Early JWST studies found an apparent population of massive, compact galaxies at redshifts z ≳ 7. Recently three of these galaxies were shown to have prominent Balmer breaks, demonstrating that their light at λrest ∼ 3500 Å is dominated by a stellar population that is relatively old (∼200 Myr). All three also have broad Hβ emission with σ > 1000 km s−1, a common feature of such “little red dots.” From Sérsic profile fits to the Near Infrared Camera images in F200W we find that the stellar light of galaxies is extremely compact: the galaxies have half-light radii of re ∼ 100 pc, in the regime of ultracompact dwarfs in the nearby Universe. Their masses are uncertain, as they depend on the contribution of possible light from an active galactic nucleus (AGN) to the flux at λrest > 5000 Å. If the AGN contribution is low beyond the Balmer break region, the masses are M* ∼ 1010–1011 M☉, and the central densities are higher than those of any other known galaxy population by 1 order of magnitude. Interestingly, the implied velocity dispersions of ∼1500 km s−1 are in very good agreement with the measured Hβ line widths. We suggest that some of the broad lines in “little red dots” are not due to AGNs, but simply reflect the kinematics of the galaxies, and speculate that the galaxies are observed in a short-lived phase where the central densities are much higher than at later times. We stress, however, that the canonical interpretation of AGNs causing the broad Hβ lines also remains viable.

中文翻译：

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带有巴尔默断裂的“小红点”的小尺寸和高隐含密度可以解释它们在没有活跃星系核的情况下宽阔的发射线


早期的 JWST 研究发现，在红移 z ≳ 7 处存在明显的大质量、致密星系群。最近，其中三个星系被证明具有明显的巴尔默断裂，这表明它们在 λrest ∼ 3500 Å 处的光是由相对古老的恒星种群（∼200 Myr）主导的。这三种星系还具有较宽的 Hβ 发射，在 1000 km s−1 处为 σ>，这是这种“小红点”的共同特征。从 Sérsic 剖面拟合到 F200W 的近红外相机图像，我们发现星系的恒星光非常紧凑：星系的半光半径为 re∼ 100 pc，处于附近宇宙中的超紧凑矮星状态。它们的质量是不确定的，因为它们取决于来自活跃星系核 （AGN） 的可能光对 λrest > 5000 Å 通量的贡献。如果 AGN 贡献在 Balmer 断裂区之外很低，则质量为 M* ∼ 1010–1011 M☉，中心密度比任何其他已知星系群的密度高 1 个数量级。有趣的是，∼1500 km s−1 的隐含速度色散与测得的 Hβ 线宽非常吻合。我们认为，“小红点”中的一些粗线不是由于 AGN 引起的，而只是反映了星系的运动学，并推测这些星系是在短暂的阶段观察到的，其中中心密度比后来的时间要高得多。然而，我们强调，导致宽 Hβ 线的 AGNs 的规范解释也仍然可行。