the ISM including star formation and stellar feedback, which drive the chemical enrichment in a galaxy. Numerical hydrodynamical simulation provides a powerful approach to studies of such nonlinear processes. Firstly, we examine whether the dust enrichment model can reproduce the extinction curves of the Milky Way and the nearby galaxies using a one- zone model. The dust model includes all processes that dominate the dust evolution in galaxies such as dust production in stellar ejecta, destruction in supernova shocks, dust growth by accretion and coagulation, and dust disrup- tion by shattering. We also treat the evolution of grain sizes distribution by representing the entire grain radius range by small (≲ 0.03 μm) and large (≳ 0.03 μm) grains. The Milky Way extinction curve is reproduced in reasonable ranges for the timescale of the above processes, which shows that our models are successful in reproducing the Milky Way dust extinction properties. The LMC/SMC extinction curves can be reproduced by adopting amorphous car- bon for the carbonaceous dust species and additionally considering selective supernova destruction in which small grains are easier to be destroyed than large grains. Our results suggest that interstellar processes, in particular, grain growth by accretion, are important in reproducing the Milky Way, LMC and SMC extinction curves. Secondly, we implement the dust enrichment model mentioned above into smoothing particle hydrodynamics (SPH) simulation of a single galaxy. In comparison with one-zone model, simulation provides much more realistic physical conditions to estimate the dust evolution processes. By further separating dust species into carbonaceous and silicate dust, we can obtain the extinction curves. For the first time, the simulation allows us to examine the dependence of extinction curves on the position, gas density, and metallicity in the galaxy. We present the evolution of dust-to-gas mass ratio, grain size distribution and extinction curve in the simulated galaxy. Our results show that at the earliest evolutionary stage (t ≲ 0.3 Gyr), dust is limited to the central region of the galaxy since star formation starts from the center and stellar dust production is the dominant source. Grain growth by accretion becomes efficient after small grains become abundant by shattering and rapidly raises the total dust abundance. Because the dust mass increase by accretion is saturated at t ≳ 3 Gyr, the total dust abundance does not evolve much. Extinction curves are flat at t ≲ 0.3Gyr because stellar dust production dominates the total dust abundance. After dust growth by accretion becomes efficient, extinction curves have a prominent 2175 Å bump and a steep far-ultraviolet (FUV) rise. At t ≳ 3 Gyr, shattering and coagulation dominate the low and high density regions, respectively. Therefore, extinction curves show a very strong 2175 Å bump and steep FUV rise in the outer disc; in contrast, the center of galaxy have extinction curves with a moderate 2175 Å bump and FUV slope. The extinction curves at t ≳ 3 Gyr are consistent with that of the Milky Way, which indicates that the included processes in our models are consistent with the dust properties indicated by the Milky Way extinction curve. Finally, we present a cosmological simulation with dust evolution. We consider the same dust enrichment model as above with additionally consid- ering sputtering in the hot circum/intergalactic gas. Our cosmological simu- lation allows us to analyze the dust abundance and dust properties in galaxies statistically. We examine the dust mass function and various dust scaling rela- tions between dust properties including dust-to-gas mass ratio, dust-to-stellar mass ratio, and small-to-large grain abundance ratio and galaxy properties like stellar mass, metallicity, gas fraction, and specific star formation rate. We broadly reproduce the observed dust mass function at z = 0, except an over-prediction at dust mass ≳ 1010 M⊙. The relation between dust-to-gas mass ratio and metallicity at z = 0 fits the local galaxy observations, which implies that the dust evolution is implemented successfully in our cosmology simulation. Besides, we investigate the redshift evolution of dust content in galaxies up to z ∼ 5. We find that the comoving dust mass density is the highest between z = 2 and 1 in the history of the Universe. The grain size distribution calculated in our simulation allows us to examine the extinction curve. Flat extinction curves was found in high z galaxies; at z = 0, galax- ies with Z ∼ 0.3 Z⊙ have the steepest FUV slope than galaxies with other metallicities.