Frequency of the dark matter subhalo collisions and bifurcation sequence arising formation of dwarf galaxies

TL;DR

This work investigates whether head-on collisions between gas-rich dark matter subhaloes (DMSHs) can produce dark-matter-deficient dwarfs within a cold dark matter framework. It combines analytical estimates of DMSH collision frequencies in an NFW host potential with a one-dimensional shock–gas cooling model to define a bifurcation: slow collisions merge into dark-matter-dominated galaxies, whereas moderate-velocity collisions yield dark-matter-deficient galaxies via gas-density–driven star formation at the collision surface; very fast collisions lead to shock-breakout and failure to form galaxies. The authors validate these regimes with three-dimensional SPH/N-body simulations that include star formation and supernova feedback, showing that outcomes depend on subhalo mass, relative velocity, metallicity, and feedback strength. The results imply collisions are a frequent channel for forming DM-deficient dwarfs, particularly in inner regions of host halos, and highlight the roles of cooling, feedback, and shocks in shaping the DM content of emergent galaxies. Overall, this collision-driven pathway complements tidal scenarios and offers a quantitative framework to interpret observed DM-deficient dwarfs and to predict their properties in different environments.

Abstract

The cold dark matter (CDM) model predicts galaxies have 100 times more dark matter mass than stars. Nevertheless, recent observations report the existence of dark-matter-deficient galaxies with less dark matter than expected. To solve this problem, we investigate the physical processes of galaxy formation in head-on collisions between gas-containing dark matter subhaloes (DMSHs). Analytical estimation of the collision frequency between DMSHs associated with a massive host halo indicates that collisions frequently occur within 1/10th of the virial radius of the host halo, with a collision timescale of about 10 Myr, and the most frequent relative velocity increases with increasing radius. Using analytical models and numerical simulations, we show the bifurcation channel of the formation of dark-matter-dominated and dark-matter-deficient galaxies. In the case of low-velocity collisions, a dark-matter-dominated galaxy is formed by the merging of two DMSHs. In the case of moderate-velocity collisions, the two DMSHs penetrate each other. However the gas medium collides, and star formation begins as the gas density increases, forming a dwarf galaxy without dark matter at the collision surface. In the case of high-velocity collisions, shock-breakout occurs due to the shock waves generated at the collision surface reaching the gas surface, and no galaxy forms. For example, the simulation demonstrates that a pair of DMSHs with a mass of 10^9 Msun containing gas of 0.1 solar metallicity forms a dark-matter-deficient galaxy with a stellar mass of 10^7 Msun for a relative velocity of 200 km/s.

Figures (15)

• Figure 1: Probability distribution of the relative velocity for $c = 7.5$ calculated by equation \ref{['eq: Prrel']}.
• Figure 2: Distributions of collision frequency of equation \ref{['eq: Collision']} between DMSHs within a host galaxy for (a) $c=14.8$, (b) $c=10.5$, (c) $c=7.50$, and (d) $c=5.33$, respectively. Top sub-panel in each panel: dependence of collision frequency on the radius of a host halo. Right sub-panel in each panel: dependence of collision frequency on the relative velocity between two DMSHs.
• Figure 3: Radial profiles of cumulative collision frequency within $r$ divided by parameters $N^2 \eta^2 /c_\mathrm{sub}^2$, which are defined as equation \ref{['eq: CumColFreq']} for $c=14.8,\,10.5,\,7.50,\text{ and } 5.33$, respectively.
• Figure 4: Results of analytical models assuming 0.1 solar metallicity. Grey region: velocity condition satisfied with the formation of the dark-matter-dominated galaxies. Blue region: velocity condition satisfied with the formation of dark-matter-deficient galaxies. Red region: no galaxy form to occur shock-breakout. The right axis indicates the temperature divided by the mean molecular weight $T/\mu$, which corresponds to the kinetic energy of the relative velocity $v_\mathrm{rel}$.
• Figure 5: Snapshots of dark matter density (top), gas density (middle) and stellar density (bottom) of collision simulation between DMSHs with $10^9\,\mathrm{M_\odot}$ at the relative velocity of $20\,\mathrm{km\,s^{-1}}$. All the colour bars for mass density range from $10^{-29}$ to $10^{-21}\,\mathrm{g\,cm^{-3}}$ . From left to right, $t=0,\,285,\,570\text{ and }884\,\mathrm{Myr}$, respectively. A dark-matter-dominated galaxy forms in the case of this velocity. The masses of star, gas and dark matter enclosed within the bound radius $r_\mathrm{bound}=16.1\,\mathrm{kpc}$ are $M_\star = 5.19\times10^6\,\mathrm{M_\odot},\,M_\mathrm{gas}=2.88\times10^7\,\mathrm{M_\odot}\text{ and }M_\mathrm{DM}=1.16\times10^9\,\mathrm{M_\odot}$ at $t=4.7\,\mathrm{Gyr}$, respectively.