Collapsed Dark Matter Structures

The distributions of dark matter and baryons in the Universe are known to be very different: The dark matter resides in extended halos, while a significant fraction of the baryons have radiated away much of their initial energy and fallen deep into the potential wells. This difference in morphology leads to the widely held conclusion that dark matter cannot cool and collapse on any scale. We revisit this assumption and show that a simple model where dark matter is charged under a “dark electromagnetism” can allow dark matter to form gravitationally collapsed objects with characteristic mass scales much smaller than that of a Milky-Way-type galaxy. Though the majority of the dark matter in spiral galaxies would remain in the halo, such a model opens the possibility that galaxies and their associated dark matter play host to a significant number of collapsed substructures. The observational signatures of such structures are not well explored but potentially interesting.

Figure 1

Range of dark matter halo masses that cool as a function of $m_L$ for $m_H=1.2\mathrm{TeV}$ (left) and 250 GeV (right). Colored regions result in halos cooling for ${\alpha }_D={10}^{-1}$ (red) and ${\alpha }_D={10}^{-2}$ (blue). Lower limits on the halo mass as a function of $\xi$ are shown as solid, dashed, and dotted lines for $\xi =0.5$, 0.1, and 0.02, respectively (red for ${\alpha }_D={10}^{-1}$, blue for ${\alpha }_D={10}^{-2}$). For $m_H=250\mathrm{GeV}$, the ${\alpha }_D=0.1$ region violates the self-scattering bound Eq. (7) but is included to demonstrate the cooling dependence on $m_H$ and ${\alpha }_D$.