Can dwarf spheroidal galaxies host a central black hole ?

Abstract

We construct mass models of Milky Way dwarf spheroidal galaxies to place constraints on the central black hole (BH) masses they can host. We model the galaxies as a three-component system consisting of the stars, dark matter halo, and a central black hole, using the Osipkov--Merritt--Cuddeford class of anisotropic distribution function. The posterior distribution of black hole mass remains flat toward the low-mass end, indicating that the kinematic data places an upper limit on the black hole mass. Our analysis yields a 95% credible upper limit of $\log (M_{\bullet }/M_{\odot })<6$. We combine our results with black hole mass measurements and upper limits from the literature to construct a unified $M_{\bullet }$--$\ddot{I}{ƒ}_{}$ relation spanning $\ddot{I}{ƒ}_{}\sim 10$--$300,\mathrm{km},{\mathrm{s}}^{-1}$, described by $\log (M_{\bullet })=8.32+4.08,\log \left(\ddot{I}{ƒ}_{}/200,\mathrm{km},{\mathrm{s}}^{-1}\right)$, with an intrinsic scatter of $\ddot{I}{ƒ}_{\mathrm{int}}=0.55$. We compare the inferred limits to models of black hole growth via momentum-driven accretion and stellar capture, which predict black hole masses in the range $10^3$--$10^4,M_{\odot }$ for the range $\ddot{I}{ƒ}_{}\sim 6$--$12,\mathrm{km},{\mathrm{s}}^{-1}$, in close agreement with the $M_{\bullet }$--$\ddot{I}{ƒ}_{\ast }$ relation within the 95% credible upper limits on the black hole masses derived in this work.

Accepted for publication in ApJ. Comments are welcome