恒星核盘的多波段光度分解
Multiband photometric decomposition of nuclear stellar disks
摘要：
小而明亮的恒星盘，尺度长度只有几十秒差距，已知存在于星系的中心。它们被认为是在耗散过程中形成的，作为恒星在气体中形成的最终结果，要么在合并（或收购）事件中吸积，要么通过核棒的长期演化堆积起来。迄今为止，只有少数几个问题得到了详细研究。利用哈勃空间望远镜（HST）的影像资料，研究了室女星系团中三个早型星系NGC 4458、NGC 4478和NGC 4570的核星盘光度参数。我们的目标是限制恒星的形成过程。采用Scorza & Bender提出的光度分解方法，假设核盘是无限薄的指数形核盘，导出了核盘的中心面亮度、刻度长度、倾角和位置角。核盘的位置，方向和大小在使用宽视场行星照相机2和高级巡天照相机获得的所有图像中都是相同的，并可在HST科学档案中获得。每个圆盘的刻度长度、倾角和位置角在所观察到的U、B、V和I通带的误差范围内是恒定的，与它们的值和宿主球体的属性无关。我们解释的颜色梯度的情况下，在核盘的恒星星族的签名，恒星形成均匀地发生所有通过其扩展。相反，预计由内而外的形成方案会产生颜色梯度，因此被排除。
英文摘要：
Small and bright stellar disks with scale lengths of few tens of parsec are known to reside in the center of galaxies. They are believed to have formed in a dissipational process as the end result of star formation in gas either accreted in a merging (or acquisition) event or piled up by the secular evolution of a nuclear bar. Only few of them have been studied in detail to date. Using archival Hubble Space Telescope (HST) imaging, we investigated the photometric parameters of the nuclear stellar disks hosted by three early-type galaxies in the Virgo cluster, NGC 4458, NGC4478, and NGC4570. We aimed at constraining the process of formation of their stars. The central surface brightness, scale length, inclination, and position angle of the nuclear disks were derived by adopting the photometric decomposition method introduced by Scorza & Bender and assuming the disks to be infinitesimally thin and exponential. The location, orientation, and size of the nuclear disks is the same in all the images obtained with the Wide Field Planetary Camera 2 and Advanced Camera for Survey and available in the HST Science Archive. The scale length, inclination, and position angle of each disk are constant within the errors in the observed U, B, V, and I passbands, independently of their values and of the properties of the host spheroid. We interpret the absence of color gradients in the stellar population of the nuclear disks as the signature that star formation homogeneously occurred all through their extension. A inside-out formation scenario is, instead, expected to produce color gradients and therefore is ruled out.
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