arXiv subjects

Nicolas Esser

Publications and source records attributed to Nicolas Esser.

7 recordsLinked to original sources

arXiv2026

Collisional Dynamics of Stars and Dark Matter in Ultra-Faint Galaxies

We use controlled N-body simulations to study the collisional exchange of energy between stars and dark matter in ultra-faint galaxies. We find that dynamical friction between stars and subsolar-mass dark matter particles results in the depletion of dark matter from the galaxies' centers, thereby transforming dark matter cusps into constant-density cores. The process is particularly effective in tidally limited galaxies with low stellar velocity dispersion. As high-mass stars sink toward the center of the dark matter halo, the dynamical-to-stellar mass ratio within the stellar half-light radius decreases monotonically. The stellar population of a dark matter-dominated galaxy is thereby compacted into a dense, baryon-dominated cluster, surrounded by a dark matter halo. Such a cluster would share the chemical composition of an ultra-faint galaxy, yet would be virtually dark matter-free within its half-light radius. We moreover find that the collisional cooling with dark matter particles provides an efficient pathway for the formation of stellar binaries in the contracting cluster. The contraction is eventually slowed down due to the decreasing central dark matter densities and the formation of stellar binaries. Our models highlight that the dynamical processes governing the faintest galaxies give rise to a rich phenomenology, blurring the line between the dynamics of globular clusters and galaxies.

astro-ph.GA↗

arXiv2026

Gravitational waves from primordial black holes passing by neutron stars: observational prospects for the Galactic center

We investigate the gravitational wave (GW) signals emitted by planetary-mass primordial black holes (PBHs) passing nearby or traversing neutron stars (NSs). While previous studies mainly focused on the detailed waveforms of the signals, we estimate the rate of PBH-NS gravitational-wave events originating from the Galactic center and compute the probability of detecting a signal over 10 years of LIGO-Virgo-KAGRA observations. We examine in detail the case of PBHs bound to NSs, focusing on eccentric orbits that give rise to repeated GW bursts emitted in correlated series, each burst corresponding to a periastron passage. Despite the enhancement from the large number of bursts produced by a single PBH-NS pair, the total number of signals produced in this way remains subdominant to those due to random unbound encounters of PBHs with NSs. We also find that both types of signals have a very small probability $P\lesssim 10^{-8}$ to be detected in a 10 year period.

astro-ph.HE↗

arXiv2026

Dynamical friction on binary stars in dark matter dominated environments

We study binary stars moving through a uniform dark matter background and experiencing dynamical friction. The centre-of-mass motion of the pairs is taken into account. We derive formulas and timescales for the secular evolution of the orbital parameters for both wide and close binaries. We apply these results to environments typical of dark matter dominated ultra-faint dwarf galaxies and show that some binaries undergo significant eccentricity oscillations, while their semi-major axes decrease more gradually. We consider a simple binary star population and find that dynamical friction, notably, can enhance the bias from unresolved binaries in velocity dispersion measurements. With future, more detailed theoretical studies and improving observational capabilities, binary stars may serve as a tool to probe the dark matter content of some of the faintest galaxies.

astro-ph.GA↗

arXiv2025

Primordial black hole-star binaries via dynamical friction

We study a new channel for binary system formation involving stars and stellar-mass primordial black holes (PBHs) embedded in dark matter (DM) minihalos. In this scenario, binaries form when a star passes through the DM minihalo surrounding a PBH and loses sufficient energy due to dynamical friction. The continued energy loss induced by this friction is expected to drive the resulting systems to merge rapidly. We estimate their merger rate and explore the implications for two observables: rapidly decaying X-ray binaries in the Milky Way and gravitational waves sourced by compact object mergers. We find that, for a PBH abundance $\Omega_\text{PBH}/\Omega_\text{DM} = 0.01$, this mechanism naturally produces a population of $\mathcal{O}(1)$ currently observable short-lived X-ray binaries. It also leads to a non-negligible gravitational wave event rate of $\mathcal{O}(0.3)$ Gpc$^{-3}$yr$^{-1}$, potentially involving high mass ratios and black holes in the lower or upper mass gap. Notably, most mergers arise in low-mass galaxies, making the latter rate sensitive to the low-mass end of the galaxy stellar mass function. The dynamical friction channel thus offers a plausible explanation for several unusual observations reported in recent years across both the X-ray and gravitational wave domains.

astro-ph.HE↗

arXiv2025

Constraints on asteroid-mass primordial black holes in dwarf galaxies using Hubble Space Telescope photometry

Primordial black holes (PBHs) in the asteroid-mass range remain a viable and until now unconstrained dark matter (DM) candidate. If such PBHs exist, they could be captured by stars in DM-dominated environments with low velocity dispersion such as ultra-faint dwarf galaxies (UFDs). The capture probability increases with the stellar mass, and captured PBHs would rapidly destroy their host stars. As a result, the presence of PBHs in UFDs would alter their stellar mass functions. Using photometric observations of three ultra-faint dwarf galaxies from the Hubble Space Telescope, we show that it is unlikely that their mass functions have been significantly modified by PBHs, and we place constraints on the PBH abundance. In the ultra-faint dwarf galaxy Triangulum II, PBHs around $10^{19}$g are excluded at the $2\sigma$ ($3\sigma$) level from constituting more than $\sim55\%$ ($\sim78\%$) of the dark matter, while the possibility that PBHs represent the entirety of the DM is excluded at the $3.7\sigma$ level.

astro-ph.GA↗

arXiv2023

The impact of primordial black holes on the stellar mass function of ultra-faint dwarf galaxies

If primordial black holes constitute the dark matter, stars forming in dark-matter dominated environments with low velocity dispersions, such as ultra-faint dwarf galaxies, may capture a black hole at birth. The capture probability is non-negligible for primordial black holes of masses around $10^{20}$g, and increases with stellar mass. Moreover, infected stars are turned into virtually invisible black holes on cosmologically short timescales. Hence, the number of observed massive main-sequence stars in ultra-faint dwarfs should be suppressed if the dark matter was made of asteroid-mass primordial black holes. This would impact the measured mass distribution of stars, making it top-light (i.e. depleted in the high-mass range). Using simulated data that mimic the present-day observational power of telescopes, we show that already existing measurements of the mass function of stars in local ultra-faint dwarfs could be used to constrain the fraction of dark matter composed of primordial black holes in the -- currently unconstrained -- mass range of $10^{19}-10^{21}$g.

astro-ph.GA↗

arXiv2022

Constraints on primordial black holes from observation of stars in dwarf galaxies

We propose a way to constrain the primordial black hole (PBH) abundance in the range of PBH masses $m$ around $10^{20}$g based on their capture by Sun-like stars in dwarf galaxies, with subsequent star destruction. We calculate numerically the probability of a PBH capture by a star at the time of its formation in an environment typical of dwarf galaxies. Requiring that no more than a fraction $\xi$ of stars in a dwarf galaxy is destroyed by PBHs translates into an upper limit on the PBH abundance. For the parameters of Triangulum II and $\xi=0.5$, we find that no more than $\sim 35\%$ of dark matter can consist of PBHs in the mass range $10^{18} - \text{(a few)}\times 10^{21}$g. The constraints depend strongly on the parameter $\xi$ and may significantly improve if smaller values of $\xi$ are established from observations. An accurate determination of $\xi$ from dwarf galaxy modeling is thus of major importance.

astro-ph.HE↗