Low Mass Black Holes from Dark Core Collapse

Unusual masses of black holes being discovered by gravitational wave experiments pose fundamental questions about the origin of these black holes. Black holes with masses smaller than the Chandrasekhar limit $\approx 1.4M_{\odot }$ are essentially impossible to produce through stellar evolution. We propose a new channel for production of low mass black holes: stellar objects catastrophically accrete nonannihilating dark matter, and the small dark core subsequently collapses, eating up the host star and transmuting it into a black hole. The wide range of allowed dark matter masses allows a smaller effective Chandrasekhar limit and thus smaller mass black holes. We point out several avenues to test our proposal, focusing on the redshift dependence of the merger rate. We show that redshift dependence of the merger rate can be used as a probe of the transmuted origin of low mass black holes.

Figure 1

DM mass and scattering cross section required for a dark core collapse and subsequent transmutation of a $1.3M_{\odot }$ NS to a comparable mass BH are shown in the red shaded regions. The two panels on the left (respectively, right) correspond to interaction between DM and stellar nuclei mediated by an infinitely heavy mediator, i.e., $m_{\varphi }\gg$ recoil momentum (respectively, 10 MeV scalar). Scenarios with nonannihilating bosonic or fermionic DM are marked. Two representative values of ambient DM density, ${\rho }_{\chi }=1$ and ${10}^3\mathrm{GeV}{\mathrm{cm}}^{-3}$ (only for the right panel, ${\rho }_{\chi }=10$ and ${10}^3\mathrm{GeV}{\mathrm{cm}}^{-3}$), are considered. Exclusion limits from underground direct detection experiments PandaX-II [84] and XENON1T [85], as well as from existence of an $\sim 7\mathrm{Gyr}$ old nearby pulsar PSR J0437-4715 [58, 59, 60, 86], are also shown by the gray shaded regions. Green hatched regions mark the parameter space where efficient Hawking evaporation stops the implosion of the NS. The region of no thermalization is many orders of magnitude below [59] and is not shown for clarity.

Figure 2

Cosmic evolution of the binary merger rates and mass distributions of the compact objects provide a simple yet novel technique to determine the stellar or primordial origin of BHs. The left panel corresponds to the cosmic evolution of the binary PBH, NS, and TBH merger rates. For the binary NS and TBH merger rate, cosmic star formation rate is adopted from [92] and they are normalized to the recent LIGO-VIRGO measurement [95]. Nonannihilating bosonic DM with mass of 10 TeV and DM-nucleon scattering cross section of ${10}^{-45}$ and ${10}^{-47}{\mathrm{cm}}^2$ in the contact approximation are assumed for the estimation of binary TBH merger rate. The PBH merger rate is estimated by considering $1.3M_{\odot }–1.3M_{\odot }$ PBH binary and a dark matter fraction $f_{\mathrm{PBH}}={10}^{-3}$, which enters as $f_{\mathrm{PBH}}^{53/37}$ [87, 88]. The middle (right) panel corresponds to the mass distribution of all observed NSs (WDs) [96, 97]. Mass distributions of the progenitors can be compared against some well-motivated PBH mass distributions to examine the origin of low mass BHs.