Impact of subhalo dynamical friction heating on the formation of the first structures in the universe

TL;DR

This paper models gas heating from subhalo dynamical friction within dark matter halos, showing its significant role in early universe structure formation, star formation suppression, and black hole seed creation.

Contribution

It introduces a model based on TNG50 simulation data to quantify dynamical friction heating effects across cosmic time and halo masses, highlighting its impact on early star formation and black hole formation.

Findings

Dynamical friction heating can suppress gas cooling in early minihalos.

Heating rates are comparable to molecular hydrogen cooling at high redshift.

This mechanism favors direct-collapse black hole seed formation.

Abstract

We present a model for gas heating, driven by dynamical friction from orbiting subhalos within dark matter halos. Using data from the TNG50 simulation, we derive the subhalo mass function and calculate the dynamical friction heating rate for a wide range of halo masses and redshifts from $z = 15$ to 0. Our results show that, by converting gravitational potential energy into thermal energy, dynamical friction is an important mechanism for galaxy quenching in massive halos at low redshifts, consistent with previous studies. Additionally, we find that in the early universe at $z \sim 15$, heating rates can be comparable to the molecular hydrogen cooling rates in metal-free minihalos. This can suppress gas cooling and fragmentation and does increase the critical molecular fraction for Pop III star formation by up to one order of magnitude, thereby making Pop III star formation more difficult. In combination with the Lyman-Werner background, the dynamical friction heating mechanism favors the formation of direct-collapse black hole (DCBH) seeds in atomic cooling halos, even when the average H$_2$ fraction is $\sim 10^{-5}$ during the minihalo progenitor phase. Dynamical friction heating at a fixed host halo mass can vary by two orders of magnitude due to the scatter in the number of subhalos. To capture dynamical friction heating in simulations, it is necessary to resolve subhalos with a subhalo to host halo mass ratio $\psi \gtrsim 0.05$.