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arXiv 2102.11936astro-ph.GA
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AGN和宇宙时间中的恒星形成

AGN and star formation across cosmic time

M. Symeonidis, M. J. Page

摘要:

通过红外星系光度函数和红外活动星系核光度函数的比较,研究了宇宙历史中恒星和活动星系核之间的能量平衡。前者对应于由恒星和活动星系核加热的尘埃的辐射,而后者只包括由活动星系核加热的尘埃的辐射。我们发现,在所有的红移(至少达到z~2.5),两个LF的高光度尾巴收敛,表明最红外发光的星系是活动星系核供电。我们的研究结果揭示了几十年来的难题,即与紫外和光学星系相比,红外星系LF中的高光度斜率更平坦。我们将这种差异归因于活动星系核占主导地位的星系的比例随着总红外光度(L_IR)的增加而增加。我们将L_IR-z参数空间划分为恒星形成区和活动星系核支配区,发现所有时期最亮的星系都位于活动星系核支配区。这为可达到的恒星形成率设定了一个潜在的“极限”,并对“极端恒星爆发”的丰度产生了怀疑:如果活动星系核不存在,L_IR>10^13 Lsun的星系将比我们目前可观测到的宇宙中的星系要少得多。我们还发现活动星系核影响了星系的平均尘埃温度(T_dust),从而影响了著名的L_IR-T_dust关系的形状。我们提出,为什么本地ULIRGs比高红移对应的热的原因是因为一个更高的分数的活动星系核占主导地位的星系之间的前组。

英文摘要:

We investigate the balance of power between stars and AGN across cosmic history, based on the comparison between the infrared (IR) galaxy luminosity function (LF) and the IR AGN LF. The former corresponds to emission from dust heated by stars and AGN, whereas the latter includes emission from AGN-heated dust only. We find that at all redshifts (at least up to z~2.5), the high luminosity tails of the two LFs converge, indicating that the most infrared-luminous galaxies are AGN-powered. Our results shed light to the decades-old conundrum regarding the flatter high-luminosity slope seen in the IR galaxy LF compared to that in the UV and optical. We attribute this difference to the increasing fraction of AGN-dominated galaxies with increasing total infrared luminosity (L_IR). We partition the L_IR-z parameter space into a star-formation and an AGN-dominated region, finding that the most luminous galaxies at all epochs lie in the AGN-dominated region. This sets a potential `limit' to attainable star formation rates, casting doubt on the abundance of `extreme starbursts': if AGN did not exist, L_IR>10^13 Lsun galaxies would be significantly rarer than they currently are in our observable Universe. We also find that AGN affect the average dust temperatures (T_dust) of galaxies and hence the shape of the well-known L_IR-T_dust relation. We propose that the reason why local ULIRGs are hotter than their high redshift counterparts is because of a higher fraction of AGN-dominated galaxies amongst the former group.

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