恆星再度生成－利用氣態金屬豐度揭開重生星系之謎

Abstract

星系的重生(Rejuvenation)過程在星系演化是一個亟需研究的問題。我們利用了兩個年齡敏感的光譜指數來挑選重生星系(Rejuvenating galaxies)，並從 SDSS MaNGA 計畫中的約一萬個星系中，辨別出了共 27 個重生星系。其中，有 25 個是利用空間解析(spatially-resolved)法挑選，有 2 個是利用集成(integrated)法挑選。這些重生星系在距今 300 百萬年內曾經經歷過第二次的恆星生成，且第二次恆星生成距離第一次恆星生成應久於 20 億年。透過分析重生星系的氣態金屬豐度(gas-phase metallicity)及其梯度，我們發現其氣態金屬豐度的值與其他恆星生成區域相比，並未顯著地較低，豐度梯度也未較平緩。我們確認了這些重生星系的氣體含量相比其他寧靜星系(quiescent galaxies)來得較多，甚至與恆星生成星系(star-forming galaxies)相比，也多落在其平均之上。另外，我們還發現重生星系受到周遭環境的潮汐影響力相較於寧靜星系及恆星形成星系來得較大，並且有6個星系都有著近且大的相鄰星系。特別的是，我們發現有 2 個重生星系顯示出氣體流入的跡象。我們推測，這些重生星系在過去雖有著一段低恆星生成的時期，但氣體仍存在其星系內部。而當鄰近的星系愈發靠近後，來自於鄰近星系的重力影響使得這些存在於重生星系內部的氣體變得不穩定，進而引發星系內部的恆星再度生成(也就是星系重生)。

Rejuvenating process remains a open question in the field of galaxy evolution. We identified 27 rejuvenating galaxies by Dn4000 and EW(Hδ) indicators in the local universe from SDSS DR17 MaNGA, which provide approximately 10,000 spatially-resolved galaxies. Among these rejuvenating galaxies, 25 were selected based on spectra from individual spaxels (spatial-pixels), and 2 were selected based on integrated spectra. These rejuvenating galaxies most likely experienced a second star-formation event longer than 2 Gyr after the first star burst. The second star-formation is most likely 1% to the first star burst and occurred within last 300 Myr. We calculate the gas-phase metallicity, and we do not find a lower metallicity for most of rejuvenating gas. Further we calculate the metallicity gradients for the rejuvenating galaxy, we still do not find a shallower gradient. These two indicate that the rejuvenating gas is inherent inside the galaxies, instead of originating from the intergalactic space. We confirm that rejuvenating galaxies have higher gas content compared to other quiescent galaxies, and their gas content is even comparable to the average gas content of star-forming galaxies. Additionally, we found that rejuvenating galaxies experienced stronger tidal influences from their surrounding environment compared to quiescent and star-forming galaxies. Six rejuvenating galaxies have close and large neighboring galaxies. Notably, we observed signs of pristine gas inflow in two rejuvenating galaxies. We propose that, for most of our rejuvenating galaxies, they experienced a period of extremely low star-formation, while retaining internal gas. When neighboring galaxies approached, their gravitational influence make the gas unstable, triggering the rejuvenation within the galaxies.