坠落的烟火：为什么坠落的星系会在星系团的外围发光？
Plunging fireworks: Why do infalling galaxies light up on the outskirts of clusters?
摘要：
本文利用SDSS DR 4的光谱数据，计算了星系的积分恒星形成率（SFR）和比恒星形成率（sSFR），结果表明，在邻近星系团（0.02<=z<=0.15）的外围（1-2 r_{200}）发现的星系（M_r<=-20.5）中，恒星形成活动增强。通过比较至少有一个星暴星系的星系团样本中星系的平均SFR，（log sSFR>=-10/yr & SFR>10 M_sun/yr）的星系团样本（“比较”集群），我们发现，尽管预期的下降，平均SFR星系对集群核心，SFR档案的两个样本是不同的，使得“比较”星系团中的星系在距离星系团中心的所有半径（<=3 r_{200}）处显示出较低的平均SFR。这种星系SFR的增加更可能在动力学非弛豫（sigma_v>~500 km/s）星系团中看到。同样明显的是，这些非弛豫的星系团目前正在通过星系通过直丝下落而组装，导致高速分散。另一方面，“比较”簇更有可能由相对低密度的长丝供给。我们发现，在星系团的外围星暴星系是在一个更高的局部密度比其他星系团星系在类似的径向距离，从星系团中心的环境。我们的结论是，一个相对较高的星系密度在星系团的下落区域促进星系之间的相互作用，导致恒星形成的瞬间爆发。这种相互作用在耗尽星系中恒星形成的燃料方面发挥着至关重要的作用，然后由于在星团致密内部运行的环境过程而被驱逐。
英文摘要：
(abridged)Integrated star formation rate (SFR) and specific star formation rate (sSFR), derived from the spectroscopic data obtained by SDSS DR4 are used to show that the star formation activity in galaxies (M_r<=-20.5) found on the outskirts (1-2r_{200}) of some nearby clusters (0.02<=z<=0.15) is enhanced. By comparing the mean SFR of galaxies in a sample of clusters with at least one starburst galaxy (log sSFR>=-10/yr & SFR>10 M_sun/yr) to a sample of clusters without such galaxies (`comparison' clusters), we find that despite the expected decline in the mean SFR of galaxies toward the cluster core, the SFR profile of the two samples is different, such that the galaxies in the `comparison' clusters show a lower mean SFR at all radius (<=3r_{200}) from the cluster centre. Such an increase in the SFR of galaxies is more likely to be seen in dynamically unrelaxed (sigma_v>~500 km/s) clusters. It is also evident that these unrelaxed clusters are currently being assembled via galaxies falling in through straight filaments, resulting in high velocity dispersions. On the other hand, `comparison' clusters are more likely to be fed by relatively low density filaments. We find that the starburst galaxies on the periphery of clusters are in an environment of higher local density than other cluster galaxies at similar radial distances from the cluster centre. We conclude that a relatively high galaxy density in the infalling regions of clusters promotes interactions amongst galaxies, leading to momentary bursts of star formation. Such interactions play a crucial role in exhausting the fuel for star formation in a galaxy, before it is expelled due to the environmental processes that are operational in the dense interiors of the cluster.
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