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

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 $ψ\gtrsim 0.05$.

Figures (23)

• Figure 1: Subhalo mass function in TNG50-1 at $z = 0$. We group the host halos in 5 mass bins separated equally in logarithmic scale, covering the minimum to maximum mass, as shown by the different colors. The solid and dashed grey lines are the evolved and unevolved subhalo mass function, respectively, in jiang_statistics_2016, vandenbosch_statistics_2016. The vertical dashed lines indicate the critical mass ratio where for the left boundary of the host mass range, its subhalo reaches the resolution limit of 50 dark matter particle mass. Using only the resolved results, i.e. the data to the right of the vertical lines, we show the best-fit subhalo mass function in the solid black line. Our best-fit subhalo mass function naturally becomes almost the same as the evolved subhalo mass function in jiang_statistics_2016, vandenbosch_statistics_2016.
• Figure 2: Same as Figure \ref{['fig:SHMF_snap_99']} but at $z = 12$. The slope of the SHMF is shallower compared to $z = 0$ because at larger $\psi$, $dN/d\lg \psi$ is closer to the unevolved SHMF in jiang_statistics_2016.
• Figure 3: The evolution of the best-fit $\alpha$ (red dots) and $\lg A$ (blue triangles) as a function of redshift. From $z = 15$ to $z = 0$, the subhalos of higher mass ratios transform from being similar to the unevolved SHMF to being similar to the evolved SHMF, leading to a steeper slope, or a larger $\alpha$. In the fitting for SHMF, $\lg A$ is anti-correlated with $\alpha$. At $z = 0$, our results agree with the evolved SHMF in jiang_statistics_2016, vandenbosch_statistics_2016, which are indicated by the filled grey circles and triangles. The dashed lines show the fit for the trend of the redshift evolution of the best-fit $\alpha$ and $\lg A$.
• Figure 4: Same as Figure \ref{['fig:SHMF_redshift_evolution_alpha_lgA']} but for the evolution of best-fit $\omega$ and $\ln \beta$ in SHMF. We use a piecewise function to fit their evolution, which is a constant for $z > 6$ and a linearly increasing line below $z = 6$.
• Figure 5: The scatter of the cumulative SHMF at $z = 0$. Same as Figure \ref{['fig:SHMF_snap_99']}, the rainbow colors represent different host halo mass bins, and the black solid line is overall best-fit SHMF. The vertical lines correspond to mass resolution. We show the scatter of the SHMF in the TNG data with the error bars. As a comparison, we use dashed lines to indicate the range of Poisson fluctuations. The scatter becomes super-Poissonian when $\psi \lesssim 10^{-2}$, especially for the lowest mass ratios.