光学静止早型星系中的恒星形成特征
Star Formation Signatures in Optically Quiescent Early-type Galaxies
摘要：
近年来，有观点认为，局部宇宙中有很大一部分早型星系经历了低水平（< 1 M_sun/yr）的恒星形成（SF），这导致了强烈的紫外光通量过剩，但光学颜色为红色。这些研究中有许多是基于SDSS星系（z~0.1）的GALEX成像，因此受到5”FWHM的限制。低的紫外线分辨率留下了紫外线过量的其他可能性，例如老年人口或活动星系核。在这里，我们研究高分辨率的远紫外HST/ACS图像的光学静止的早期型星系与强紫外过剩。新的图像显示，这些中等质量（~5x10^10 M_sun）的早期类型星系中有四分之三显示出明显的扩展SF证据，通常以宽或同心的UV环的形式，在某些情况下，引人注目的螺旋臂。SDSS光谱可能会错过这些功能，由于小的纤维尺寸。与其他绿谷星系相比，紫外线过量的早型星系平均有更少的尘埃和更大的紫外线尺寸（D>40 kpc），这表明驱动SF的气体来自外部。因此，这些星系中的大多数看起来“恢复了活力”（例如，通过少量的富气合并或IGM增生）。对于样本的较小子集，不能排除SF下降（来自原始内部气体）。SF在非常大质量的早期类型（M_* > 10^11 M_sun）中是罕见的，这可能是活动星系核反馈的结果。除了扩展的紫外辐射，许多星系还显示出一个紧凑的中心源，这可能是一个弱的，光学上不显眼的活动星系核。
英文摘要：
In recent years argument has been made that a high fraction of early-type galaxies in the local universe experience low levels (< 1 M_sun/yr) of star formation (SF) that causes strong excess in UV flux, yet leaves the optical colors red. Many of these studies were based on GALEX imaging of SDSS galaxies (z~0.1), and were thus limited by its 5" FWHM. Poor UV resolution left other possibilities for UV excess open, such as the old populations or an AGN. Here we study high-resolution far-ultraviolet HST/ACS images of optically quiescent early-type galaxies with strong UV excess. The new images show that three-quarters of these moderately massive (~5x10^10 M_sun) early-type galaxies shows clear evidence of extended SF, usually in form of wide or concentric UV rings, and in some cases, striking spiral arms. SDSS spectra probably miss these features due to small fiber size. UV-excess early-type galaxies have on average less dust and larger UV sizes (D>40 kpc) than other green-valley galaxies, which argues for an external origin for the gas that is driving the SF. Thus, most of these galaxies appear `rejuvenated' (e.g., through minor gas-rich mergers or IGM accretion). For a smaller subset of the sample, the declining SF (from the original internal gas) cannot be ruled out. SF is rare in very massive early-types (M_* > 10^11 M_sun), a possible consequence of AGN feedback. In addition to extended UV emission, many galaxies show a compact central source, which may be a weak, optically inconspicuous AGN.
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