NGC 404，一个在合并引起的蓝移到绿谷的复兴的透镜状星系
NGC 404, A Rejuvenated Lenticular Galaxy on a Merger-Induced, Blueward Excursion into the Green Valley
摘要：
我们已经发现了最近的恒星形成的最外层部分（1-4x R_25）附近的透镜状（S 0）星系NGC 404使用GALEX紫外成像。FUV亮源强烈集中在星系的HI环（根据del Rio等人的说法，由合并事件形成），即使平均气体密度是动态亚临界的。存档的HST成像显示了已分辨的上主序星，并最终证明紫外线来自最近的恒星形成活动。给出了NGC 404的FUV、NUV径向表面亮度廓线和积分星等。在环内，平均恒星形成率表面密度（Sigma_{SFR}）为2.2 × 10 ^-5 Msun/yr/kpc ^2。在FUV总通量中，70%来自HI环，该环以2.5 × 10 ^-3 Msun/yr的速度形成恒星。气体消耗的时间尺度，假设一个恒定的SFR和没有气体循环，是宇宙年龄的几倍。在紫外光学星系CMD的背景下，SF HI环的存在将NGC 404置于分隔红色和蓝色序列的绿色山谷中。恢复活力的透镜星系经历了一个合并引起的，从红色序列向蓝色序列的盘状构造偏移，在那里它可能会安静地进化，或者（如果适当地触发）经历一次爆发，能够将其置于蓝色/恒星形成序列上长达~1 Gyr。绿谷星系的人口是异质的，大多数系统从蓝色过渡到红色，但其他系统由于通过各种渠道获得新鲜气体而朝着相反的方向发展。
英文摘要：
We have discovered recent star formation in the outermost portion (1-4x R_25) of the nearby lenticular (S0) galaxy NGC 404 using GALEX UV imaging. FUV-bright sources are strongly concentrated within the galaxy's HI ring (formed by a merger event according to del Rio et al.), even though the average gas density is dynamically subcritical. Archival HST imaging reveals resolved upper main sequence stars and conclusively demonstrates that the UV light originates from recent star formation activity. We present FUV, NUV radial surface brightness profiles and integrated magnitudes for NGC 404. Within the ring, the average star formation rate surface density (Sigma_{SFR}) is 2.2x10^-5 Msun/yr/kpc^2. Of the total FUV flux, 70% comes from the HI ring which is forming stars at a rate of 2.5x10^-3 Msun/yr. The gas consumption timescale, assuming a constant SFR and no gas recycling, is several times the age of the Universe. In the context of the UV-optical galaxy CMD, the presence of the SF HI ring places NGC 404 in the green valley separating the red and blue sequences. The rejuvenated lenticular galaxy has experienced a merger-induced, disk-building excursion away from the red sequence toward bluer colors, where it may evolve quiescently or (if appropriately triggered) experience a burst capable of placing it on the blue/star-forming sequence for up to ~1 Gyr. The green valley galaxy population is heterogeneous, with most systems transitioning from blue to red but others evolving in the opposite sense due to acquisition of fresh gas through various channels.
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