STAR FORMATION SIGNATURES IN OPTICALLY QUIESCENT EARLY-TYPE GALAXIES
STAR FORMATION SIGNATURES IN OPTICALLY QUIESCENT EARLY-TYPE GALAXIES
复制标题
光学静止早型星系中的恒星形成特征
DOI：
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发表时间：
2010
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通讯作者：
R. Rich
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作者：
S. Salim;R. Rich
近年来，有一种观点认为，本地宇宙中大部分早期型星系（ETG）经历了低水平（≲1 M☉ yr−1）的恒星形成（SF），导致紫外线通量强烈过剩，但光学颜色呈红色。其中许多研究都是基于斯隆数字巡天 (SDSS) 星系 (z ∼ 0.1) 的星系演化探测器成像，因此受到其 5 英寸半高宽的限制。紫外线分辨率差导致紫外线过量的其他可能性存在，例如老年群体或活跃的银河核（AGN）。在这里，我们研究了具有强烈紫外线过剩的光学静止早期型星系的高分辨率远紫外 HST/ACS 图像。新图像显示，这些中等质量（∼5 × 1010 M☉）的 ETG 中有四分之三显示出扩展的 SF 的明显证据，通常呈宽或同心的 UV 环形式，在某些情况下，呈引人注目的旋臂。由于光纤尺寸较小，SDSS 光谱可能会错过这些特征。与其他绿谷星系相比，紫外线过量的 ETG 平均具有更少的尘埃和更大的紫外线尺寸 (D > 40 kpc)，这表明驱动 SF 的气体有外部来源。因此，这些星系中的大多数看起来“重新焕发活力”（例如，通过少量富含气体的合并或星系间介质吸积）。对于较小的样本子集，不能排除 SF（来自原始内部气体）下降的可能性。 SF 在非常巨大的早期类型中很少见（M* > 1011 M☉），这可能是活动星系核反馈的结果。除了扩展的紫外线发射之外，许多星系还显示出一个紧凑的中心源，这可能是一个微弱的、光学上不明显的活动星系核。
In recent years, an argument has been made that a high fraction of early-type galaxies (ETGs) in the local universe experience low levels (≲1 M☉ yr−1) of star formation (SF) that causes strong excess in UV flux, yet leaves the optical colors red. Many of these studies were based on Galaxy Evolution Explorer imaging of Sloan Digital Sky Survey (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 active galactic nucleus (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 (∼5 × 1010 M☉) ETGs 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 ETGs 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 intergalactic medium 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* > 1011 M☉), 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.