超深成像时代星系大小的物理定义
A physically motivated definition for the size of galaxies in an era of ultra-deep imaging
摘要：
目前的多波长深度成像调查允许以前所未有的精度探测星系的外围。利用这种情况下，我们定义了一个新的物理动机的测量星系的大小的基础上预期的位置的气体密度阈值恒星形成。采用理论和观测参数，我们使用恒星质量密度等值线在1 $M_{\rm \odot}$ pc $^{-2}$作为代理这个密度阈值的恒星形成。这种选择使我们的大小定义具有可操作性。有了这个新的尺寸措施，内在的全球恒星质量（$M_{\rm \star}$）的大小关系（探索超过五个数量级的恒星质量）的散射减少到$\sim$ 0.06 dex。该值比使用有效半径（$\sim$ 0.15 dex）测量的散射小2.5倍，比使用其他传统尺寸指标（如$R_{\rm 23.5,i}$（$\sim$ 0.09 dex）、Holmberg半径$R_{\rm H}$（$\sim$ 0.09 dex）和半质量半径$R_{\rm e,M_{\star}}$（$\sim$ 0.11 dex））测量的散射小1.5至1.8倍。此外，具有10 $^7$ $M_{\rm \odot} <$ $M_{\star} < 10^{11}$ $M_{\rm \odot}$的星系在大小上遵循幂律单调增加，斜率非常接近1/3，相当于在这个质量范围内的星系的平均恒星质量3D密度为$\sim$ 4.5 $\times$ 10 $^{-3}$ $M_{\rm \odot}$ pc $^{-3}$。具有$M_{\rm \star}$ $>$ 10 $^{11}$ $M_{\rm \odot}$的星系显示出与恒星质量不同的斜率，这表明在它们的恒星形成高峰期，恒星形成的气体密度阈值更大。
英文摘要：
Present-day multi-wavelength deep imaging surveys allow to characterise the outskirts of galaxies with unprecedented precision. Taking advantage of this situation, we define a new physically motivated measurement of size for galaxies based on the expected location of the gas density threshold for star formation. Employing both theoretical and observational arguments, we use the stellar mass density contour at 1 $M_{\rm \odot}$ pc$^{-2}$ as a proxy for this density threshold for star formation. This choice makes our size definition operative. With this new size measure, the intrinsic scatter of the global stellar mass ($M_{\rm \star}$) - size relation (explored over five orders of magnitude in stellar mass) decreases to $\sim$0.06 dex. This value is 2.5 times smaller than the scatter measured using the effective radius ($\sim$0.15 dex) and between 1.5 and 1.8 times smaller than those using other traditional size indicators such as $R_{\rm 23.5,i}$ ($\sim$0.09 dex), the Holmberg radius $R_{\rm H}$ ($\sim$0.09 dex) and the half-mass radius $R_{\rm e,M_{\star}}$ ($\sim$0.11 dex). Moreover, galaxies with 10$^7$ $M_{\rm \odot} <$ $M_{\star} < 10^{11}$ $M_{\rm \odot}$ increase monotonically in size following a power-law with a slope very close to 1/3, equivalent to an average stellar mass 3D density of $\sim$4.5$\times$10$^{-3}$ $M_{\rm \odot}$ pc$^{-3}$ for galaxies within this mass range. Galaxies with $M_{\rm \star}$$>$10$^{11}$ $M_{\rm \odot}$ show a different slope with stellar mass, which is suggestive of a larger gas density threshold for star formation at the epoch when their star formation peaks.
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