Collapse versus Disruption: The Fate of Compact Stellar Systems in Ultralight Dark Matter Halos

Abstract

Interference of the ultralight dark matter (ULDM) field generates time-varying gravitational potential fluctuations, which stochastically heat stellar systems embedded in ULDM halos. Small-sized stellar systems are therefore often used to set stringent constraints on ULDM. However, the evolution of systems with sizes well below the ULDM de Broglie wavelength remains poorly explored. Using numerical simulations, we show that the evolution of compact stellar systems in ULDM halos is governed by the interplay between internal stellar relaxation and ULDM-induced heating. We find the following main results. First, in sufficiently compact systems, relaxation-driven core collapse dominates, allowing the system to remain bound and dense, while ULDM-induced stripping of outer stars further accelerates the collapse. Second, in more extended systems, ULDM heating dominates and ultimately disrupts the system. Near the disruption threshold, we identify systems resembling ultra-faint dwarfs like Segue 1. Third, we further introduce a dimensionless parameter to quantify the relative importance of heating and relaxation and finally lead to an evolutionary phase diagram. Our results reveal the rich and nontrivial dynamics of compact stellar systems in ULDM halos, indicating that precise system modeling is essential for robust ULDM constraints.

Figures (5)

• Figure 1: Circularly averaged stellar surface density profiles of six simulation systems at different snapshots, obtained by projecting onto the $z=0$ plane and azimuthal averaging. Insets show the corresponding velocity dispersion profiles along the $z$-direction. Colors indicate evolutionary time, while solid and dashed lines denote evolution with and without ULDM, respectively. In the last two panels, the light-red and light-green shaded regions represent fluctuations arising from 100 randomly selected viewing directions. Observational data for Segue 1 are also shown, including the stellar surface density (purple points) Martin_2008 and line-of-sight velocity dispersion (red and black) profiles Simon_2011.
• Figure 1: Upper left: orbit of the ULDM halo during infall into the MW following an outside evolution of 3 Gyr. Lower left: temporal evolution of the distance between the stellar system mass point and the soliton center. Right: three-dimensional ULDM density distributions on the $z=0$ plane at four different simulation snapshots.
• Figure 2: Stellar surface density maps of S1 at different simulation times, obtained by projecting stellar particles onto the $z=0$ plane and constructing the density using a particle-mesh method Vogelsberger:2019ynw. The upper and lower panels show evolution without and with ULDM, respectively.
• Figure 3: Time evolution of the core radius $r_c$ and Lagrangian radii of S1, with colors indicating different enclosed mass fractions. Solid and dashed lines denote evolution with and without ULDM, respectively. The inset shows the ratio of the averaged relaxation time within $r_{0.05}$ for the cases with and without ULDM.
• Figure 4: Phase diagram of stellar systems within ULDM halos for $m_a=10^{-22}$eV. Filled circles, crosses, and pentagrams denote systems undergoing core collapse, disruption, and negligible evolution, respectively. Squares mark systems with strong collapse that are susceptible to numerical artifacts. Upward triangles, downward triangles, and diamonds indicate intermediate cases between collapse and no evolution, disruption and no evolution, and collapse and disruption, respectively, and are therefore difficult to classify unambiguously. Colors encode different $R_\mathrm{h}$. The six simulation sets S1-S6 are indicated separately.