Little Red Dots and Supermassive Black Hole Seed Formation in Ultralight Dark Matter Halos

Abstract

We explore a possible mechanism for the formation of supermassive black hole (SMBH) seeds at the centers of ultralight dark matter (ULDM) halos in the early Universe. We investigate the conditions under which high-redshift baryonic gas, strongly confined by central solitonic cores of the halos, undergoes direct and monolithic collapse. The solitonic core sets characteristic mass and length scales for the confined baryons. Once the confined gas becomes self-gravitating, rapid inflow and shock heating may drive it into a high-temperature and high-density regime favorable for suppressing molecular cooling, without requiring a strong external UV background. We present semi-analytic scaling relations for the halo mass, soliton mass, baryonic core radius, and characteristic thermodynamic state of the gas, parametrizing the possible effects of baryonic contraction. These relations provide order-of-magnitude estimates of the characteristic range of SMBH seed masses as a function of redshift. In this framework, pristine gas clouds satisfying the adopted thermal criterion may avoid efficient fragmentation and undergo rapid central collapse, potentially forming massive black hole seeds with characteristic masses of order $10^5 M_\odot$, while systems below the threshold may form compact star clusters instead. The ULDM particle mass required to reproduce the inferred seed mass scale, $m \simeq O(10^{-22}){\rm eV}$, lies in a range favored by galactic-scale observations, suggesting a possible connection between the characteristic scales of galactic cores and early SMBH seeds. Our estimates indicate that favorable conditions for SMBH seed formation may arise at redshifts $z \gtrsim 10$. Such conditions may be relevant to the young SMBHs inferred in some little red dots, which appear to be embedded in compact, dense, ionized gas.