宇宙大尺度結(jié)構(gòu)對星系形成和空間排列的影響

王鵬>

(中國科學院紫金山天文臺南京210023)

星系的空間排列以及指向并非隨機的. 星系的主軸和角動量與物質(zhì)的大尺度分布存在一定的指向相關(guān). 這一方向的研究將可以促進我們對結(jié)構(gòu)形成機制的認識， 并提供一個獨立的視角去檢驗星系形成過程中的環(huán)境效應(yīng). 本文就相關(guān)問題做了詳細的探討.

首先， 我們提出了一種有別于傳統(tǒng)的大尺度結(jié)構(gòu)分類方法. 該方法具有非常高的精度和效率.

其次， 通過考察數(shù)值模擬中暗物質(zhì)暈(暗暈)在大尺度結(jié)構(gòu)中的并合歷史， 我們研究了暗暈吸積方向相對于暗暈形狀和大尺度方向(物質(zhì)塌縮最慢的方向)之間的關(guān)系. 我們發(fā)現(xiàn)， 子暗暈傾向于沿著宿主暗暈的主軸方向被吸積. 這一模式是普適的， 其一致程度隨著宿主暗暈的質(zhì)量增加逐漸變強. 我們還發(fā)現(xiàn)， 子暗暈在被吸積時， 其相對于大尺度方向之間的關(guān)系不是普適的. 小質(zhì)量宿主暗暈主要傾向于垂直于大尺度方向吸積子暗暈， 而大質(zhì)量宿主暗暈傾向于平行于大尺度方向吸積子暗暈. 這種“兩階段”的吸積模式打破了前人一直認為的只存在單一的“平行”吸積模式， 為解釋角動量方向和大尺度方向翻轉(zhuǎn)問題提供了思路.

同時， 我們還仔細研究了暗暈角動量-大尺度結(jié)構(gòu)相關(guān)關(guān)系的形成和演化歷史. 我們發(fā)現(xiàn)暗暈角動量-大尺度結(jié)構(gòu)相關(guān)性中的翻轉(zhuǎn)現(xiàn)象不僅存在于纖維狀(Filament)結(jié)構(gòu)中， 還存在于團塊結(jié)構(gòu)中(Knot)，對于纖維狀結(jié)構(gòu)中的暗暈， 其角動量-大尺度相關(guān)性與暗暈的形成時間、遷移時間以及暗暈在纖維狀結(jié)構(gòu)中的質(zhì)量增長率有關(guān). 我們進一步發(fā)現(xiàn)， 統(tǒng)計上， 所有暗暈在早期的角動量方向都是傾向平行于大尺度方向， 隨后逐漸向著垂直方向演化. 大質(zhì)量的暗暈演化得較快， 而小質(zhì)量的暗暈演化得較慢. 因此，如今小質(zhì)量暗暈表現(xiàn)出角動量方向和大尺度方向的平行趨勢， 而大質(zhì)量暗暈則表現(xiàn)為垂直趨勢. 這一演化模式與暗暈在大尺度上吸積物質(zhì)方式的改變有關(guān): 早期暗暈在垂直大尺度方向上吸積物質(zhì)， 從而使得所有暗暈角動量平行大尺度方向， 隨著平行大尺度方向物質(zhì)吸積逐漸加劇， 暗暈角動量逐漸向著垂直大尺度方向轉(zhuǎn)變.

最后， 我們考察了流體數(shù)值模擬中星系角動量-大尺度的相關(guān)性. 我們的主要發(fā)現(xiàn)是: 低質(zhì)量、藍色、扁圓形星系的角動量方向傾向與大尺度潮汐場中物質(zhì)塌縮最慢的方向平行， 而大質(zhì)量、紅色、長橢形星系的角動量方向往往與大尺度方向垂直. 這些因素中，質(zhì)量是主因. 在紅移z=0時，從平行到垂直趨勢過渡位置對應(yīng)的恒星質(zhì)量約為109.4h-1M ⊙、g-r顏色約為0.62、3軸形狀參數(shù)約0.4. 隨著紅移增加， 過渡質(zhì)量逐漸變小.

The spatial distribution and orientation of galaxies are not random. The direction of galaxy shape and spin are correlated with the direction of the distribution of mass on large scale. Research in this field will promote our understanding of the mechanism of structure formation and provide an independent perspective to examine the environmental effects of galaxies formation. We have discussed in detail with related issues.

First of all，we introduced an improved algorithm of classifying the large scale structure.

We then investigated the accretion history of dark matter halos in large scale structure.We studied the tendency of accretion relative to halo’s shapes and large scale structure.We find that subhalos tend to be accreted along the direction of the major axis of the host halo. This accretion pattern is universal and depends on the virial mass of the host halo.We also find that the accretion relative to the direction of large scale (the slowest direction of material collapse) is not universal. Low-mass host halos tend to accrete subhaloes from the direction that’s perpendicular to the direction of large scale structure， while high-mass hosts tend to accrete subhaloes from the direction that is aligned with the direction of large scale structure. This “two-stage” accretion mode breaks the “only parallel” accretion mode suggested in previous works， providing a solution to the problem of halo spin-flip.

Additionally，we also studied the formation and evolution of the correlation between the halo spin and the direction of the large scale structure(spin-LSS).We find that the spin flips are not only occurring in filaments but also in knots. For halos in filament， the spin-LSS correlation is related to their formation time， migration time and mass growth rate. We further found that， statistically， all halos have their spins aligned with the direction of LSS at the very early time， and then gradually evolve toward the vertical direction. High-mass halo evolves fast，while low-mass halo evolves slowly. Therefore，atz=0，low-mass halo still shows a parallel trend while high-mass halo shows a vertical trend. This evolution pattern depends on the mass accretion history during halo formation.

At last，the correlation between galaxy spin and the large scale tidal field is investigated.We mainly find that the spins of low-mass， blue， oblate galaxies are preferentially parallel to the direction of the large scale tidal field， while the spins of high-mass， red， prolate galaxies tend to be perpendicular to the direction of the large scale tidal field. Of these three properties， galaxy stellar mass is the main factor. The transition from parallel to perpendicular trend occurs at about 109.4h-1M⊙in the stellar mass，～ 0.62 in g-r color，and～0.4 in triaxiality.