Improved constraints from ultra-faint dwarf galaxies on primordial black holes as dark matter

Abstract

Soon after the recent first ever detection of gravitational waves from merging black holes it has been suggested that their origin is primordial. Appealingly, a sufficient number of primordial black holes (PBHs) could also partially or entirely constitute the dark matter (DM) in the Universe. However, recent studies on PBHs in ultrafaint dwarf galaxies (UFDGs) suggest that they would dynamically heat up the stellar component due to two-body relaxation processes. From the comparison with the observed stellar velocity dispersions and the stellar half-light radii it was claimed that only PBHs with masses 10 M can significantly contribute to the DM. In this work, we improve the latter constraints by considering the largest observational sample of UFDGs and by allowing the PBH masses to follow an extended (log-normal) distribution. By means of collisional Fokker-Planck simulations, we explore a wide parameter space of UFDGs containing PBHs. The analysis of the half-light radii and velocity dispersions resulting from the simulations leads to three general findings that exclude PBHs with masses ∼ O(1-100) M from constituting all of the DM: (i) We identify a critical sub-sample of UFDGs that only allows for ∼ O(1) M PBH masses; (ii) for any PBH mass, there is an UFDG in our sample that disfavours it; (iii) for a majority of UFDGs, dynamical heating by PBHs would be too efficient to match the observed stellar half-light radii.

The MUSE-Faint survey

“…Other constraints of various strengths do exist in this mass range, from lack of mass segregation in Segue 1 (Koushiappas & Loeb 2017), the distribution of widebinary separations (Monroy-Rodríguez & Allen 2014), and lack of an imprint on the cosmic microwave background from accreting primordial black holes (Ali-Haïmoud & Kamionkowski 2017). Another constraint (Stegmann et al 2020), comparing the observed half-light radii of a large sample of UFDs to simulations, also rules out this mass range for dark matter consisting completely of primordial black holes, even when considering extended mass distributions, but the methodology is unable to provide answers for lower abundances. As each of these astrophysical constraints involves assumptions, corroboration from different sources is still valuable, especially considering this mass range is the last window for dark matter purely consisting A107, page 13 of 16 of MACHOs that is not robustly closed by more fundamental physics.…”

INTEGRALconstraints on primordial black holes and particle dark matter

“…PBHs, formed from SM plasma that collapsed due to its own gravity in the very early Universe, are a possible solution to the DM puzzle [7][8][9][10][11][12][13]. The fraction f PBH of DM in the form of PBHs is constrained to be below unity for PBH masses M PBH ≳ 10 23 g (≈5 × 10 −11 M ⊙ ) via various observations, such as gravitational lensing [14][15][16][17][18][19][20][21][22][23][24], stellar dynamics [25][26][27][28][29][30][31], gravitational waves [32][33][34][35][36][37][38][39][40][41], and the cosmic microwave background (CMB) [42][43][44][45]. The situation is different for lower-mass PBHs, as their gravitational signatures are not strong enough to cause a measurable effect in existing data.…”

Generation of gravitational waves and tidal disruptions in clumpy galaxies

“…In order to build a realistic model for the star-forming clumps in which IMBHs may reside, we rely on the numerical work of Tamburello et al (2015), where the properties of massive star-forming clumps in isolated galaxies were studied in detail. They performed a large suite of galaxy simulations, to assess the dependence of gas fragmentation properties in a galaxy on sub-grid physics, galaxy mass, structural parameters, and resolution, using the smoothed particle hydrodynamic code gasoline2 (Wadsley et al 2017), an updated version of gasoline (Stadel 2001;Wadsley et al 2004). In this paper, we specifically analyse the clumps forming in their run 27 (see tables 1 and 2 of Tamburello et al 2015), in which they simulated a dark matter halo with an embedded stellar and gaseous disc, with a virial and a baryonic mass of 2.5 × 10 12 and 7.8 × 10 10 M , respectively, a gas fraction of 50 per cent, and a virial concentration of 6.…”

“…Using skid (Stadel 2001), a group finder for N -body simulations, we extracted the mass and half-mass radius of the largest clumps formed in the simulation, finding a typical stellar clump mass…”

“…8 phaseflow has been successfully tested in the context of e.g. nuclear star clusters (Gen- Bahcall & Wolf (1976) cusps (Vasiliev 2017), primordial BH mass functions (Zhu et al 2018;Stegmann et al 2020), and TDE rates in galaxy merger remnants and nuclear star clusters (Pfister et al 2019(Pfister et al , 2020.…”