Galaxy mergers are fascinating because galaxies grow and evolve through a long history of such events. When galaxies collide, they not only appear distorted but also undergo profound internal changes. Using detailed observations from the Sloan Digital Sky Survey MaNGA project, astronomers have examined how interactions reshape the distribution of chemical elements, especially oxygen, across galactic disks. The results show that soon after two galaxies make their first close encounter, streams of low-metallicity gas from the outskirts rush toward the center. This inflow dilutes the central regions and flattens the usual chemical gradient, where metals are normally most concentrated in the core and decrease toward the edges. Later, intense bursts of star formation enrich the gas again with fresh heavy elements, sometimes steepening the gradient instead of flattening it. In the final merging phase, galaxies experience another strong dilution, leaving their cores with lower metal content than expected. These findings challenge the long-standing assumption that mergers always smooth out chemical differences. Instead, the process is dynamic and stage-dependent: dilution and enrichment happen in cycles, depending on when and how galaxies interact. By analyzing hundreds of galaxies across different merger stages, this study offers a detailed view to date of how cosmic encounters reshape galactic chemistry and influence their long-term evolution.
星系合併之所以重要，是因為星系本身就是在持續的合併中形成的。當星系發生互擾與碰撞時，不僅外觀會被拉扯變形，內部也會經歷深刻的改變。利用史隆數位巡天計畫（SDSS）中的 MaNGA 觀測數據，天文學家研究了星系交互作用如何影響盤面上化學元素（特別是氧）的分布。研究結果顯示，當兩個星系第一次近距離接觸時，外圍的低金屬豐度氣體會湧入核心，稀釋中心的金屬含量，使原本由內而外遞減的金屬豐度梯度變得平坦。隨後，劇烈的恆星形成釋放出新的重元素，使金屬含量再度累積，有時甚至讓梯度比之前更陡。在最終合併階段，星系又會經歷一次強烈的氣體稀釋，使核心的金屬含量比預期更低。這些發現挑戰了「星系合併必然均化金屬分布」的傳統看法。實際上，金屬分布的演化是動態的，會隨交互作用階段在「稀釋與累積」之間循環。透過對數百個不同互擾及合併階段星系的分析，研究顯示了星系互擾與碰撞如何重塑星系的化學結構，並推動其長期演化。
