星系的分類中,橢圓星系被視為恆星形成活動較不活躍的族群。我們針對三千多 個鄰近的橢圓星系進行恆星形成活動的分析。我們藉由史隆數位化巡天(SDSS) 的影像資訊以及星系動物園(Galaxy Zoo)的統計資料定義橢圓星系的形態。接著, 我們將廣域紅外線巡天探測衛星(WISE)的近紅外以及中紅外波段觀測資料分別轉換為星系所含的恆星質量以及恆星形成率。在我們的樣本中,有大約 4.4%的橢圓星系含有非常強的中紅外波段輻射,我們定義這些樣本為恆星形成活躍橢圓星系(star-forming elliptical galaxy; SFEG)。為了探究在這些橢圓星系中不尋常 的恆星形成原因,我們針對一些可觀測性質進行分析,例如:光學以及中紅外波 段的顏色、恆星形成率、星系所含恆星質量、大尺度環境分布、以及表面顏色分 布,結果指出,SFEG 在這些性質的表現與其他橢圓星系非常不同。此外,我們進一步的分析橢圓星系的恆星形成歷史。SFEG 在恆星形成歷史的表現上,與螺旋星系非常類似,這樣的結果指出,某些 SFEG 具有持續的恆星形成活動,同時暗示著這些橢圓星系中的恆星形成活動並不是由於近期的星爆事件,而是來自某種較溫和的恆星形成機制。這些橢圓星系可能在近期曾與比較小的星系有交互作用,以維持/增加內部的恆星形成活動;此外,另一種可能,如同螺旋星系中的 密度波,這些橢圓星系內部可能有某些未知的機制,來幫助橢圓星系維持內部的恆星形成活動。

We investigated the star formation activities for a sample of nearby elliptical galaxies. From the Sloan Digital Sky Survey (SDSS) and the Galaxy Zoo, we selected a sample of morphologically-identified elliptical galaxies and considered their near-infrared (NIR) and mid-infrared (MIR) emission data from the Wide-field Infrared Survey Explorer (WISE) as indices of their stellar masses and star formation rates (SFRs) respectively. We found that there are about 4.4% of nearby elliptical galaxies showing very strong MIR emission and high specific star formation rates (sSFRs), which are comparable to the sSFRs of spiral galaxies. To probe the causes of the unexpected star formation activities, we studied some observable characters such as optical and MIR colors, SFRs, stellar masses, large-scale environments, and surface color distributions. We found that these characters of the star-forming elliptical galaxies (SFEGs) are very different from normal non-star-forming ellipticals. Furthermore, we analyzed the star formation history of our sample galaxies. The star formation history of the SFEGs is similar to that of spiral galaxies; the SFEGs show nearly constant SFRs at different epochs. Our results indicate that some of the SFEGs have continuous star formation activities and the star formation in the SFEGs might not be caused by recent burst events. To maintain/enhance the star formation activities, the SFEGs might have experienced some multiple minor-wet mergers, or there might be another mechanisms to maintain the star formation activities in the elliptical galaxies similar to the density waves in spiral galaxies.

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