Constraining galactic structures of mirror dark matter

The simplest model of mirror sector dark matter maintains exact mirror symmetry, but has a baryon abundance $Ω_{b'} = βΩ_b$ and a suppressed temperature $T' = x T$ in the mirror sector; hence it depends only on two parameters, $β,x$. For sufficiently small $x$, early cosmological observables may not constrain mirror baryons from constituting all of the dark matter despite their strong self-interactions, depending on the unknown details of structure formation in the hidden sector. Here we close this loophole by simulating mirror structure formation, mapping out the allowed regions of parameter space using cosmological and astronomical data. We find that the Milky Way disk surface density and bulge mass constrain $Ω_{b'}\lesssim 0.3 Ω_{b}$ at the highest $T'$ allowed by BBN and CMB ($T'=0.5 T$), or $Ω_{b'}\lesssim 0.8 Ω_{b}$ at lower values of $T'$. We also briefly discuss the realization of the necessary temperature asymmetry between the SM and the mirror sector in our model with unbroken mirror symmetry.

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