The Unusual Tidal Dwarf Candidate in the Merger System NGC 3227/3226: Star Formation in a Tidal Shock?
The Unusual Tidal Dwarf Candidate in the Merger System NGC 3227/3226: Star Formation in a Tidal Shock?
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合并系统 NGC 3227/3226 中不寻常的潮汐矮星候选者：潮汐冲击中的恒星形成？
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发表时间：
2004
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影响因子：
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通讯作者：
D. Forbes
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作者：
C. Mundell;P. James;N. Loiseau;E. Schinnerer;D. Forbes
我们报告在与相互作用的Seyfert系统NGC 3227/3226相关的H I云中发现了活跃的恒星形成，该系统最初被蒙代尔等人确定为候选潮汐矮星系（TDG），我们将其命名为J1023+1952。我们提供的宽带光学图像B、R、I（来自艾萨克牛顿望远镜）和紫外线图像（来自xmm -牛顿望远镜）显示，H I云与正在形成的大质量恒星有关，被一个漫射紫外晕包围的蓝色结团（MB > -15.5等），在云的南半部有一个高中性氢柱密度的脊（NH ~ 3.7 × 1021 cm-2）。我们还探测到Hα的辐射，其通量密度为FHα ~ 2.55 × 10-14 ergs -1 cm-2，对应的恒星形成速率为SFR(Hα) ~ 10.6 × 10-3 M☉y -1。J1023+1952位于北潮尾的底部，尽管它在空间上与NGC 3227盘的边缘重叠，但Mundell等人表明，它的H I云在运动学上是不同的，其H I平均速度比NGC 3227高150 km s-1。比较电离（Hα）和中性（H I）气体的云的运动学显示出密切匹配的后退速度，提供了强有力的证据，表明恒星形成结嵌入在J1023+1952中，而不仅仅是NGC 3227背景盘中的光学结，从而证实J1023+1952是一个富气体（MH/LB > 1.5）矮星系。在云的北半部没有发现恒星形成，尽管有相似的H I柱密度；相反，我们新的高分辨率H - I图像显示了一个高柱密度的山脊，与B - I图像中最红的结构相吻合。我们认为这些结构是由于NGC 3227盘的背景恒星连续体被J1023+1952固有的尘埃吸收，从而将J1023+1952置于NGC 3227的视线前方。关于J1023+1952的起源，我们讨论了两种可能的情况：一种是与NGC 3227和NGC 3226相互作用的第三个先前存在的矮星系，另一种是由NGC 3227的气态盘上的潮汐碎片凝聚而成的新形成的矮星系。第一种情况是可行的，因为NGC 3227是一个星系群中最亮的成员，在这个星系群中，先前存在的矮星系预计会很常见。然而，在J1023+1952中缺乏可探测的老恒星群，使得潮汐起源的可能性更大。如果J1023+1952是一个由返回的气态潮汐尾物质形成的束缚天体，那么它位于北尾底部的不寻常位置意味着它的动态年轻年龄与其恒星形成年龄相似，并表明它处于TDG演化的最早阶段。无论J1023+1952的起源是什么，我们认为它的恒星形成是由潮汐碎片坍塌引发的。
We report the discovery of active star formation in the H I cloud associated with the interacting Seyfert system NGC 3227/3226 that was originally identified as a candidate tidal dwarf galaxy (TDG) by Mundell et al. and that we name J1023+1952. We present broadband optical B, R, I (from the Isaac Newton Telescope), and ultraviolet images (from XMM-Newton) that show that the H I cloud is associated with massive ongoing star formation seen as a cluster of blue knots (MB ≲ -15.5 mag) surrounded by a diffuse ultraviolet halo and cospatial with a ridge of high neutral hydrogen column density (NH ~ 3.7 × 1021 cm-2) in the southern half of the cloud. We also detect Hα emission from the knots with a flux density of FHα ~ 2.55 × 10-14 ergs s-1 cm-2 corresponding to a star formation rate of SFR(Hα) ~ 10.6 × 10-3 M☉ yr-1. J1023+1952 lies at the base of the northern tidal tail, and, although it spatially overlaps the edge of the disk of NGC 3227, Mundell et al. showed that the H I cloud is kinematically distinct with an H I mean velocity 150 km s-1 higher than that of NGC 3227. Comparison of ionized (Hα) and neutral (H I) gas kinematics of the cloud shows closely matched recessional velocities, providing strong evidence that the star-forming knots are embedded in J1023+1952 and are not merely optical knots in the background disk of NGC 3227, thus confirming J1023+1952 as a gas-rich (MH/LB > 1.5) dwarf galaxy. No star formation is detected in the northern half of the cloud, despite similar H I column densities; instead, our new high-resolution H I image shows a ridge of high column density coincident with the reddest structures evident in our B - I image. We suggest that these structures are caused by the background stellar continuum from the disk of NGC 3227 being absorbed by dust intrinsic to J1023+1952, thus placing J1023+1952 in front of NGC 3227 along the line of sight. We discuss two scenarios for the origin of J1023+1952: as a third, preexisting dwarf galaxy involved in the interaction with NGC 3227 and NGC 3226, or as a newly forming dwarf galaxy condensing out of the tidal debris removed from the gaseous disk of NGC 3227. The first scenario is feasible given that NGC 3227 is the brightest member of a galaxy group, an environment in which preexisting dwarf galaxies are expected to be common. However, the lack of a detectable old stellar population in J1023+1952 makes a tidal origin more likely. If J1023+1952 is a bound object forming from returning gaseous tidal tail material, its unusual location at the base of the northern tail implies a dynamically young age similar to its star formation age, and suggests it is in the earliest stages of TDG evolution. Whatever the origin of J1023+1952, we suggest that its star formation is shock-triggered by collapsing tidal debris.