Robust bounds on MACHOs from the faintest galaxies

TL;DR

The paper tackles whether MACHOs can make up a significant fraction of dark matter by using dynamical heating of stars in ultrafaint dwarfs as a gravitational probe. It develops a framework that maps MACHO-induced heating rates to bounds on the MACHO fraction $f_{ m MACHO}$ as a function of mass $M_{ m MACHO}$, explicitly accounting for uncertainties in UFD properties and testing both cored Dehnen and cuspy NFW halos. By incorporating multiple UFDs, including the recently discussed Ursa Major III/UNIONS 1, the authors show that bounds are robust across profiles and that Ursa Major III could yield the strongest constraints in the $1-10^5\,M_\odot$ range. The results highlight the power of UFDs as gravitational probes of DM microphysics and motivate future observations of ultra-compact UFDs to tighten MACHO/PBH limits.

Abstract

We use the dynamical heating of stars in ultrafaint dwarf (UFD) galaxies to set limits on Massive Compact Halo Objects (MACHOs). In our analysis we study the robustness of the bounds under uncertainties in key UFD parameters, such as the half-light radius, stellar velocity dispersion, total halo mass and dark matter and stellar density profiles. We apply this framework to both well-established UFD candidates, as well as the recently discovered UFD candidate Ursa Major III/UNIONS 1. We find that multiple UFDs yield consistently strong limits in the mass range $10\, M_\odot \lesssim M_{\rm MACHO} \lesssim 10^9\, M_\odot$, underscoring the robustness of a previous analysis solely based on Segue I. We also demonstrate that Ursa Major III, if confirmed as an UFD, would improve the constraints significantly, providing the strongest constraints on MACHO dark matter in the mass range $1\, M_{\odot}\lesssim M_{\rm MACHO} \lesssim 10^5\, M_\odot$.

Figures (8)

• Figure 1: DM density (blue) and radial velocity dispersion (red) for Segue I, for an NFW (solid) and a cored Dehnen (dashed) profile. The profile parameters are fixed using the total halo mass $M_{\rm UFD} = 10^9\, M_\odot$ and the estimate of the stellar line-of-sight velocity dispersion in Eq. \ref{['eq:MassEstimator']}. The velocity dispersions assume hydrostatic equilibrium and emphasize the different behavior of the velocity dispersion in NFW and cored Dehnen. While the velocity dispersion grows as $\sqrt{r}$ for an NFW profile, it is approximately constant within the core of a cored Dehnen profile.
• Figure 2: MACHO Density enhancement factor $\eta$ for Segue I. The solid (dashed) lines show contours of constant $\eta$ assuming a cored Dehnen (NFW) DM profile. The parameter region below the black diagonal line corresponds to a scenario with on average less than $3$ MACHOs in the whole UFD and is therefore not experimentally accessible.
• Figure 3: Direct heating rate multiplied by the stellar distribution function for Segue I assuming a cored Dehnen (solid) and an NFW (dashed) DM profile. The MACHO fraction is fixed to $f_{\rm MACHO} = 10^{-2}$ and we use a Plummer stellar profile with $M_* = 5.7\cdot 10^2\, M_\odot$ and $R_{0,*}=24.7$ pc in both cases.
• Figure 4: Profile dependence of the limits on the fraction of MACHO DM $f_{\rm MACHO}$ as a function of the MACHO mass $M_{\rm MACHO}$ from dynamical heating of stars in Segue I. In red (blue) we show the limits assuming an NFW (cored Dehnnen) profile for DM. The solid and dashed curves show results for a Plummer and exponential profile for the stars, respectively.
• Figure 5: Dependence on the uncertainty of the astrophysical input parameters of the limits on the fraction of MACHO DM $f_{\rm MACHO}$ as a function of the MACHO mass $M_{\rm MACHO}$ from dynamical heating of stars in Segue-I. The figure shows the variation of the bound within the uncertainties of the stellar half-light radius (red), the line-of-sight velocity dispersion (green) and the total halo mass (orange). The solid black line assumes the central values of all parameters from Stegmann:2019wyz. The solid orange lines correspond to the total halo mass uncertainties provided in Stegmann:2019wyz. The dashed black and orange lines are the central value and uncertainties of the total halo mass from 10.1093/mnras/sty2505. See the text for details. The left panel shows the variation of the bound in a cored Dehnen profile, whereas the right panel assumes an NFW DM profile. The bounds in both plots are for a Plummer stellar profile.