标题： 早期类型系统中颜色梯度的起源及其在高红移处的紧凑性 显示英文标题

标题： The Origin of Color Gradients in Early-Type Systems and Their Compactness at High-z

摘要： 来信中，我们展示了基于斯隆数字巡天（SDSS）的 griz 波段和英国红外深空巡天（UKIDSS）的 YJHK 波段的 5080 个早期型星系（ETGs）的平均光学+NIR 色彩梯度估计值。 色彩梯度被定义为早期型星系中径向色彩分布曲线的对数斜率。 通过如此庞大的样本量，我们以史无前例的精度研究了平均色彩梯度随波段的变化。 我们发现：(i) 平均而言，色彩梯度主要由星系半径每增加一个数量级金属丰度减少约 -0.4 dex 引起；(ii) 在早期型星系中，存在一个小但显著的正年龄梯度，内层恒星群体比外层恒星群体平均年轻约 0.1 dex 每径向数量级。 此外，我们表明，在早期型星系中存在正的平均年龄梯度（如同本研究发现的那样），意味着它们在高红移时有效半径较小，与观测结果一致。 显示英文摘要

摘要： In this Letter, we present mean optical+NIR color gradient estimates for 5080 early-type galaxies (ETGs) in the grizYJHK wavebands of the Sloan Digital Sky Survey (SDSS) plus UKIRT Infrared Deep Sky Survey (UKIDSS). The color gradient is estimated as the logarithmic slope of the radial color profile in ETGs. With such a large sample size, we study the variation of the mean color gradient as a function of waveband with unprecedented accuracy. We find that (i) color gradients are mainly due, on average, to a metallicity variation of about -0.4dex per decade in galaxy radius; and (ii) a small, but significant, positive age gradient is present, on average, in ETGs, with the inner stellar population being slightly younger, by ~0.1dex per radial decade, than the outer one. Also, we show that the presence of a positive mean age gradient in ETGs, as found in the present study, implies their effective radius to be smaller at high z, consistent with observations.