Dark Minihalos with Intermediate Mass Black Holes

We argue that the Milky Way (MW) contains ${10}^3–{10}^4$ intermediate mass black holes (IMBHs) of mass $\sim {10}^{2–3}{M}_{\odot }$. Some IMBHs are naked, and some are enshrouded by dense dark minispikes and by minihalos of $({10}^6–{10}^7)M_{\odot }$. The IMBH is formed off-center by gas accretion in a minihalo. These dense minihalos (the nearest at about 2 kpc) survive mostly without tidal stripping by the MW, and are largely invisible except that their pointlike neutralino annihilation signals (with bolometric luminosities of ${10}^{3–5}{L}_{\odot }\frac{50\mathrm{GeV}}{m_{\chi }}$ within $0.01–0.1\mathrm{pc}$ of their centers) stand out well above the MW background and are more luminous than outer dwarf satellite galaxies.

Figure 1

The log-log radial profiles of the rescaled halo density $\rho (r)/\rho (r_c)$, escape velocity $V_{\mathrm{esc}}(r)/(K\sqrt{2})$, dispersion $2\sigma (r)/K$, circular velocity $V_{\mathrm{circ}}(r)/K$, and enclosed mass $M(r)/100\rho (r_c)r_c^3$ in our cored halo model (crosses), NFW model (blue dashed lines), and N04 model (red lines). Here $K=\sqrt{G\rho (r_c)r_c^2}$, and $r_c$ is the core radius.

Figure 2

The log-log plot of the radial profiles of spherically averaged density (black lines) in $M_{\odot }{\mathrm{pc}}^{-3}$ inside a minihalo in three cases: a strong minispike around a $1000M_{\odot }$ IMBH (thick line), a moderate minispike around a $100M_{\odot }$ IMBH, or a pure cored minihalo where the IMBH either was never formed or was ejected (thin line). Lines (green parallel) of constant enclosed mass $1M_{\odot },10M_{\odot },\dots ,{10}^{12}{M}_{\odot }$ are shown to guide the eye. For comparison are the average densities inside a dwarf galaxy (lower thick line, with a dispersion of $10\mathrm{km}{\mathrm{s}}^{-1}$ and a core of 200 pc) and inside the pristine MW halo (lower right thin line). Also shown is the logarithm of the rescaled annihilation (bolometric) luminosity $\frac{L_{\chi \overline{\chi }}(<r)}{L_{\odot }}\times \frac{m_{\chi }}{50\mathrm{GeV}}$ (nearly horizontal red lines to the right) of these models. Also shown are the total luminosity of ${10}^3$ minihalos with $100M_{\odot }$ IMBHs (thickest black at top middle), and the luminosity of the MW as a whole (thick red line) or scaled down by a factor of $\frac{\Delta \Omega }{4\pi }$ for a cone of $\Omega ={10}^{-3}$ (thin red line). The minihalos stand out as clumps above the MW background.