Ultra-Strongly Self-Interacting Dark Matter: From Phenomenology to Observables*

With the advent of the James Webb Space Telescope (JWST), observational probes of the structure of objects in the early universe are more readily available. In particular, the discovery of high-redshift (z ~ 10) Supermassive Black Holes (SMBHs) challenges the typical formation channels of these objects which cannot form the SMBH seeds within these timescales, and cannot grow them to the observed masses without significant periods of Super-Eddington accretion. Similarly, in the past couple of years, a brand new class of high-redshift objects has been discovered, called “Little Red Dots” (LRDs) - which are thought to be heavily dust enshrouded SMBHs (10⁶ -10⁸ M_⊙) hosted in a highly compact galaxy approximately 50 parsec in size. 

To address the potential formation of these objects, we have introduced a model of self-interacting dark matter (SIDM), where a small fraction of the SIDM is ultra-strongly self-interacting (uSIDM). The typical cross-sections of SIDM are on the order of 0.1-50 cm²/g, uSIDM cross-sections are on the order of 1,000-10,000 cm²/g. With such high cross-sections, the uSIDM undergoes rapid gravothermal evolution leading to a full core-collapse of the innermost portion of the dark matter halo. The rapid timescale for this collapse allows for the formation of a heavy SMBH seed well within the observed formation times. Subsequent sub-Eddington accretion grows the seeds to match the observed SMBH masses. Because of this formation mechanism, uSIDM is a prime candidate for the creation of LRDs. We demonstrated in two previous works, that through uSIDM, we can form high-redshift quasars and using the constraints on the uSIDM model parameters from fitting to these quasars, we produce the correct mass function for LRDs.

In this talk, I will present the phenomenology of uSIDM, which sets the stage for constraining the underlying particle physics of both uSIDM and SIDM through early universe probes as well as potential late universe signatures. The uSIDM model is a powerful tool that has observational impacts at nearly all epochs, and allows for a powerful explanation of exotic astrophysical objects.