Black hole surrounded by a dark matter halo in the M87 galactic center and its identification with shadow images

In this paper we present a new black hole solution surrounded by dark matter (DM) halo in the galactic center using the mass model of M87 and that coming from the universal rotation curve (URC) dark matter profile representing family of spiral galaxies. In both cases the DM halo density is cored with a size ${r}_{0}$ and a central density ${\ensuremath{\rho}}_{0}$: $\ensuremath{\rho}(r)={\ensuremath{\rho}}_{0}/(1+r/{r}_{0})(1+(r/{r}_{0}{)}^{2})$. Since ${r}_{0}{\ensuremath{\rho}}_{0}=120\text{ }\text{ }{\mathrm{M}}_{\ensuremath{\bigodot}}/{\mathrm{pc}}^{2}$ [Mon. Not. R. Astron. Soc. 397, 1169 (2009)], then by varying the central density one can reproduce the DM profile in any spiral. Using the Newman-Jains method we extend our solution to obtain a rotating black hole surrounded by dark matter halo. We find that the apparent shape of the shadow beside the black hole spin $a$, it also depends on the central density of the surrounded dark matter ${\ensuremath{\rho}}_{0}$. As a specific example we consider the galaxy M87, with a central density ${\ensuremath{\rho}}_{0}=6.9\ifmmode\times\else\texttimes\fi{}{10}^{6}\text{ }\text{ }{\mathrm{M}}_{\ensuremath{\bigodot}}/{\mathrm{kpc}}^{3}$ and a core radius ${r}_{0}=91.2\text{ }\text{ }\mathrm{kpc}$. In the case of M87, our analyses show that the effect of dark matter on the size of the black hole shadow is almost negligible compared to the shadow size of the Kerr vacuum solution hence the angular diameter $42\text{ }\text{ }\ensuremath{\mu}\mathrm{as}$ remains almost unaltered when the dark matter is considered. For a small totally dark matter dominated spiral such as UGC 7232, we find similar effect of dark matter on the shadow images compared to the M87. However, in specific conditions having a core radius comparable to the black hole mass and dark matter with very high density, we show that the shadow images decreases compared to the Kerr vacuum black hole. The effect of dark matter on the apparent shadow shape can shed some light in future observations as an indirect way to detect dark matter using the shadow images.

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