新猝灭星系是星族平均大小表观演化的原因
Newly-quenched galaxies as the cause for the apparent evolution in average size of the population
摘要：
节录。利用COSMOS自洽地研究了给定大小的淬灭早型星系（Q-ETG）在0.2 < z < 1.0区间内的数密度变化，以研究这些星系的大小演化。在10^10.5<M星系<10^11 M太阳时，我们发现紧凑型Q-ETG的数密度没有变化，而在>10^11 M太阳时，我们发现减少了30%。在两个质量箱中，Q-ETG的中值尺寸随时间的增加主要是由于更大和更扩散的Q-ETG的尺寸函数的增加。在所有质量下，紧凑型Q-ETG在以后的时期变得系统性地更红，（U-V）差异与它们的恒星星族的被动演化一致，表明它们是一个在大小上没有明显演化的种群。在所有时期，较大的Q-ETG（至少在较低的质量箱）的平均休息帧颜色系统蓝比那些更紧凑的Q-ETG，这表明前者比后者年轻。新的，大的，Q-ETG负责在给定质量下观察到的人口中值大小的增长，这一观点得到了恒星形成星系的大小和数量的支持，这些星系预计将成为新Q-ETG的祖先。在低质量仓中，新的Q-ETG比它们的恒星形成祖先小30%。这很可能是由于它们停止恒星形成后，它们的圆盘会褪色。与更高z值的比较表明，在质量不变的情况下，新猝灭星系的中值大小大致为（1+z）^-1。因此，在Q-ETG星群中看到的尺寸演化的主要原因是，单个Q-ETG的平均尺寸与它们被淬灭时宇宙的平均密度成比例，随后通过次要的合并，特别是在质量<10^11 Msun的情况下，单个物体的尺寸发生了变化。
英文摘要：
Abridged. We use COSMOS to study in a self-consistent way the change in the number densities of quenched early-type galaxies (Q-ETGs) of a given size over the interval 0.2 < z < 1.0 to study the claimed size evolution of these galaxies. At 10^10.5<Mgalaxy<10^11 Msun, we see no change in the number density of compact Q-ETGs, while at >10^11 Msun we find a decrease by 30%. In both mass bins, the increase of the median sizes of Q-ETGs with time is primarily caused by the addition to the size function of larger and more diffuse Q-ETGs. At all masses, compact Q-ETGs become systematically redder towards later epochs, with a (U-V) difference consistent with passive evolution of their stellar populations, indicating that they are a population that does not appreciably evolve in size. At all epochs, the larger Q-ETGs (at least in the lower mass bin) have average rest-frame colors systematically bluer than those of the more compact Q-ETGs, suggesting that the former are younger than the latter. The idea that new, large, Q-ETGs are responsible for the observed growth in the median size of the population at a given mass is supported by the sizes and number of the star-forming galaxies that are expected to be progenitors of the new Q-ETGs over the same period. In the low mass bin, the new Q-ETG have 30% smaller sizes than their star-forming progenitors. This is likely due to the fading of their disks after they cease star-formation. Comparison with higher z shows that the median size of newly-quenched galaxies roughly scales, at constant mass, as (1+z)^-1. The dominant cause of the size evolution seen in the Q-ETG population is thus that the average sizes of individual Q-ETGs scale with the average density of the Universe at the time when they were quenched, with subsequent size changes in individual objects through eg merging of secondary importance, especially at masses <10^11 Msun.
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